community acceptance of helicopter noise criteria and application

NASA CR-132430

COMMUNITY ACCEPTANCE OF HELICOPTER

NOISE: CRITERIA AND APPLICATION

By Charles L. Munch

Robert J. King

Prepared Under Contract No. NAS1-12495 By Sikorsky Aircraft

Division of United Aircraft Corporation Stratford, Connecticut

for

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

This page intentionally left blank.

ii

TABLE O F CONTENTS

PAGE TABLES.. v . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

v i ILLUSTRATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v i i i SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 S U M M A R Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

INTRODUCTION.

NOISE ACCEPTANCE CRITERIA 3 . . . . . . . . . . . . . . . . . . . . . . . 3 Noise Criteria Development . . . . . . . . . . . . . . . . . . . 3 Select ion of a r a t i n g scale

Tone correct ions. . . . . . . . . . . . . . . . . . . . . . 5 Duration e f f e c t s . . . . . . . . . . . . . . . . . . . . . .

Select ion of a common comparison b a s i s for r a t i n g schemes .5 6 Evalu&tisn of c - i r e n t c r i t e r i a . . . . . . . . . . . . . . .

The impact of impulsive noise on acceptab i l i ty of helicop-

. . . . . . . . . . . . . . . ' 4

t e r s . . . . . . . . . . . . . . . . . . . . . . . . . . 9

11 Select ion of Criteria f o r Civ i l Helicopter Operations. . . . . . Computation of LbN. 11 . . . . . . . . . . . . . . . . . . . . Acceptabili ty c r i t e r i a i n terms of bN. 12

1 4 Use of the C r i t e r i a . . . . . . . . . . . . . . . . . . . . . . . Select ion of Typical Locations and Operations for Evaluation of

15 Baseline Helicopter Noise . . . . . . . . . . . . . . . . . .

. . . . . . . . . .

C I V I L TRANSPORT HELICOPTER NOISE EVALUATION . . . . . . . . . . . . . . 1 5

1 6 Charac te r i s t ics of t h e Basic Helicopter. 16 Helicopter Description . . . . . . . . . . . . . . . . . . . . . 16 Noise Predict ion Method. . . . . . . . . . . . . . . . . . . . .

Basic Helicopter Noise Character is t ics . . . . . . . . . . . . . 18 Noise Reduction Requirements . . . . . . . . . . . . . . . . . . 18 Effec t of Impulsive Noise. . . . . . . . . . . . . . . . . . . .

Noise Reduction t o Meet t h e Community Acceptance C r i t e r i a . . . .I9

. . . . . . . . . . . .

HARDWARE TESTS REQUIRED . . . . . . . . . . . . . . . . . . . . . . . .20

iii

TABLF: OF CONTENTS (continued)

PAGE . Noise Acceptance C r i t e r i a . . . . . . . . . . . . . . . . . . . 21 C i v i l Helicopter Operations . . . . . . . . . . . . . . . . . . 22

RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A 1

Example Use of Noise Acceptance C r i t e r i a . . . . . . . . . . . . A 1

TABLES

PAGE 7

I.

11.

Frequency Weighted Terms Used t o Define Judged Annoyance Loudness .37

Simi la r i ty of SPL(A) and PNL Unit Accuracy for Assessing Noise Spectrum Annoyance . . . . . . . . . . . . . . . . . . . . . . . .38

111. Conversion of Federal Noise C r i t e r i a Units t o LA . . . . . . . . .39

IV. Measured Community Noise Levels. . . . . . . . . . . . . . . . . .bo

V. Data Used i n Preparing Blade Slap Crest Factor Information . . . .bo

VI. Recommended C i v i l Helicopter Operations for Determining Ai rc ra f t Compliance with Noise Criteria . . . . . . . . . . . . . . . . . .41

V I I . Summary. of Ai rcraf t Hardware Changes Evaluated . . . . . . . . . .42

LIST OF FIGURES

Comparison of Domestic and Foreign Federal Community Acceptance PAGE -

1. Noise Guidelines . . . . . . . . . . . . . . . . . . . . . . . . 43

2.

3.

4.

Comparison of Clear ly Acceptable L i m i t s of Federal Regulations . 44

Comparison of Res ident ia l Community Noise Regulations. . . . . . 45

Effec t of Blade Slap on Judged Annoyance of Helicopters. . . . . 46

5. Instrumentation Used i n the Inves t iga t ion of Impulsive Noise Annoyance. . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Blade Slap Annoyance as a Function of Signal Crest Factor. . . . 48 6.

7. Oscillograph Traces of Helicopter Sounds Used i n Impulsive Noise T e s t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . k g

8. Possible Preliminary Impulsive Noise Annoyance Penal ty C r i t e r i a . 50

9. Comparison of Federal, Camznity, and S%a%e Noise Regulations and Guidelines with the Proposed Community Acceptance Cr i te r ion . . . 51

10. Recommended Community Noise Acceptance Criteria. . . . . . . . . 52

11. Effec t of Ambient Noise on t h e Calculated LDN f o r a Given A i r -

12 . CH-53D Helicopter Description. . . . . . . . . . . . . . . . . . 54

c r a f t Noise Signature. . . . . . . . . . . . . . . . . . . . . . 53

13. S-67-40 Commercial Transport Helicopter. . . . . . . . . . . . . 55

1 4 . Typical Correlat ion of Predicted and Measured Performance of an H-53 Helicopter. . . . . . . . . . . . . . . . . . . . . . . . . 56

15. Comparison of Measured and Predicted Rotor Noise . . . . . . . . 57

16. Comparison of Calculated and Measured PNLT Time History f o r the CH-53D Helicopter. . . . . . . . . . . . . . . . . . . . . . . . 58

17. Four Takeoff P r o f i l e s for t h e S-65-40 Helicopter Used for Noise Evaluation . . . . . . . . . . . . . . . . . . . . . . . . 59

18. Comparison of Actual and Approximate F l igh t P r o f i l e s . . . . . . 60

19. Equal Noise Ground Contours for Takeoff 1 - Ver t i ca l Climb t o 500 Feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G I

v i ,

LIST OF FIGURES (continued) PAGE -

20. Equal Noise Ground Contours f o r Takeoff 2 - Ver t i ca l Climb t o 250 F e e t . . , . . . . . . . . . , . . . . . . . . . . . . . . . 6 2

21. Equal Noise Ground Contours fo r Takeoff 3 - Oblique. . . . . . . 63

22. Equal Noise Ground Contours f o r Takeoff 4 - Horizontal . . . . . 64

23. Comparison of t h e 95 SENEL Ground Noise Contours f o r Four Differ- en t Takeoffs of t he Baseline Helicopter . . . . . . . . . . . . 65

24. Noise Footpr int Area Character is t ic f o r Four Baseline Helicopter T a k e o f f s . . . . . . . . . . . . . . . . . . . . . , . . . . . . 66

Equal Noise Ground Contours f o r Landing - 10' Glide Slope. . . . 67 25.

26. Comparison of Baseline Helicopter Noise Charac te r i s t ics With Community Acceptance C r i t e r i a . . . . . . . . . . . . . . . . . . 68

27. Contribution of Each Noise Source t o t h e Baseline Helicopter N o i s e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

28. Baseline Helicopter Cruise Noise Compared With t h e Noise Accep- tance C r i t e r i a . . . . . . . . . . . . . . . . . . . . . . . . . 70

29. Effec t of Impulsive Noise on a Helicopter 's Noise Annoyance Charac te r i s t ics . . . . . . . . . . . . . . . . . . . . . . . . . 71

30. Comparison of t h e Noise Footprints of t h e Baseline S-65-40 Helicopter and Six Modified S-65-40 Helicopters. . . . . . . . . 72

SENEL Footpr int Area Character is t ic f o r t h e Modified S-65-40 Helicopters. . . . . . . . . . . . . . . . . . . . . . . . . . . 73'

31.

32. Octave Band Noise Reduction Resulting i n Modification F H e l i - c o p t e r . . . . . . . . . . . . . . . . . . . . . . . , . . . . . 74

33. F l igh t P ro f i l e s of the Modified S-65-40 Helicopters. . . . . . . 75

v i i

C

CNEL

CNR

EPNL

EPNdB

L

LA

LB

LC

L~~

%I

LNN

LLS

LT

LLZ

Ln

L90

LA

T -3

50 L

0 L

U t ) N

SYMBOLS

To ta l number of a i r c r a f t types

Community Noise Exposure Level

Composite Noise Rating

Effec t ive Perceived Noise Level ( tone and durat ion corrected PNL)

Units of Ef fec t ive Perceived Noise Level

Overall Sound Pressure Level

A-weighted sound pressure leve l ; A-weighted sound pressure l e v e l f o r 1 exposure of 10 second duration

B-weighted sound pressure l e v e l

C-weighted sound pressure l e v e l

Daytime noise l e v e l

Day-Night noise l e v e l

N-weighted sound pressure leve l ; nighttime noise l e v e l

NN-weighted sound pressure l e v e l

Stevens' loudness l e v e l

Tota l accumulated noise exposure during a day

Zwicker's loudness l e v e l

Noise l e v e l of t h e n-th period of the day

Level of ambient noise t h a t i s exceeded 50% of t h e t i m e

Level of ambient noise t h a t i s exceeded 90% of t h e t i m e

Steady blade air loading; Ambient noise l e v e l

A-th blade a i r loading harmonic

A-weighted sound pressure l e v e l as a funct ion of time

Number of operations per day or night ; number of A t increments contained i n t h e i n t e r n a l t t o t + AT

0 0

v i i i

SYMBOLS (continued)

NC Noise Cr i te r ion l e v e l

N E F Noise Exposure Forecast

N I Noisiness Index

N N I Noise and Number Index

Number of constant noise energy periods per day

Tota l number of f l i g h t paths over which "C" a i r c r a f t types f l y

Nt

P

PNL Perceived Noise Level

PNdB Units of Perceived Noise Level

PNLt

Q Mean Annoyance Level

Tone corrected Perceived Noise Level -

SENEL Single Event Noise Exposure Level ( tone and duration corrected dBA)

SIL Speech Interference Level

SPL(A) A-weighted Sound Pressure Level

AT Time i n t e r v a l during which the a i r c r a f t noise l e v e l i s above the ambient level

dB Decibel

dBA Units of A-weighted sound pressure l e v e l

Tone corrected dBA dBAt

k Airloading harmonic delay fac tor

A t

t

A t

t Time

Duration of t h e n-th period of t h e day

Time when a i r c r a f t noise l e v e l f i rs t exceeds the ambient

Time increment between successive values of L(t)

n

0

x Airloading harmonic number

i x

SYMBOLS ( c ont inued )

0 Standard deviat ion

Isopheric Index

X

COMMUNITY ACCEPTANCE OF HELICOPTER NOISE: CRITERIA AND APPLICATION

By Charles L. Munch and Robert J. King Sikorsky Aircraf t Division United Aircraf t Corporation

SUMMARY

A study w a s conducted t o define those c r i t e r i a necessary fo r c i v i l he l icopter operations t o be acoust ical ly acceptable t o t h e communities from which they operate and over which they f l y . The study involved surveying ex i s t ing domestic and foreign Federal regulat ions and guidel ines , s t a t e and l o c a l noise ordinances, r e s u l t s of community noise annoyance s tudies , and r e s u l t s of individual a i r c r a f t noise annoyance s tudies i n order t o e s t a b l i s h t h e c r i t e r i a .

The f i n a l c r i t e r i a se lec ted are based on t h e Day-Night Noise Level, %u, a measure of t o t a l noise exposure. weighted sound pressure l e v e l (dBA) which has accuracy comparable t o other u n i t s cur ren t ly used fo r a i r c r a f t . An L t e r i o n f o r areas where t h e ambient noise i s below 58 dBA. 2 dBA above t h e l o c a l ambient is recommended f o r a reas where the ambient i s above 58 dBA. source (such as a i r c r a f t operations) is less than the ex is t ing ambient noise energy. Charac te r i s t ics found important f o r a i r c r a f t noise r a t ing such as tone content , durat ion, and number of operations have been accounted f o r . I n addi t ion t o t h e capabi l i ty f o r ra t ing individual and cumulative a i r c r a f t operations f o r community acceptab i l i ty , these c r i t e r i a include t h e e f f e c t s of other non-aircraft noise sources and t h e ambient noise environment. This broad capab i l i t y makes the c r i t e r i a widely appl icable .

The bas ic r a t i n g u n i t i s t h e "A"

of 60 i s recommended as a c r i - DN An LDN value

This assures t h a t the energy contributed by any new noise

As p a r t of t h e study a current generation 50 passenger c i v i l t ranspor t he l icopter developed from a 18100 t o 22700 kg (40,000 t o 50,000 pound) s ing le main r o t o r mi l i t a ry t ranspor t was acous t ica l ly evaluated f o r t y p i c a l commercial se rv ice using t h e recommended LDN c r i t e r i a . t he unmodified a i r c r a f t meets t h e c r i t e r i o n l eve l s i n c ru i se f l i g h t . For t y p i c a l takeoffs and landings, some modifications t o t h e main and t a i l r o t o r were found t o be necessary f o r t h e hel icopter t o meet t he c r i t e r i a , however these changes do not g rea t ly a l t e r t h e a i r c r a f t ' s performance.

It w a s found t h a t

1

The modifications found necessary include t i p speed reductions and use of advanced design blades. suppression w a s found t o be necessary, however, t h e penalty i n weight and power l o s s i s q u i t e s m a l l .

Some turbine engine i n l e t and exhaust noise

INTRODUCTION

The current study was undertaken i n recogni t ion of t h e f a c t t h a t t h e r e is a growing need f o r s ign i f icant improvements i n c i v i l short-haul air t ranspor ta t ion systems. With ma jo r a i r p o r t s moving fu r the r and fu r the r from t h e c i t y center (o r business d i s t r i c t ) t he re i s an obvious need f o r r e l i a b l e , e f f i c i e n t c i t y center t o c i t y center short haul a i r t ranspor t systems t h a t can be good neighbors t o the r e s iden t s of t h e communities they serve. I n t h i s regard, t he hel icopter i s an i d e a l candidate as a vehicle t o use i n such a t ranspor t system. It i s capable of reasonably high speeds, it can ca r ry 50 t o 100 passengers or more, and it can operate from s m a l l terminals , a necess i ty i n c i t y centers where land i s a t a premium. Perhaps most importantly, the he l icopter i s general ly quie te r t h a n o ther V/STOL systems (for a given s i z e ) because it has a much lower disk loading tiiui tiie other systems.

I n undertaking a study t o evaluate he l icopters i n a c i v i l t ranspor t system it i s necessary t o have avai lable a noise acceptance c r i t e r i a aga ins t which t h e i r acoust ic performance can be measured. There a re i n exis tance today as many as 25 t o 30 descr iptors f o r sca l ing an ind iv idua l ' s annoyance t o noise and perhaps 8 t o 1 2 methods f o r describing community annoyance and/or reac t ion t o a l l types of noise. mary a i m s of t h e present study is t o evaluate a l l of these measures along with ex i s t ing or proposed f ede ra l , s t a t e , and l o c a l noise guidel ines and regula t ions and f r m them evolve workable, accurate noise c r i t e r i a t o pred ic t t he accep tab i l i t y of projected hel icopter operations t o a community.

Therefore, one of t h e p r i -

The other main object ive of t h i s study i s t o compare t h e noise c h a r a c t e r i s t i c s of a current generation 50 passenger he l icopter with t h e c r i t e r i a and then determine hardware changes t h a t can be made t o the a i r c r a f t t o allow it t o meet t h e c r i t e r i a . For t h e purposes of the-s tudy , t h e c i v i l he l icopter i s considered t o be a der ivat ive of a m i l i t a r y t ranspor t helicop- t e r i n t h e 18100 t o 22700 kilogram (40,000 t o 50,000 pound) gross weight category. Hardware changes considered are those t h a t a r e developed enough t o be applied t o t h e hel icopter i n t h e 1975-1976 time frame with l i t t l e or no add i t iona l development t i m e required. Preliminary estimates of changes i n a i r c r a f t performance due t o t h e hardware changes are t o be made.

2

NOISE ACCEPTANCE CRITERIA

Select ion of a community noise acceptance c r i t e r i a involves severa l d i s c r e t e s teps s t a r t i n g with se lec t ion of t he basic u n i t r e l a t ing physical sound t o human react ion. Once t h e ra t ing sca l e i s se lec ted , t h e number on t h i s s c a l e t h a t corresponds t o a noise exposure acceptable t o the average member of t h e cornunity must be determined. Further , it must be determined wnether t'ne environment t o which t h i s average commw-ity member has become aclimated has an influence on his "acceptabie" noise l e v e l m d i f s o what t he r e l a t ionsh ip i s between t h i s ambient noise envieonment and h i s tolerance t o new noise exposure. exposure duration on the acceptab i l i ty of noise. exposure as w e l l as t h e number of exposures per day has been shown t o influence noise acceptab i l i ty and t h i s too must be included i n the f i n a l c r i t e r i a . F ina l ly , t he re i s t h e question of t h e annoyance of per iodic impulsive noise (known as "blade slap" on he l i cop te r s ) . The annoyance of t h i s type of noise i s not adequately accounted f o r i n any ex is t ing r a t i n g sca l e and therefore requi res a correct ion t o properly def ine i t s annoyance.

Another fac tor t o be considered i s t h e e f f e c t of The duration of each

Each of t h e considerations iden t i f i ed above w i l l be dea l t with i n d e t a i l below. f o r each of t h e f ac to r s i n a conservative manner and w i l l , i f observed, r e s u l t i n community/helicopter compatibil i ty. reso lu t ion , however, i s t h a t of t he blade s l a p annoyance penalty. There i s not s u f f i c i e n t r e l i a b l e data avai lable a t t h i s time t o f i n a l l y resolve e i t h e r t h e method of quantifying existence of blade s l a p i n a noise s igna l or i n measuring an observer 's annoyance reac t ion t o it.

The r e su l t i ng community noise accep tab i l i t y c r i t e r i a accounts

One top ic requir ing fu r the r

Noise C r i t e r i a Development

Select ion of a r a t i n g sca le . - There a r e three basic considerations i n choosing a sca l e fo r r a t i n g the annoyance of noise. F i r s t i s t h e precis ion of t he scale . choice of a sca l e because it determines t h e degree t o which a calculated r a t i n g matches t h e subject ive r a t i n g of a t y p i c a l member of t he community. Scale inaccuracies could render an en t i r e r a t i n g system worthless. The second consideration is t h a t of commonality with other systems. t rend of developing a new annoyance scale f o r each new noise source has l ed

This is perhaps t h e most important fac tor i n t h e

The current

t o some confusion, hence t o non-acousticians must use. A simple weighting meter i s preferable t o a ca lcu la t ion t o produce a

a sca i e t h a t i s e a s i l y recognized and i s of use be selected. The t h i r d consideration i s ease of sca le t h a t produces a d i r e c t readout on a simple method which requi res a computer or long hand r e s u l t .

3

The "A" weighted sound pressure l e v e l (SPL(A) i n dBA) has been se lec ted as t h e basic u n i t of annoyance measurement f o r the community annoyance c r i t e r i a because of t he above considerations. Candidate u n i t s t h a t w e r e surveyed are l i s ted i n Table I. l e v e l w a s found i n severa l s tud ies t o be as accurate as any of t h e other u n i t s . types including a i r c r a f t , motor vehicles, and community ambients, and it i s e a s i l y measured using a standard sound l e v e l meter.

The "A" weighted sound pressure

It i s t h e most commonly used uni t f o r r a t i n g a va r i e ty of noise

The s u i t a b i i i t y of SPL(A) for rating t h e annoyance of a i r c r a f t noise i s subs tan t ia ted i n severa l s tud ies (eg. - References 1-7). SPL(A) w a s compared i n these s tud ies with severa l other measures of a i r c r a f t noise annoyance and found t o be s t a t i s t i c a l l y as good o r b e t t e r than t h e rest . A study performed by Ollerhead (Reference 3) on general av ia t ion a i r c r a f t noise r e su l t ed i n co r re l a t ion coef f ic ien ts between measured and computed subjec t ive r a t ings of 0.867 f o r SPL(A) and 0.88 f o r PNL. This and other data are l i s t e d i n Table 11. The other r a t i n g u n i t s had co r re l a t ion coe f f i c i en t s i n t h e range 0.714 t o 0.879. The s imi l a r i t y i n cor re la t ion between PNL, t h e commonly used un i t f o r a i r c r a f t noise annoyance, and SPL(A) i s t y p i c a l and shows t h a t t he two measures d i f f e r in s ign i f i can t ly i n t h e i r a b i l i t y t o rate a i r c r a f t noise f o r annoyance. Perhaps t h e la rge amount of study regarding t h e effect iveness of these s i m i l a r r a t i ng un i t s i s summed best by D. M. Green i n Reference 6: a t least i n my opinion, a r e s o c lose t h a t it i s r e a l l y r a the r po in t l e s s t o argue about t h e super ior i ty of one or another. r e s u l t of an experiment using a carefu l ly se lec ted set of spec t ra where the d i f fe rences i n predict ion are very g rea t . Otherwise, I think we w i l l continue t o f ind t h a t a l l of t h e methods are about equally good and while one, on t h e average, may be somewhat b e t t e r , no one i s c l ea r ly superior t o a l l t h e others". performed subsequent t o h i s remarks by J. B. Ollerhead (Reference 1). Using spec t ra which var ied from pure j e t a i r c r a f t t o ro to rc ra f t he found t h a t t he re w a s l i t t l e difference among t h e r a t ing u n i t s .

"The ex is t ing procedures,

What is needed is t h e

An experiment such as t h a t mentioned by M r . Green w a s

I n summary, t h e SPL(A) u n i t , although not subs t an t i a l ly b e t t e r than any of t h e other u n i t s ava i lab le , i s of comparable accuracy t o them and o f f e r s major advantages of commonality with non-aircraft r a t i n g schemes, ease of measurement, and a v a i l a b i l i t y of measurement equipment.

Tone correct ions. - There i s subs t an t i a l evidence (References 1, 2, 8, 9 ) t o support t he need fo r a correct ion t o account f o r t h e increase i n annoyance of s igna ls containing pure tones. This subject ive increase i n annoyance i s over and above the calculated contr ibut ion made by t h e tone t o t h e ove ra l l annoyance r a t ing . s tud ies (Reference 3 f o r one) indicate t h a t such a correct ion enhances annoyance predict ion only f o r tones above 500 Hertz i n frequency.

The most recent and wel l documented

The Federal Aviation Administration has adopted a tone cor rec t ion i n i t s noise standards f o r t ranspor t category a i r c r a f t (Reference 1 0 ) .

4

Although applied i n t h i s case t o t h e ?NL r a t i n g u n i t , t he correct ion i s independent of t h e annoyance r a t ing uni t ca lcu la t ion procedure and hence i s appl icable t o other un i t s . PNL and SPL(A) f o r most noise spectra so it w i l l be assumed t h a t because appl icat ion of t h e tone correct ion enhances t h e annoyance predict ion accuracy of PNL it w i l l a l s o enhance t h e annoyance predict ion accuracy of SPL(A). The procedure t o be used t o determine t h i s cor rec t ion i s t h a t described i n Reference 11, with the exception t h a t only tones with frequencies above 500 Hz a r e included.

There i s a nearly constant difference between

Duration e f f ec t s . - Nearly a l l ava i lab le evidence ind ica tes t h a t t h e time t o which a subject is exposed t o noise a f f e c t s h i s judgement as t o i t s annoyance. The consensus of t h i s evidence fu r the r ind ica tes t h a t t h e time-annoyance r e l a t ionsh ip i s a d i r ec t acoust ic energy summation; i . e . annoyance increases 3 dB per doubling o f exposure time. Other re la t ionships have evolved from t h e many experimental inves t iga t ions on t h e subject . These include doppler s h i f t correct ions, onset correct ions, and higher and lower r a t e s of accumulated annoyance with time. The more sophis t icated of these other re la t ionships have been developed f o r special ized c lasses of noise sources; i n any case, they have not achieved general acceptance. The instances of other than d i r e c t energy summation f o r accumulating annoyance with time a r e i n t h e minority and have been adopted for regulatory use only i n a f e w foreign countr ies . The current f ede ra l regulat ion f o r t ransport a i r c r a f t includes t h e d i r e c t energy summation method of accumulating annoyance with t i m e .

The near un iversa l acceptance of t h e energy summation procedure

Not only may t h e durat ion of a s ing le event as wel l as i t s s impl ic i ty of use has lead t o i t s se l ec t ion f o r use i n the c i v i l he l icopter c r i t e r i a . ( f lyover , t akeoff , or landing) be ra ted accurately f o r annoyance, but a l s o e f f e c t s of mult iple sources and events a r e accurately and simple included.

Select ion of a common comparison bas i s fo r r a t i n g schemes. - It is necessary t o reduce t o a common bas is a l l of t h e r a t i n g schemes t o be evaluated i n order t o develop t h e c i v i l t ranspor t he l icopter community noise acceptance c r i t e r i a . The common u n i t se lec ted is L weighted sound pressure l e v e l (SPL(A)) of a s ing le event with a constant noise l e v e l and a duration of 1 0 seconds. is comon t o most non-aircraft annoyance r a t i n g schemes, SPL(A) is t o be used i n the developed c r i t e r i a , t h e 10 second durat ion i s common t o most a i r c r a f t r a t i n g schemes, and use of a s ing le event eliminates any confusion which might be caused by the various summation methods used i n a i r c r a f t noise annoyance r a t i n g schemes.

t h e "A" A'

It w a s se lec ted because SPL(A)

LA is defined as:

LA = 10 loglo (3 an t i log rSPL(Alat 1 \ 10 sec

5

. L* = 10 loglo I"/ f T ant i log spL(A)d$ 10 - 10

where JT antilog{-$tt represents t h e t o t a l energy i n t h e s i g n a l over a F u l l 24 hour period and 10 sec i s the normalizing t i m e . A l l c r i t e r i a t o be compared must now be converted t o t h i s u n i t . This is done r a t h e r simply by noting (References 3, 12, 13) t h a t t h e difference between a spectrum's SPL(A) and i t s PNL i s , on t h e average, 13 dB and t h a t a l l c r i - t e r i a considered use one of these two basic uni t s . ship:

So noting t h i s re la t ion-

SPL(A) = PNL - 1 3 ( 2 )

t h e various c r i t e r i a may be compared. be converted t o L durat ion of 10 seconds:

Composite Noise Rating (CNR) w i l l as an example. From Reference 12 f o r an exposure A

CNR = 1 0 Loglo (lOpNL'lo) + 10 LOGIO(n) - 12 ( 3 )

Here t h e 10 second duration has a l r eady been accounted f o r hence LA=PNL-13. The number of f l i g h t s , n, i s s e t equal t o one r e s u l t i n g i n t h e folowing r e i a t i o n for CNi i :

CNR = 10 loglo (1 10 -12 = L A + 13-12 = LA + 1 ( 4 )

or : LA = CNR - 1 ( 5 )

Evaluation of current c r i t e r i a . - The same conversion process given i n t h e example above w a s applied t o several domestic and foreign federa l noise standards r e s u l t i n g i n Table 111. t o t h e various standards t o define the range of acceptable l e v e l s f o r each standard i n terms of L To insure t h a t t h e l e v e l derived i s conservative i n terms of being acceptable t o t h e exposed community t h e various standards were t r e a t e d as shown i n Figure 1. For each standard l i s t e d a range of l e v e l s i s blocked out. p r e t a t i o n of t h e marginally acceptable range of l e v e l s t h a t separate the c l e a r l y acceptable and c l e a r l y unacceptable l e v e l s i n t h e p a r t i c u l a r standard. Some i n t e r p r e t a t i o n was necessary because of t h e var ia t ions i n language used t o def ine t h e degree of a c c e p t a b i l i t y of noise i n t h e standards. t h e average center f a l l i n g i n t h e LA range of 100 t o 110.

These conversions were applied

A'

This range defines t h e inves t iga tors ' best in te r -

There i s a l a r g e range of marginally acceptable l e v e l s with

The lower end of t h e shaded regions i n Figure 1, which is t h e upper boundary of t h e c l e a r l y acceptable region, w a s se lec ted f o r fur ther consid- e ra t ion . This l e v e l , ra ther than t h e center of t h e marginally acceptable range, w a s chosen i n an e f f o r t t o bias t h e ul t imate c r i t e r i a i n favor of t h e community. acceptable t o t h e community.

This decis ion w a s made t o provide r e s u l t i n g c r i t e r i a l e v e l s

6

Figure 2 shows t h e "c lear ly acceptable" l e v e l s of Figure 1, again i n terms of t h e common r a t i n g f ac to r L . The mean of a l l t h e standards i s 97.5 dB with a standard deviat ion 04 6 . 5 . t i v e HUD t r a f f i c noise (possibly out of t h e range of t h e others because of d i f f i c u l t y i n in te rpre t fng it i n L process t h e numbers become 100 and 4.4 dB respect ively. t h i s 100 dB i n L u n i t s represents a conservative estimate of noise exposure A which would be considered c l e a r l y acceptable according t o t h e c r i t e r i a evaluated.

When t h e obviously conserva-

u n i t s ) i s removed from t h e average It i s f e l t t h a t

Community c r i t e r i a : Twenty-two community noise regulat ions were evaluated on t h e same basis as t h e federa l c r i t e r i a . The L values computed from them are shown i n Figure 3 . The mean and standard deviation of t h e regula t ions described by the open c i r c l e s are 93.5 and 3.0 respect ively. Regulations corresponding t o the darkened c i r c l e s were considered out of l i n e with t h e main body of data . The e n t i r e data set had a mean of 93 dB and a standard deviat ion of approximately 6 ind ica t ing t h a t t he main body, or two th i rds of a standard set , of data w a s between 87 and 99 dB; data poin ts ou ts ide of these bounds were not considered fu r the r and were dropped from t h e average.

B

The s ix teen community noise regulat ions considered i n t h e Figure 3 average include severa l l a rge c i t i e s such as Chicago, Los Angeles, and M i a m i and some smaller c i t i e s and towns. Figure 3 because regulat ions f o r a i r c r a f t noise are j u s t now being considered and t e n t a t i v e regulat ions are not yet ava i lab le . i n t h e f igu re are f o r r e s i d e n t i a l zones except where a d i s t inc t ion w a s made i n t h e regula t ion . Normally, commercial and i n d u s t r i a l zones are allowed t o be 5 t o 10 dB higher than those leve ls shown.

New York City is not shown i n

The LA numbers indicated

The L values f o r community regulat ions were derived using t h e same A grnund r u l e s and sca l e de f in i t i on as fo r t h e f ede ra l c r i t e r i a t r e a t e d e a r l i e r . However, t h e nature of these community ordinances required a s l i g h t l y d i f f e r e n t method of deriving the LA value. considerat ion regulate t h e allowable noise over t h e t o t a l 24 hour per iod of a day. Therefore t o develop t h e comparable 10 second allowable noise exposure ( L ) t he t o t a l noise over t h i s 24 hour per iod must be summed and converted back t o an equivalent 10 second durat ion of constant l eve l . The procedure i s as follows:

The ordinances under

A

The t o t a l noise exposure L, accumulated during a day is:

$ = 10

where L is t h e noise l e v e l The Nt ger iods cons t i t u t e a minus 10 ( t o account f o r

f o r t he nth period with durat ion A t seconds. f u l l day. t he 1 0 second durat ion assumed). Hence:

The LA term is then equa? t o LT

7

I Ln LA = 10 Loglo an t i l og ( ~ ) x Atn -10 (7 )

I n cases where an allowable l e v e l i s no t given f o r night time periods t h e l e v e l i s assumed t o be f i v e d B below tha t allowed f o r daytime periods. Night i s assumed t o have a durat ion of 9 hours and day i s t h e remaining 15 hours.

A s an example of t h e computation consider t h e case of Farmington, Connecticut. given f o r nighttime. be 5 1 dB, which i s a penalty of 5 dB for t h i s period. The ca lcu la t ion i s as follows.

The allowable daytime noise l e v e l is 46 dBA with no correct ion I n ca lcu la t ing L t h e nighttime noise l e v e l w i l l then A

= 10 Loglo {antilog(%)x 9 x 3600 + a n t i l o g ( T ) x 46+5 1 5 \

LA

L = 99 - 10 38004 = 8 dBA -lo --

S t a t e Cr i t e r i a : S t a t e c r i t e r i a f o r allowable noise exposure i n r e s i d e n t i a l areas are not common. However, those f o r t h e th ree states f o r which such information w a s avai lable were t r e a t e d i n t h e same manner as t h e community ordinances. The r e su l t i ng average i s an LA of 96.3 dBA.

Studies such as References 1 4 , 1 5 and Impact" type c r i t e r i a : 11

16 ind ica t e t h a t t h e most equi table method of determining the amount of noise t o which a community can be exposed i s t o relate it t o the ex i s t ing ambient noise conditions. This appears t o be a reasonable approach t o t h e problem because of t h e w e l l known f a c i l i t y of individuals t o become acclimated t o t h e i r environment. Those l i v i n g i n high noise areas have become accustomedlto it and are less disturbed by a noise of a given l e v e l than those l i v i n g i n a lower ambient noise area.

A s a check on t h i s theory and as an addi t iona l check on t h e t o l e r a b l e noise l e v e l s i n various communities, severa l categories of ambient noise l e v e l were evaluated i n terms of t h e common u n i t L Ambient

17 (Donley). These two references u t i l i z e l a rge quan t i t i e s of measured data t o der ive average quan t i t i e s and they agree subs t an t i a l ly on l eve l s . Data from Reference 18 i s fo r spec i f ic loca t ions and a l s o general ly agree. For instance, i n the case of suburban r e s i d e n t i a l noise l e v e l s during t h e daytime hours Reference 1 5 c i t e s L l e v e l s of 45.6 and 50.9 dBA respec t ive ly . The correspondi8: levelzOfrom Reference 17 are 43 and 50 dBA. because they w e r e presented i n a more e a s i l y used format. The spec i f i c data used i s contained i n Table N. The L and L l e v e l s r e fe r r ed t o i n t h e table and above a r e t h e "A" weightea0ambien?'sound pressure l e v e l s which are exceeded during t h e measurement period 50 and 90 percent of t h e time respect ively. due t o i t s s t a t i s t i c a l nature.

noise l e v e l s were determined from Reference 15 (Ollerhead) and 5 eference

and L

The data of Reference 1 5 w a s used f o r t h e following computations

This method of presenting ambient noise i s necessary

8

The L values f o r these ambient noise conditions were determined A with t h e same procedure used fo r community ordinances. LA values f o r t h e c i t y center and suburban r e s i d e n t i a l areas a r e 110 and 85.3 respect ively. They cover t h e f u l l range of ex is t ing ordinances and provide at l e a s t a p a r t i a l explanation f o r t h e wide spread i n these regulat ions. noise exposure environment of t h e various areas probably influence t h e l eve l s which r e s iden t s consider reasonable.

The previous

The impact t o ambient t m e c r i t e r i a take t h e ex i s t ing ambient noise l e v e l s , sometimes bounded by a lower l i m i t , and state t h a t new noise i n t h e a rea can increase t h i s ambient only by "X" dB. I n a c t u a l p rac t i ce , t h i s number "X" ranges from two t o f i v e dB f o r t h e regulat ions surveyed. Although not mentioned i n previous sect ions a s m a l l percentage of t h e community noise regulat ions surveyed were i n f a c t impact t o ambient types and t h e i r allowable l e v e l s were determined by applying the allowable impact t o t h e ambients t y p i c a l of t h e i r location.

The impact of impulsive noise on acceptab i l i ty of he l icopters . - Very l i t t l e information i s ava i lab le i n t h e l i t e r a t u r e regarding the e f f e c t of repeated impulses (blade s l a p ) on the acceptab i l i ty of hel icopter noise. Many s tud ie s on t h e annoyance of sonic booms have been performed, but t h e r e s u l t s are not d i r e c t l y appl icable t o blade s l ap conditions where the impulses are r e p e t i t i v e and where t h e ove ra l l amplitude increases and then decreases as t h e he l icopter passes over. Based on t h e sketchy information ava i lab le from t h e l i t e r a t u r e (References 19, 20, 21, 22, 23) it appears (as shown i n Figure 4 ) t h a t t he re should be a 4 t o 6 PNdB penalty when impulsive noise i s present i n a s ignal . I n terms of A-weighted sound l e v e l t h e penal ty appears t o be somewhat l a rge r , on t h e order of 8 t o 1 3 dBA.

One of t he major d i f f i c u l t i e s i n applying a penalty f o r impusive noise i s i n es tab l i sh ing an object ive means t o def ine i t s presence and seve r i ty . A study performed i n 1963 (Reference 23) ind ica tes t h a t impulses repeated i n excess of 18.80 per second are perceived as steady (continuous) noise. Most inves t iga tors , including t h e authors , have concluded t h a t phase information is necessary t o t h e determination of whether a spectrum has impulsive content. Others, i n t he minority, contend t h a t phase information is i r r e l e v a n t t o the determination of t h e nois iness of impulsive noise . Leverton (Reference 20) defines no object ive method of defining t h e presence of impulsivi ty i n a s igna l but contends t h a t i f it i s subjec t ive ly present a penal ty of 12 dB should be imposed r e l a t i v e t o t h e nois iness computed by conventional object ive means.

Because of t h e lack of consis tent information on t h e annoyance of impulsive noise and on object ive means of determing i t s presence, a l imi ted t e s t w a s conducted. This t e s t w a s meant t o be no more than a crude beginning toward defining an accurate blade s l a p annoyance assessment method. Consequently, only preliminary recommendations can be made a t t h i s po in t .

9

The c r e s t f a c t o r of a s igna l , which, i n dB, is 20 t i m e s t h e common logarithm of t h e r a t i o of peak t o root-mean-square ( r m s ) sound pressure l e v e l , w a s se lec ted f o r evaluation as an impulse noise indicator because it has known values f o r common noise types ( 3 dB f o r s inusoida l or pitched noise , 10 dB f o r Gaussian or broadband noise) and seems l o g i c a l for t he des- c r i p t i o n of blade s l ap noise which i s characterized by a highly peaked t i m e h i s tory . It i s a l s o a parameter which i s r a the r e a s i l y measured. Selec- t i o n of a frequency spectrum based descr iptor was avoided because of t h e need t o include phase information necessary t o d is t inguish t h e blade s l a p c h a r a c t e r i s t i c . The meter used t o determine t h e peak s igna l l e v e l had a time constant T of 0.005 seconds. The ear has a t i m e constant of about 0.05 seconds. The e f f ec t of t i m e constant i s not known and should be t h e subject of fu r the r study. The instrumentation used f o r t h e inves t iga t ion i s described i n Figure 5 .

Unfortunately the peak meter used was a "capture-and-hold" type peak impulse meter which r e g i s t e r s t he highest peak l e v e l obtained. In r e t rospec t , t h e peak l e v e l required i n ca lcu la t ing t h e c r e s t f ac to r should have been t h e average peak l e v e l because a random peak or two i n an otherwise r e p e t i t i v e s igna l would produce too high a peak reading f o r t he sigrlal in gerreral, and hence, an abnormally high c r e s t f ac to r .

Nine recorded hel icopter noise samples were evaluated during t h e study. They were subject ively c l a s s i f i e d as t o t h e exis tence and extent of blade s l ap by t h e inves t iga tors (admittedly not an unbiased or naive evaluat ion) during a l i s t en ing tes t . l e v e l s w e r e then determined with t h e use of t h e Figure 5 instrumentation. Resul ts a r e tabulated i n Table V . The da ta i s p lo t t ed as c r e s t f ac to r versus subject ive blade s l ap r a t i n g i n Figure 6. The t e n t a t i v e boundary f o r blade s l ap exis tence a t a c r e s t fac tor of 13 dB i s shown i n t h e f igu re . The boundary i s not based on a l e a s t squares f i t l i n e crossing t h e blade s l a p h o blade s l a p ordinate d iv is ion . Ins tead , it w a s merely se lec ted as t h e point below which a l l da ta were subjec t ive ly r a t e d as having no d e f i n i t e l y d iscern ib le blade s l a p content and above which a l l da ta w a s rated as having blade s l ap t o some degree. boundary c r e s t f a c t o r value the re a r e two data poin ts with d i f f e r i n g judgements as t o blade s l ap content. This occurrence w a s not unexpected i n a preliminary t e s t such as t h e one conducted and serves t o ind ica t e t h a t more d a t a of higher qua l i t y is needed and the dividing l i n e between t h e slap and non-slap condition i s not c l e a r cut.

The t r u e rms and peak sound pressure

It should be noted t h a t at t h e

Oscillograms of t he ove ra l l sound pressure of t h e acous t ic s igna l s I n general those s igna ls which were sub- evaluated a r e shown i n Figure 7.

j e c t i v e l y r a t ed as having blade s l a p appear impulsive i n nature i n t h e t r a c e s . The two cases which w e r e shown i n Figure 7 t o have t h e same c r e s t f a c t o r , t h e Vert01107 and t h e Sikorsky S-~I-F, have e n t i r e l y d i f f e ren t pressure s ignatures . determining peak sound pressure l eve l as discussed above. It poin ts ou t ,

The reason fo r t h i s apparent anomaly may l i e i n t h e method of

10

i n any case , t h a t t he re i s considerably more t o be learned about t h e problem.

A s noted i n Figure 4, when blade s l a p i s present there i s an associated annoyance penalty. t h a t pena l t i e s of 8 t o 13 dBA a r e typ ica l . It is possible t h a t a preliminary impulsive noise annoyance penalty could be t h a t shown i n Figure 8. penal ty i n dBA i s added d i r e c t l y t o the ca lcu la ted (or measured) a i r c r a f t SENEL value t o a r r i v e at t h e corrected SENEL value f o r t h e a i r c r a f t . The corrected value i s 'clsed i n determining i f t h e a i r c r a f t meets t he t e r ion . It i s recognized t h a t t h i s penalty is , a t bes t , crude. T e purpose of t h i s l imi ted study was not t o define a spec i f i c c r i t e r i o n t o measure t h e exis tance and extent of blade s lap , but r a the r w a s t o determine t h e po ten t i a l of a simple measure ( t h e c r e s t f a c t o r ) t o objec t ive ly ind ica te the presence of blade s lap . Much work remains t o be done i n t h i s area t o c l e a r l y def ine t h e presence of blade s l a p and t h e associated penalty t o SENEL.

While the da t a used is very l imited, it appears

The

%N

Select ion of C r i t e r i a f o r C i v i l Helicopter Operations

Computation of IT. A combination of t h e foregoing discussions ind ica t e s t n a t t n e 1-oilowing cha rac t e r i s t i c s should be incorpcrated i ~ ? a_

c i v i l he l icopter noise c r i t e r i a :

- The bas ic r a t i n g u n i t should be dBA - Duration should be considered over a f u l l day of exposure. - Exposure accumulation should be by the energy summation method. - The annoyance e f f e c t of tones above 500 H e r t z should be considered. - Ambient noise l e v e l s must be included.

These c h a r a c t e r i s t i c s are included i n t h e LDS measure (Day-Night Noise Level) . t e c t i o n Agency f o r a i r c r a f t annoyance r a t i n g i n i t s recent de l ibera t ions i n the Aircraf t /Ai rpor t Noise Study. The d r a f t repor t which describes t h i s recommendation i n grea t d e t a i l and with f u l l t echnica l subs tan t ia t ion i s l i s t e d as Reference 24. The basic L u n i t has been transformed somewhat i n format t o fit the requirements of t h i s study as shown below:

This u n i t has recent ly been recommended by the Environmental Pro-

DN t + AT

SENEL = 10 Loglo an t i l og L ( t ) dt L o 1 OR

LD = 10

N C antiloglO p*] A t

C

SENEL = 10 log 'Ok=o

SENEL. P an t i l og [ i d + (54,000 -igl

an t i l og - 47.3

( 9 )

P C AT ) j=1 i j

(10)

11

LN = 10 C P

SENEL 5 . 3 ) + (32,000 -igj j41 ATij) (11) P c 10 ant i log (-

j =1 \ lo = a n t i l o g ($$ -45.1

Equations (8) and ( 9 ) define the Single Event Noise Exposure Level (SENEL). The SENEL i s t h e t o t a l noise dose f o r a s ingle takeoff , landing, or f lyby. It i s the parameter which corresponds t o Effec t ive Perceived Noise Level (EPNL) i n t h e current FAA c e r t i f i c a t i o n procedure and it i s t h e one which would be used for a i r c r a f t c e r t i f i c a t i o n under the proposed procedure. It has been used i n the S ta te of Cal i forn ia Noise Standards (Reference 25).

Equations (10) and (11) describe t h e average day and night noise l e v e l s respect ively. They c a l l fo r summation of t he SENELs generated by t h e var ious a i r c r a f t on t h e i r various f l i g h t paths at t h e point of obser- vat ion, t h e addi t ion of ambient noise exposure over t h e time when it i s iiot exceeded 5J t h e a i rcmft noise, and normalization t o t h e t i m e duration of t h e day or night periods t o obtain an average r a the r than a t o t a l exposure l eve l . The day and night periods a r e from 0700 t o 2200 hours and from 2200 t o 0700 hours respect ively i n accordance with t h e EPA recommendation (Reference 24) and the current NEF exposure c r i t e r i a (Reference 12 ) . t h a t t h e night SENEL numbers a re multiplied by a f ac to r of 10 t o a t t a i n t h e 10 dB penalty associated with t h i s time period.

Note

Equation (12) describes t h e method of combining the day and night average noise l eve l s t o obtain t h e day-night (LDN) noise l e v e l which i s t o be t h e c r i t e r i o n f o r acceptab i l i ty . The day and night periods a r e . weighted according t o t h e f r ac t ion of t h e f u l l 24 hour day they occupy. Hence t h e 10 dB penalty imposed on the night period i s p a r t i a l l y balanced because of t h e smaller percentage of the t o t a l r a t i n g period it occupies.

Acceptabi l i ty c r i t e r i a i n terms of LDN. - LDN has been defined as t h e

It now remains t o determine the ac tua l LbN m b e r which co r re l a t e s u n i t whicn rei aLes m e a - m e x p D s u r L I,U h m a n annoyance t o t h e same noise. with community acceptance.

A s a s t a r t i n g point i n defining t h e acceptable L it should be DN mentioned t h a t t h e Environmental Protection Agency i n i t s draf t repor t (Reference 24) recommended a constant f o r human compatibi l i ty i n t h e a reas o annoyance, speech in te r fe rence , and hearing damage r i s k . by EPA i n t h e i r document makes t h i s l eve l worthy of strong consideration.

l e v e l of 60 as meeting requirements $N

The excellent technica l subs tan t ia t ion offered

12

The average community acceptable noise l e v e l s spec i f ied by t h e

A three categories of standards were determined i n terms of L section. c r i t e r i o n value t o be used. mean value shown i n t h e f i g u r e is t h a t f o r community r e s i d e n t i a l area standards, an

i n a previous These w i l l now be converted i n t o t h e LDN sca le t o determine t h e

The r e s u l t s are shown i n Figure 10.

The other two categories f a l l at LDN

The lowest

l e v e l of 59.5 dBA.

The Figure 9 sunmary i n combination with t h e EPA recommendation leads t o the se lec t lon of L = 60 as the basic c r i t e r i o n f o r cornunity acceptance of c i v i l helicop%r noise. This i s f e l t t o be a conservative choice because it is deslgned t o meet community noise standards as wel l as standards f o r other ty-pes of nolse instead of being aimed pr inc ipa l ly a t a i r c r a f t noise. Other a i r c r a f t oriented standards allow higher noise exposures than do t h e community regulations.

Select ion of L = 60 i s not the f i n a l s t e p i n community noise TEYs noise exposure i s ul t imately desireable , but c r i t e r i a se lec t ion .

cannot be a t ta ined under a l l circumstances a t t h e present time. for t h i s i s t h e existence of ambient noise levels" which already exceed t h e c r i t e r i o n (60 added. This would YGeclude any a i r c r a f t operations at a l l i n areas where high ambient noise l e v e l s e x i s t . This s i t u a t i o n can not be allowed t o occur because it would eliminate a i r c r a f t operations i n j u s t those locat ions where a i r c r a f t noise would be t h e least noticeable.

The reason

and t o which, presumably, no f u r t h e r noise may be

To overcome t h i s problem, t h e complete c r i t e r i o n i s comprised of

The allowable LDN i s 60 dBA two segments based on ambient noise levels i n t h e area under consideration. This c r i t e r i o n i s i l l u s t r a t e d i n Figure 1 0 . f o r areas wfth ambient noise l e v e l s of 58 dBA and below. Where ambient noise exceeds 58 dBA t h e allowable hN equals t h e ambient l e v e l plus 2 dBA. type. Because of t h i s l i m i t new noise sources i n an area add l e s s energy than t h e ex is t ing amblent noise.

The la t te r port ion of t h e c r i t e r i o n may be termed an impact t o ambient

The impact type standard does a number of things t o make t h e proposed c r i t e r i o n workable as f a r as both t h e community and t h e a i r c r a f t industry a r e concerned. F i r s t , it allows operation of a i r c r a f t i n areas where ambient noise i s high and t h e public is acclimated t o it. Second, it allows only a small impact t o t h e ambient so t h a t t h e increased noise exposure i n these areas w i l l not be so l a r g e as t o draw s i g n i f i c a n t not ice . Third, automatic de-escalation of noise l e v e l s is bui l t - in by way of t h e ambient noise l e v e l . A s t h i s ambient decreases over t h e years due t o s t r i c t e r controls on noise sources other than a i r c r a f t , t h e a i r c r a f t noise w i l l have t o be lowered accordingly. Perhaps appl icat ion of such a c r i t e r i o n could c a l l for periodic

"Ambient noise is defined as the 24 hour L l e v e l plus 0.115 t i m e s 50

the standard deviat ion about t h i s level .

updating of t h e ambient noise statistics and corresponding adjustments of a i r c r a f t operations and equipment use a t a spec i f ic s i t e . held t o reasonable periods, t h e a v a i l a b i l i t y of new,quieter, a i r c r a f t and equipment would presumably keep pace with t h e ambient noise reductions. A i r se rv ice would not then be severely impacted by ambient noise reductions.

If the updating i s

U s e of the Criteria

U s e or' the c r i t e r f o n I s r e l a t i v e l y simple md Is similar t o current a i r c r a f t noise annoyance evaluation procedures. SENEL is calculated from the measured or predfcted SPL(A) time h is tory . The values of SPL(A) used may be tone corrected depending on t h e spec i f ic appl icat lon. All of the SENELs from a l l a f r c r a f t f o r a day or night time period a r e summed and combfned with the ambient noise and duration data t o determine t h e day and night notse leve ls . LDN is then computed as t h e f i n a l s tep .

F i r s t , t h e a i r c r a f t

Figure 11shows an example of the SENEL and corresponding hN computations. Four simulated noise l e v e l - time h i s t o r i e s a r e shown. For Event 1, t h e peak noise l e v e l i s 25 d B above t h e ambient. I n t h i s case t h e SENEL i s t h e summation of energy between t h e points 10 dBA down from t h e peak (an approximation made with l i t t l e e r r o r t o save computation time f o r predicted da ta ) and has a duration of 10 seconds.

value f o r a t o t a l of 100 f l i g h t s with t h e i d e n t i c a l time h is tory i s $9 dB above t h e ambient leve l . Event 2 has t h e same maximum SPL(A) as t h e previous (and t h e o ther ) events except t h a t t h e ambient l e v e l i s now only 10 dB below t h e peak. 100 f l i g h t s a t t h i s l e v e l on the L ambient l e v e l . For Event 3 t h e amgyent i s only 5 dB below the peak and t h e duration i s consequently reduced t o 5 seconds ( t h e time t h e s igna l i s above the ambient). The SENEL i s reduced s l i g h t l y compared t o t h e two previous cases and the L For t h e case ofD#vent 4 the ambient i s higher than t h e peak of the noise t i m e h i s t o r y , t h e SENEL i s zero, and the LDN i s equivalent t o the ambient. A complete example showing use of t h e c r i t e r i a i s given i n t h e Appendix.

The calculated

The SENEL is unchanged, however the impact of is now only 0.2 dB due t o the increased

is approximately equal t o t h e ainbient noise l e v e l .

Some comment i s i n order regarding t h e consequences of modifying the c r i t e r i o n t o a t t a i n lower community noise exposure. It should be pointed out t h a t t h e r e i s no need seen f o r such modification because t h e c r i t e r i o n as constructed meets nearly a l l of t h e c r i t e r i a and community annoyance s tudies surveyed and has an extremely high probabi l i ty of community acceptance. However, if it were thought necessary t o impose a r e s t r i c t i o n of L perm?!ted. establishment of a i r c r a f t (or any noise producing) locat ions i s therefore precluded, a s i t u a t i o n which should not be allowed t o occur. A second a l t e r n a t e modification t o the c r i t e r i o n i s an extension of t h e impact t o ambient type of ordinance t o ambient l e v e l s below 58 dB.

= 60 f o r ambients above 60 dBA absolutely no new noise would be

?P Of 60* The I n f a c t , even the ambient would exceed t h e

f a c i i t i e s a t high ambient

Its consequence i s t h a t no reasonable a i r c r a f t (or perhaps even ca r s or t rucks) could operate i n ambfent locations much below 60 dBA because t h e impact of a f ixed SENEL would become so l a r g e t h a t an extremely low number of events would consume t h e 2 dBA impact allowed. be allowed t o happen, espec ia l ly i n view of t he bulk of information (Reference 24) indicating 60 LDN t o be a p r a c t i c a l lower l i m i t .

Th is a l s o should not

Select ion of Typical Locat$ons and Operations f o r EvaLuatlsn of Baseline Belicopter Nolse

Because is an in tegra ted measure including e f f e c t s such as ambient noise leve!!, t i m e of day, and number of operations, it i s necessary t o spec i fy certalln ground r u l e s t o be used i n evaluating the accep tab i l i t y of c e r t a i n he l icopter tyyes and/or aperatfona. A set of spec i f i c conditions were compiled f o r evaluatfon of he l icopters I n t h i s study. These conditions were chosen wllth t h e assfs tance of operations analysis personnel t o make them r e a l i s t i c In terms of required he l ipor t (or c l e a r zone) s i z e and number of operations required for economic v i a b i l i t y .

Heliport loca t ions and per t inent information are summarized i n Table V I . The f i rs t three a r e c f t y type operating loca t ions , a i l with ambient noise l e v e l s above 58 dEA. high which would be expected f o r locat ions i n high population areas. operat ions are l imlted severely as they would have t o be i n a r e a l operation because of t h e 10 dBA penalty associated with them, The allowable foo tp r in t a rea of Table V I i s an est imate of what could be s e t aside f o r a he l ipo r t o r ava i l ab le f o r a c l e a r (noise in sens i t i ve ) area f o r t h a t loca t ion and the allowable L ambient. case of t h e urban shopping center where l a rge areas s e t aside f o r automobile parking are ava i lab le . The ambient no i se l e v e l f o r t h e urban r e s i d e n t i a l h e l i p o r t , which i s less l i k e l y t o be used i n ac tua l p rac t i ce than t h e o thers , i s w e l l below 58 dB. The number of operations is s m a l l and i s l imited t o daytime only. The last two loca t ions are not takeoff/landing loca t ions , but r a the r they a r e f o r f lyovers of normally quie t areas at 1500 and 3000 f e e t a l t i t u d e . The allowable footpr in t area i s zero ind ica t ing t h a t t h e bN c r i t e r i a may not be exceeded a t any point on the ground when t h e he l i - copter passes overhead.

The number of operations a r e r e l a t i v e l y Night

is from t h e recommended c r i t e r i o n l e v e l a t t he spec i f ied T% areas are small f o r c i t y center operation, but l a rge for t h e

C I V I L TRANSPORT HELICOPTER NOISE EVALUATION

A s a first s t e p i n assessing the a p p l i c a b i l i t y of he l icopters t o t h e ci ty-center shor t haul market, the noise c h a r a c t e r i s t i c s of current he l icopters t h a t a r e l i k e l y candidates f o r c i v i l t ranspor t must be determined and compared with the community noise acceptance c r i t e r i a developed

15

i n t h e previous sect ion. Po ten t i a l candidate a i r c r a f t are those m i l i t a r y t ranspor t helrcopters of 18100 t o 22700 Kg (40,000 t o 50,000 pounds) gross weight capable of carrying approxrmately 50 people i n a c i v i l confi- gurat ion. m i l e s ) . i n t h i s study i s t h e Marlne Corps CH-53D he l icopter . This a i r c r a f t , t o be described i n d e t a i l below, meets a l l of t h e basic s i z e , weight, and range c r i t e r i a .

Nominal range fo r these a i r c r a f t i s a t l e a s t 371 Km (200 nau t i ca l The pa r t i cu la r a i r c r a f t i n th5s category se lec ted f o r evlauation

Charac te r i s t ics of the Basic Helicopter

Helicopter descr ipt ion. - The hel icopter se lec ted f o r evaluation as a c i v i l t r anspor t , t he CH-53D, is deacrlbed i n d e t a i l i n Figure 12. Br ie f ly , it i s a-s ingle main ro to r mi l i t a ry t ranspor t hel icopter of 16750 Kg (37,000 pounds) mission gross weight (18600 Kg (41,000 l b ) maximum gross weight). a design gross weight of 18600 Kg (41,000 pounds), achieved by using uprated engines and improved ro to r blades, Figure 13 shows t h e general arrangement of t h i s a i r c r a f t . evaluated against t h e developed noise acceptance c r i t e r i a .

It 's commercial der ivat ive , herein ca l l ed t h e S-65-40, has

The S-65-40 i s t h e base l ine a i r c r a f t which w l l l be

Noise predict ion method. - The method used t o pred ic t the he l icopter noise f o r t y p i c a l operations is based on t h e procedures presented i n Reference 26. t h e noise from V/STOL propulsion components such as r o t o r s , p rope l le rs , turboshaft engines,fan engines, and j e t s and combine them t o produce a time h i s to ry of t h e a i r c r a f t noise a t an observation s t a t i o n on t h e ground f o r a prescr ibed f l i g h t p ro f i l e . For t he present study, a modified vers ion of t h e program w a s used. The modified vers ion .is spec ia l ized t o he l icopters By including i n the-program only rotors and turboshaft-engines as noise producing components. and t o speed processing t i m e .

The Reference 26 computer program i s designed t o ca l cu la t e

The purpose of t h i s i s t o make t h e program more compact

The t i m e h i s tory of PNLT and dBA needed t o computethe EPNL and SENEL respec t ive ly i s constructed a t an observer loca t ion by ca lcu la t ing (for successive a i r c r a f t loca t ions) t h e noise from each of t h e components, summing t h e components' t o produce the vehicle spectrum, and then converting t h i s spectrum t o PNLT and dBA leve l s . The t i m e h i s to ry i s then in tegra ted over t h e appropriate time i n t e r v a l t o produce t h e EPNL and SENEL values. A i rc ra f t locat ions along t h e f l i g h t path a re computed by a separate sub- rout ine. The complete f l i g h t path i s s implif ied t o a s e r i e s of s t r a i g h t l i n e and. h e l i c a l , segments along any -on e of .which a l l aperat ing parameters are calculated By a helecopter low. speea dynamic-perf ormance program. This program has been shown t o be accurate f o r speeds up t o 77 m/sec (150 knots) by cor re la t ion with f l i g h t t es t data . Figure 1 4 shows a comparison of predicted and measured f l i g h t parameters f o r t h e CH-53D indica t ing t h e accuracy of t he program.

_____ ~ ~~

I 16

Main ro to r and t a l l ro to r noise I s calculated using t h e s implif ied I n t h i s method approach suggested by Lowson and Ollerhead (Reference 27).

t h e loading i s considered t o be concentrated a t an "effect ive" rad ius and t h e harmonics of a i r loading a r e assumed t o follow a uniform exponential f a l l -o f f i a m l i t u d e based on t h e steady loading. This i s represented as

oc L X where A i s t h e harmonic number, L i s t h e amplitude of t he X-th harmonic, L i s t h e steady load, the k determines t h e r a t e a t which t h e loading amplftude f a l l s off with increasing A . of t h e spectrum i s calculated using a version of t he method presented by Schlegel, King and Mull i n Reference 28. accuracy of t h e r o t o r noise predict lon method.

-k 0 X

The broadband port ion

Figure 15 demonstrates t h e

The procedure f o r calculat ing turboshaft engine noise i s e n t i r e l y empirical . It i s based on noise data fo r severa l d i f f e r e n t engines. Both sound power and d i r e c t i v i t y are accounted f o r i n t h e method. A modification t o t h e program now allows use of measured engine da ta when it i s ava i lab le i n s i f f i c i e n t d e t a l l . For t h e present study t h e generalized procedure w a s found t o be acceptable as shown by t h e Figure 17 comparison of t he measured and predicted time h i s to r i e s of noise f o r t h e C H - 5 3 he l icopter .

Detailed descr ipt ions of procedures used i n t h i s study can be found i n Reference 26.

Basic hel icopter noise cha rac t e r i s t i c s . - The basic c i v i l helicop- t e r , t h e S-65-40, w a s acous t ica l ly evaluated f o r severa l d i f f e r e n t takeoff and landing f l i g h t p r o f i l e s t o es tab l i sh i t s acoust ic cha rac t e r i s t i c s . Four of t h e takeoff p r o f i l e s are shown i n Figure . 1 7 Two of them are ver- t i c a l climb-outs t o a pre-determined a l t i t u d e and t h e other two are more

of s t r a i g h t l i n e segments i n the acoustic pred ic t ion program, as discussed above. Comparisons of t h e ac tua l and approximate f l i g h t p r o f i l e s a re shown i n Figure 18.

normal" type takeoffs . These f l i g h t p r o f i l e s were approximated by a s e r i e s 11

Figures 19, 20, 21 and 22 show t h e ca lcu la ted constant SENEL ground contours f o r each of t he four f l i g h t s . It is r ead i ly apparent t h a t t h e shapes of t h e contours a r e d i f f e ren t for each f l i g h t . This i s more e a s i l y seen i n t h e Figure 23 comparisons of the four 95 SENEL contours. The noise contours f o r t h e f l i g h t s with an i n i t i a l v e r t i c a l climb do not extend as far down range as the more normal type takeoffs , however they do extend fu r the r t o t h e s ide and r ea r of t h e takeoff point . This i s t o be expected because of t he time in tegra t ion cha rac t e r i s t i c of t h e SENEL. For the ver- t i c a l t akeoffs t h e he l icopter spends more t i m e i n t h e v i c i n i t y of t he o r ig in hence t h e durat ion cor rec t ion a t a point near t h e o r ig in w i l l be la rger than for t he oblique takeoffs .

Although contour shape i s important, t h e u n i t of i n t e r e s t i n regard t o t h e noise c r i t e r i a i s the t o t a l area encompassed by a spec i f ied SENEL contour. Figure 24 shows t h i s cha rac t e r i s t i c f o r the four takeoffs of Figure .l7. It i s in t e re s t ing t o note t h a t although t h e contours have

q u i t e d i f f e r e n t shapes, Figure 24 indicates that t h e t o t a l area encompassed by a given SENEL contour i s about t h e same f o r all f l i g h t s . down range d is tance i s given up for increased s ide l ine and rearward d is tance i n performing a v e r t i c a l takeoff. Thus while t o t a l a rea does not change s i g n i f i c a n t l y with changfng f l l g h t p r o f i l e , It i s possible t o al ter t h e shape of t h e noise contour on t h e ground t o s u i t s p e c i a l conditions, such as p a r t i c u l a r l y noise sens i t i ve areas located near or under the f l i g h t path.

Apparently,

The same cha rac t e r i s t i c of equal contour a rea regard less of f l i g h t p r o f i l e w a s found t o e x f s t f o r dfff'erent landings. noise contours f o r a t y p i c a l landfng, A f a c t t o be noted i s t h a t a given SENEL contour on landing i s much smaller than f o r takeoff , thus when takeoff and landing operattons are conducted from t h e same d i r ec t ion t h e t o t a l ground area encompassed by a given SENEL contour i s about t he same as f o r t h e takeoff alone.

Figure 25 shows t h e

Noise reduction requirements. - Because t h e t o t a l a r ea enclosed by a given SENEL contour i s independent of t h e takeoff p r o f i l e a l l comparisons with c r i t e r i o n requirements and a l l hardware ch-wes w i l l be evaluated xith mljr m e t&enff t.ype, t h e obllque takeoff (which i s a standard he l i - copter maneuver). Figure 26 compares the SENEL foo tp r in t area character- i s t i c with the c r i t e r i a l e v e l s developed above. It is obvious t h a t t he noise l e v e l ( t h e SENEL) must be reduced by a t l e a s t 7 dBA t o meet t he c r i t e r i o n l e v e l a t loca t ion 4 (urban r e s i d e n t l a l area).

Figure 27 shows t h e contribution of each noise producing component (main r o t o r , t a i l rotor, engines) t o t o t a l noise a t severa l observer locaL t ions . To achieve a t o t a l reduction of 7 dBA the noise of a l l sources must be reduced although t h e main ro tor no ise must be reduced more than t h e other two sources.

Cruise noise i s a l s o a po ten t i a l problem because f l i g h t s may be routed over r e s i d e n t i a l a reas at r e l a t i v e l y low a l t i t u d e s (1500 f t and up) . Figure 28 shows t h e c ru i se noise cha rac t e r i s t i c on the ground f o r l e v e l f l i g h t a t 77 m/sec (150 knots ) . because t h e basel ine a i r c r a f t already i s quie te r than the recommended c r i t e r i o n l eve l .

No modifications a re necessary here

Effect of impulsive noise. - Figure 29 has been prepared t o demon- strate t h e e f f ec t impulsive noise can have on the community annoyance of he l icopter noise. If severe impulsive noise were present i n t h e sound generated by t h e basel ine he l icopter performing t h e oblique takeoff maneuver t h e SENEL foo tp r in t a r ea cha rac t e r i s t i c would be as shown i n Figure 29 assuming the 1 0 dBA penalty discussed previously. a given SENEL l e v e l increases d ra s t i ca l ly . It i s qu i t e obvious t h a t every e f f o r t should be expended t o eliminate impulsive noise.

The foo tp r in t a r e a f o r

Noise Reduction t o Meet the Community Acceptance C r i t e r i a

The basel ine hel icopter was found (above) t o require a noise l e v e l reduction of up t o 7 dBA i n order t h a t it meet t h e recommended community acceptance c r i t e r i a . evaluated i n an e f f o r t t o f i n d t h e best compromise between acoustic and aerodynamic performance. The techniques included reducing main and t a i l r o t o r t i p speeds, reducing blade loadlng, increasing ro tor s o l i d i t y , using advanced blade designs, and s i lenc ing engine i n l e t and exhaust noise.

Several d i f fe ren t techniques t o reduce t h e noise were

Table V I 1 summarizes the design parameters f o r t h e basel ine CH-53D and 6 modified a i r c r a f t evaluated f o r t h e i r noise reduction poten t ia l . Figure 30 compares t h e noise footpr in t shapes of t h e modified and basel ine a i r c r a f t and Figure 31 compares the noise c h a r a c t e r i s t i c s of the modified a i r c r a f t with t h e recommended noise c r i t e r i a and Figure 32 shows t h e noise reduction by octave band f o r Modification P. Modifications A and B do not meet t h e c r i t e r l a a t Location 4 (urban r e s i d e n t i a l ) , but Modifications C , D , E and F meet a l l of t h e cr2terTa. It remains t o s e l e c t t h e bes t a i r c r a f t from among t h e C,D,E, o r F ModifTcations.

The prime considerations i n seiect ing one or more of' the uodif ied a i r c r a f t f o r f u r t h e r study a r e mtnimizing t h e performance losses and mini- mizing t h e hardware changes requTred. r e t a i n as much of the design range/payload as possible . changes.to a minimum, expensive .(.both i n d o l l a r s and i n weight) design changes can be avoided. Modifications E and F, therefore , appear t o be t h e most promising because t h e changes required are minimized, especial ly f o r t h e main r o t o r , and becuase as shown i n Figure 33 f l i g h t performance is degraded t h e l e a s t .

Performance must be maintained t o By keeping hardware

O f t h e two modified a i r c r a f t , E and F, Modification F i s t h e most a t t r a c t i v e because very l i t t l e i n t h e way of design changes are required. The 10% t i p speed reduction required for the main and t a i l r o t o r s can be accomplished by lowering the engine speed r a t h e r than by designing a new transmission. This i s espec ia l ly a t t r a c t i v e because r o t o r speed can be increased t o 100% t o improve performance once the hel icopter i s c l e a r of noise s e n s i t i v e areas. I n f a c t , the only hardware changes required are the addi t ion of englne s i lencers and use of advanced design r o t o r blades.

Modification F , however , is a somewhat higher r i s k design than i s Modification E. Tip speed reduction, engine s i lencing, and increased r o t o r s o l i d i t y have a l l been previously v e r i f i e d as e f f e c t i v e noise reduction methods (Reference 2 9 ) . The noise reduction p o t e n t i a l of t h e advanced design main ro tor blades has been demonstrated by whir l tower and f l i g h t t e s t i n g . Unfortunately, the acoustic performance predicted f o r the advanced design blades on t h e t a i l r o t o r has not ye t been completely experimentally v e r i f i e d . Recent preliminary unpublished f u l l sca le t e s t data, however, show promising r e s u l t s i n t h i s area. On t h e other hand, ~ _ _

19

Modification E which a lso meets t he noise c r i t e r i a , w i l l suf fe r a loss of payload, w i l l r equi re new hardware (gearbox, wider blades, hub t o accept 6 blades, e t c . ) , and long, expensive design and t e s t p rogramsto substan- t i a t e t h e new hardware and receive FAA c e r t i f i c a t i o n .

HARDWARE TESTS REQUIRED

The previous sect ion described two modified a l r c r a f t t h a t meet t he requirements of t h e community noise acceptance c r i t e r i a and po ten t i a l ly r e s u l t i n t he l e a s t performance degradatfan. I n order t o develop the noise reduct ion concepts I n t o f l i g h t warthy hardware a s e r i e s of ground and f l i g h t t e s t s must be performed f o r each f t e m i n addt t ion t o de t a i l ed s t r u c t u r a l and dynamic analyses. must be both acous t ica l ly and aerodynamically substant ia ted.

I n pa r t f cu la r , t he Modiffcation F t a i l ro to r design

The required engine noise reductions must be achieved through use of acous t ica l ly t r e a t e d i n l e t and exhaust ducts . The technology necessary t o design i n l e t noise suppression l a avai lable from NASA sponsored s tudies of acous t i ca l ly t r e a t e d nace l les f o r turbofan engines and the re i s some information ava i lab le on desrgn of exhaust nofse treatments. must be done t o adapt t h i s technology t o turboshaft engines and methods of incorporat ing an t i - ic ing i n t o t h e design must be developed. A preliminary study by t h e authors ind ica tes t h a t the necessary noise Educt ion can be achieved within t h e current duct outer envelope.

Eowever , work

Most of t h e work necessary t o incorporate t h e advanced design main r o t o r blades on t h e a i r c r a f t has already been completed and t h e acoust ic performance measured. Acoustic substant ia t ion of t h e blades on t h e a i r c r a f t remains t o be completed. dynamic s t a b i l i t y a t t h e reduced operating speed.

AnaLyses must be performed, however, t o determine

The t a i l r o t o r i s t h e one piece of hardware r equ i r ing t h e most extensive ana lys i s and t e s t i n g . The Modification E design with more blades, wider chord, and subs t an t i a l ly reduced t i p speed must be analyzed and then t e s t e d ( s t a t i c and f l i g h t ) t o be sure of dynamic s t a b i l i t y i n a l l f l i g h t regimes. A new gearbox must be designed and t e s t ed . The Modification F design incorporating advanced design blades and blade t i p s must be acous- t i c a l l y as wel l as dynamically substant ia ted.

~

Gearbox noise has not been discussed i n t h i s repor t because it i s of secondary importance compared with t h e engipes and ro to r s , , There i 8 t h e p o s s i b m f , however, t h a t i f gearbox'nolse is-found t o be a problem a s other sources of noise a r e suppressed it can be d e a l t with i n a s t ra ightforward manner.

20

CONCLUSIONS

The fo l lowim conclusions r e s u l t from t h e current study:

Noise Acceptance C r l t e r i a

LD 1. The recammended noise acceptmce c r i t e r i a (based on t h e l e v e l of 60) a r e conservative from t h e community's p i n t 8 v i e w . They are well below t h e average of Federal (domestic and fore ign) guidel ines 'and s l i g h t l y below t he average of current s t a t e and l o c a l noise limits. community annoyance.

They a r e a l s o i n l f n e with EPA f indings regarding

2. The recommended c r i t e r i a a r e f a i r t o t h e camunity as wel l as t h e he l icopter operator. C r i t e r i a l eve l s were se lec ted t o be within t h e "campletely acceptable" range of comuni ty reac t ion t o noise but a t t h e same time t h i s study showed t h a t current generation a i r c r a f t , with some modrfications , can be made acceptable f o r typ i - ca41 commercial t ranspor t operations.

3. The recommended c r i t e r h a re not unduly r e s t r i c t i v e t o t h e commer- c i a 1 t ranspor t operator, Because of t h e nature of t h e c r i t e r i a he has t h e a b i l i t y t o vary schedulfng, afrcraf ' t type , f l i g h t pro- f i l es , takeoff and landing routes , and even t h e he l ipo r t loca t ion i n order t o design a n operation tha t i s acceptable both t o t h e cammun- i t y and t o himself.

4. The recammended c r i t e r i a are preferable t o other a i r c r a f t noise annoyance c r i t e r i a because they consider t h e t o t a l noise load on a ccmnmunity. A s such, a l l noises including ambient are accounted f o r i n the calculated annoyance l e v e l , . Other measures a r e designed f o r a i r c r a f t only, or for

5 . Use of t h e c r i t e r i a encourages de-escalation of a i r c r a f t noise i n t h e fu tu re . A i rc ra f t noise will automatically be de-escalated as other sources contr ibut ing t o c e r t a i n ambients a r e reduced. This occurs because t h e c r i t e r i o n l e v e l f o r a given area i s a d i r e c t funct ion of t h e l o c a l ambient down t o a "f loor" l e v e l of %N = 60.

6 . Although very l imited r e s u l t s are ava i lab le , impulsive noise g r e a t l y increases perceived annoyance i n a community. It appears t h a t a penal ty of 5 t o 1 0 dBA should be added t o t h e measured ( o r computed) Single Event N o i s e Exposure Level (SENEL) f o r an a i r c r a f t producing impulsive noise.

21

7. Tone correct ions, as they are calculated f o r Tone Corrected Perceived Noise Level, should be applied t o the A-weighted sound l e v e l (dRA), but only f o r tones above 500 Hz f o r hel icopters and propel ler driven a3rcrd-k.

C i v i l Helicopter Operations

A current generation 50 passenger he l icopter , derived f r a a s i n g l e main rotor m i l i t a r y t ranspor t helicopter i n t h e i8iOO t o 22700 Kg (40,000 t o 50,000 pound) weight c lass w a s evaluated t o determine those hardware changes necessary f o r it t o meet the recommended community acceptance c r i t e r i a :

1. The basic helicopter I n t h e c i v i l t ranspor t configuration meets the c ru ise nofse c r i t e r i a w T t h no changes.

2. The basic c i v i l hel icopter meets a l l of t h e c r i t e r i a with l i t t l e modification. Main r o t o r changes involve use of advanced design blades and a 10% t i p speed reduction, t h e t a i l r o t o r incorporates a 10% t i p speed reduction and use of advanced design blades, and t h e engines require a s m a l l amount of i n l e t and exhaust noise s i lencing.

3. The modified a i r c r a f t s u f f e r s l i t t l e degradation of performance. Additional weight added for engine s i lenc ing i s l e s s than 100 pounds, there i s no l o s s of engine performance, and takeoff per- f omance i s v i r t u a l l y unchanged. Detailed s tudies are necessary t o def ine actuaJ performance and t o determine any possible s t a b i l i t y or other problems.

4 . By paying c a r e f u l a t ten t ion t o d e t a i l r o t o r and blade design and t o engine se lec t ion and i n s t a l l a t i o n d e t a i l s it i s possible t o design a 100 passenger c i v i l t ransport hel icopter (or compound he l icopter ) f o r the 1980-85 time frame t h a t w i l l m e e t t h e noise acceptance c r i t e r i a as w e l l as reasonable performance object ives .

RECOMMENDAT I ON S

1. Subjective t e s t i n g should be carr ied out t o determine t h e annoyance penalty f o r the presence of impulsive noise. A t t h e same time a method should be establ ished t o quant i ta t ive ly def ine the existance of impulsive noise and t o indicate i t s sever i ty . The c r e s t f a c t o r of t h e sound (Peak Level/RMS Level i n dB) has shown some promise as the required impulsive noise measure. The subject ive t e s t i n g should include impulsive noise "severity" as wel l as peak amplitude as a parameter.

22

2. Further communlty subject2ve s tudies should be conducted t o ve r i fy t h e absolute lower l i m i t of 60 LDN f o r areas with ambient l eve l s below 58 dBA.

3. Additional psycho-acoustic t e s t i n g should be ca r r i ed out t o ve r i fy t h e appl icatfon of tone correctfons t o t h e A-weighted sound l e v e l (dBA) and t o verffy t h a t only tones above 500 Hz should be included i n the ca lcu la t fon of tone corrected dBA (dBAt) f o r he l icopters and propel le r a i r c r a r t .

4. The indicated hel icopter component t e s t s should be conducted t o v e r i f y t h e predicted noPse reduction and t o e s t ab l i sh aerodynamic per f ormanc e.

5. The basic hel icopter should be modified t o t h e recammended configuratlon and f l f g h t tes ted t o ve r l fy t h e predicted noise l e v e l s and aerodynamlc performance.

6. Usfng advanced desfgn concepts f o r the main and t a f l r o t o r s , auxi l - l i a r y propulslon ( i f any) , and engine in s t a l l a t3on a 1980-85 t i m e frame 100 passenger c f v l l t ransport he l icopter should be designed t o meet t h e recammended nofse c r i t e r i a as w e l l as reasonable performance goals.

Sikorsky Ai rc ra f t , United Ai rc ra f t Corporation

S t r a t fo rd , Connecticut, February 27 , 1974.

23

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Mi l le r , L. M. and Ver, I. L. , "Noise Study i n Manhattan, New York City f o r t h e Evaluatlon of Domlnant Noise Sources Including Helicopter Traf f ic" , Bolt Beranek and Newman Report No. 1610, 4 August 1967.

Moreira, N. M. and Bryan, M. E . , "Noise Annoyance Suscept ib i l i ty" J1. Sound Vib. , Vol. 21, No. 4, 1972, pp. 449-462.

Morse, K. M . , "Community Noise - The S t a t e of t h e A r t , I n d u s t r i a l Aspect", 74th Annual Meetlng, Acoustical Sooiety of America, November 1967.

Mull, H. R., "Washington D. C . Helicopter Demonstration", Harold R . Mull and Associates Report R-6606, September 1969.

Mull, H. R . , "External Noise Charac te r i s t ics of Two Commercial Transport Helicopters", United Acoustic Consultant Report R-642, January 1964.

Muller, J. L . , "Calculation of A i rc ra f t Noise Duration", J1. Sound Vib., Vol. 16, No. 4 , 1971.

N a g e l , D. C . , Parne l l , J . E. and Parry, H. J . , "Procedure f o r Correcting Perceived Noise Level f o r t h e Effec t of Background Noise", Transportation Noises, University of Washington Press , 1970.

32

80.

81.

82.

83.

84.

85

86.

87

88.

89.

90.

91.

92.

93.

National Bureau of Standards, "The Social Impact of Noise", Environmental Protect ion Agency PB 206 724, December 1971.

National E l e c t r i c a l Manufacturers Association, "Gas Turbine Sound and Its Reduction", NEMA Pub. N O . SM133-1964.

Northeastern I l l i n o i s Planning Commission, "Metropolitan Aircraf t Noise Abatement Pollcy Study, O ' H a r e I n t e rna t iona l Airport , Chicago, I l l i n o i s " , PB 203618, 1971.

Ollerhead, J. B. , "Scaling Aircraf t Notse Perception", J1. Sound Vib. , Vol. 26, No. 3, 1973, pp. 361-388

Ollerhead, J . B. , "An Evaluation of Methods for Scaling Aircraf t Noise Perceptfon", NASA CR-1883, October 1971.

Ollerhead, J . B. , "Subjectlve Evaluation of General Aviation Ai rc ra f t Noise", FAA-NO-68-35, Apr i l 1968.

Ostergaard, P. B. and Donley, R . , "Background Noise Levels i n Suburban Communities", J1. Acoustical SOC. Am., Vol. 36, No. 3 , March 1964.

I 1 Parry, H. J . and Parry, J . K . , The In t e rp re t a t ion and Meaning of Laboratory Determtnations of t h e Effect of Duration on t h e Judged Acceptabi l i ty of Noise", J1. Sound Vib., Vol. 20, No. 1, 1972.

Pau l l in , R . L. and Miller , J.S.F. , "Aircraf t Noise Abatement - The Prospects f o r a Quieter Metropolitan Environment'' , A I M Paper No. 69-800, Ju ly 1969.

Paul l in , R . L . , "Aviation and Environmental Qual i ty" , A I M Paper No. 71-729, June 1971.

Pearsons, K. S . , "Laboratory Studies on t h e Effec ts of Duration and Spec t ra l Complexity on Subjective Ratings of Noise", ASHA Reports No. 4, 1969, pp. 228-237.

Pearsons, K. S . , "Combination Effec ts of Tone and Duration Para- meters on Perceived Noisiness", NASA-CR-1283 , February 1969.

Peysons , K. S . , "Noisiness Judgements of Helicopter Flyovers", FAA DS-67-1 , January 1967.

Pearsons , K. S. , Bennett , R . L. and F i d e l l , S. , "Study of t h e Effec ts of t h e Doppler S h i f t on Perceived Noisiness", Acoustical Society of America, Houston, Texas, November, 1970.

33

94 ( 1 Pearsons, K. S . , HoronJeff, R . D . , and Bishop, D. E . , The Noisiness of Tones Plus Noise", NASA CR-1117, August 1968.

Pearsons, K . S. and Kryter, K. D . , "Laboratory Tests of Subjective Reactions t o Sonic Boom", NASA CR-187, March 1965.

Powers , J. K. , "Aircraf t Noise Standards and Regulations", FAA- RD-71-24, March-April 1971.

Reichardt, W . , "Subjective and Objective Measurement of t h e Loudness Level of Sfngle and Repeated Impulses", J1. Acoustical SOC. AIII., V O ~ . 47, No. 6 , 1970, pp. 1557-1562.

Robinson, D. W . , "A New Basis f o r A i rc ra f t Noise Rating" National Physical Laboratory NPL Aero Report AC 49, March 1971.

Robinson, D. W. and Bowahcr, J. M. , "A Subject ive Experiment with Helicopter Noises", J1. Royal Aero. SOC. , September 1961, pp. 635-637.

Rylander, R . , Sorenson, S. , Kajland, A , , "Annoyance Reaction From A i r c r a f t iu'oise Exposure" , $1. Soimd & Vlb. , V d . 2 b , 51s. 4 , 1972, pp. 419-444.

Rylander, R . , Sorensen, S. , Alexander, A., G i lbe r t , Ph., "Determinants f o r A i rc ra f t Noise Annoyance - A Comparison Between French and Scandanavian Data", J1. Sound Vib., Vol. 28, No. 1, 1973. pp. 15-21.

Schlegel, R . G . , "HUSH F ina l Report - Quie t Helicopter Program", Sikorsky Engineering Report SER 611478, January 1970.

Schul tz , T. J . , "Noise Assessment Guidelines Technical Background", HUD Report No. TE/NA 172, 1972.

Semotan, J . and Semotanova, "Startle and Other Human Responses t o Noise", J1. Sound Vib. , Vol. 10 , NO. 3, 1969, pp. 480-489.

Serendipi ty Inc. , "A Study of t h e Magnitude of Transportation Noise Generation and Potent ia l por ta t ion Report No. OST-ONA-71-1, November 19-70.

Society of Automotive Engineers , "Technique f o r Developing Noise Exposure Forecasts" , FAA Ds-67-14 , August 1967.

Abatement", Department of Trans-

Sperry, W. C . , ''Noise Source Abatement Technology and Cost Analysis Including Ret rof i t t ing" , Task Group 4 Draf t Report of EPA Ai rc ra f t / Airport Noise Report Study, June 1, 1973.

34

108.

log .

110.

111.

112.

113.

1 1 4 .

115.

116.

117.

118.

119.

120.

Sperry, W . C . , "Aircraft Noise Evaluation" , FAA-NO-68-34 , September 1968.

S t a t e of Cal i forn ia Department of Aeronautics, "Noise Standards" , T i t l e 4 , Register 70, No. 48-11.28.70.

Stave, A. M. and King, R . J., "The Annoyance of V/STOL Noise - Phase I , Impulse Nofse" , Sfkorsky Engineering Report SER 50678 , October 1970.

Stepniewski, W . Z . , Schm3tz, F. H . , "Poss ib i l i t i e s and Problems of Achieving Community Acceptance of VTOL", In te rna t iona l Council of t h e Aeronautical Sciences, ICAS Paper No. 72-34, Amsterdam, August/September 1972.

S te rnfe ld , H . , Hinterkeuser, R . B. , Hackman, R . B . , Davis J., "Acceptabili ty of VTOL Aircraf t Noise Determined by Absolute Subjective Testing", NASA CR-2043, June 1972.

Tanner , C . S. and Glass, R . E. , "Analysis of Operational Noise Measurements i n Terms of Selected Human Response Noise Evaluation i.ieas-u-es" , Fj?&-iiD-7i-ii2, Decaber 1971.

Tracor Inc. , "Community Reaction t o Airport Noise", NASA CR-1761, Ju ly 1971.

Wells, R . J . ,"Jury Ratings of Complex Ai rc ra f t Noise Spectra Versus Calculated Ratings", Presented at 80th Meeting of t he Acoustical Society of America, November 1970.

W i l l i a m s , J . M . and Berthoud, R . , "Helicopter Noise i n Central London", Social & Community Planning Research Paper P. 184, November 1970.

W i l l i a m s , H. L. and Majer, A . J . , "Is It Impactor Continuous Noise?" , Archives Environmental Health, October 1963 , pp. 37-40.

W i l l i a m s , C . E . , Pearsons, K. S . , and Hecker, M. H. L . , "Speech I n t e l l i g i b i l i t y i n the Presence of Time-Varying Ai rc ra f t Noise" J1. Acoustical SOC. Am. , Vol. 50, No. 2 , 1971, pp. 426-434.

W i l l i a m s , C . E. , Stevens, K . N . , Hecker, M. H. L. , and Pearsons, K. S . , "The Speech Interference Ef fec t s of A i rc ra f t Noise", FAA T .R. Ds-67-19 , September 1967.

Wyle Labor a t or i e s , "Community Noi s e" Environment a1 Protect ion Agency PB 207 124, December 1971.

35

I? 121. Yeowwt, N . S., An Acceptable Exposure Level f o r A i rc ra f t Noise i n Res ident ia l Communities", J1. Sound Vib. Vol. 25, No. 2, 1972.

122. Yeowart , N . S. , Bryan, M. E. , and Tempest , W. , "Low-Frequency Noise Thresholds", J1. Sound V i b . , Vol. 9, NO. 3, 1969 , pp. 447-453.

123. Young. T . C . , "Nolse Abatement - A Balanced Approach", Proceedings of t h e SAE-EPA-DOT Tnternational Conference on Transportation and t h e Environment, Paper 720626, Washington D . C., June 1972.

124. Zepler, E. E . and Harel, J . P. P., "The Loudness of Sonic Booms and Other Impulsive Sounds", J1. Sound Vib., Vol. 2 , No. 3, 1965 pp. 249-256.

125. Zepler, E. E. , Sul l ivan , B. M. , Rice, C . G . , G r i f f i n , M. J., Aldman, M . , Dickinson, P. J., Shepherd, K. P . , Ludlow, J . E. ,

bration" , J1. Sound Vib. , Vol. 28, No. 3, 1973, pp- 375-4010

and Large, J. B. , ? ? Human Response t o Transportation Noise and V i -

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FEDERAL CRITERIA

6 (GERMANY)

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N (FRANCE)

N E F ( U S A )

CNR (USA)

HUD -NONAIRCRAFT NOISE

N N I (GREAT BRITAIN)

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HUD - TRAFFIC NOISE

MEANl HUD -TRAFFIC NOISE INCL.

0 0 0

0 0

0 " 0

0

80 90 i 00 I IO

A-WEIGHTED S P L FOR ONE IO-SEC EXPOSURE PER DAY 1

Figure 2. Comparison o f Clearly Acceptable L i m i t s of Federal Regulations.

44

COMMUNITY DARKEN ED C I R C L ES IN D I CAT E REG U L AT ION S OUT OF L I N E WITH THE MAIN BODY OF DATA

INGLEWOOD, CALIF. BOULDER, CO.

BOSTON, MASS.

ANAHEIM , CALIF

SPRINGFIELD, MASS

BINGHAMTON , N.Y.

BEVERLY H I L L S , CALIF.

DENVER, CO.

FAIR LAWN , N.J.

WARWICK , R. I . FARMINGTON , CT.

D A L L A S , TX.

NEW HAVEN, CT.

COLUMBUS, OH.

DAYTON ,OH.

M I A M I , FL.

MINNEAPOLIS , MI.

PEORIA , IL.

TUCSON, AR.

CHICAGO, IL.

BALTIMORE, MD.

LOS ANGELES ,CAL IF .

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Figure 3. Comparison of Residential Community Noise Regulations.

45

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Figure 4. Effect of Blade Slap on Judged Annoyance of Helicopters.

46

RECORDER

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47

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DOMESTIC AND FOREIGN F E D E R A L STANDARDS

COMMUNITY STANDARDS ( RES1 D E N T I A L AREAS)

S T A T E STANDARDS ( R E S I D E N T I A L AREAS)

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AMBIENT SOUND PRESSURE LEVEL- dB(A) ref: 0.0002 pbar

Figure 1 0 . Recommended Community Noise Acceptance C r i t e r i a .

52

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40

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57

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59

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W cr

0 ‘a .. a

* w o n a

I- 2 U - a

N I - ? g LL

-I W Z

- w ? v ,

m 0 0

N 3 3

9 -

In

N

-

10 0

(v 0

- 0

m 0 0

(v 0 9

- 0 0

k 0

E: 0 . m a l

0 u 4

68

I O 0

90

80

70

60

50

90

80 z 1 ‘0

In

70 N + F a

60

50

I I + - + - + OBSERVER I

x = 3 1 0 M ( I O O O F T ) y = 155M ( 5 0 0 F T )

S R = 174M ( 5 6 1 F T )

3

0 8 16 24 32 T I M E , SEC

0 8 16 24 T I M E , SEC

Figure 27. Contribution of Each Noise Source t o t h e Baseline S-65-40 Helicopter Noise.

0 0 -

1- I 1 I I I

I I I I I I I 0 m 0

(0 0 IC

0 0 0 rr,

0 0 0 N

0 5: -

0 O m o m

W I- W 2

W

3 I-

c

n

- 0 5 O a In

0 0 e

0 0 rr)

0 0 N

0- (D

ld *ri k aJ *ri k V

a3 cu ‘I

I30

I 2 0 N

E z \ 10

I

0 110 X N

rc

2 a g 100

#.

1 w z w v)

90

80

I I I I I I

- - - - NO B L A D E S L A P S E V E R E B L A D E S L A P ASSUMED

I I I I I I 1

.005 -01 .02 .05 . I .2 .5 I S E N E L FOOTPRINT AREA , SQUARE K ILOMETERS

I 1 I I I I I .002 .005 .01 .02 .05 . I .2

S E N E L FOOTPRINT A R E A , SQUARE M I L E S

Figure 29. Effect of Impulsive Noise on a Helicopter's Noise Annoyance Characteristics.

(u

5- M

0 M

a,

72

IR

I I I I I I I N 3

0 0 0 00 - 0 - a O D ' - 0

CU -

- C U 0

- - 0

0 - 0

? -

-

(u - 0

0

- - 0

0

4 M

a,

,WIN , - O l x Z WBP ' 13N3S

73

B A S E L I N E H E L I C O P T E R

80

70

60

50

40

30

M O D I F I E D M A I N ROTOR I N S T A L L E D

- T O T A L d B A

- - -

- - - - - - -

-

-

OCTAVE BAND NUMBER ! ! =63Hz RAND)

M O D I F I E D M A I N AND T A I L ROTORS I N S T A L L E D

OCTAVE BAND NUMBER ~

MODIFICATION F H E L I C O P T E R i 1

r I , -TOTAL d B A

OCTAVE BAND NUMBER OCTAVE B A N D N U M B E R 1

Figure 32. Octave Band Noise Reduction Resulting in Modification F Helicopter.

74

- A L L w - m u

z +-

a m m

0 a 0 LL a 0 H

I I I I I I I I 0 0 0 0 0 0 0 0 0 0 0 0 0 pc W In d M

0 - 0 N

0

0 1 o j

0 ' 0 In

3 m m

I 4 3 a n L I i i v

75

APPENDIX

SAMPLE CALCULATION OF LDN

A sample ca lcu la t ion of t h e value a t a s ing le point on the ground w i l l be computed using t h e ions of Figure 9 i n the main body of t h e repor t . The necessary fnfcmnation is giver, belew:

Number of d i f f e r e n t a i r c r a f t : 2 Number of f l i g h t s : 50 day 1 night f o r a i r c r a f t 1

25 0 nfght for a l r c r a f t 2 Number of f l i g h t paths: 1 T i m e ASrcraft Sound l a above ambient: 15 sec f o r a i r c r a f t 1 and

Ambient Noise Level: 75 dBCl 10 sec f o r a i r c r a f t 2

110

8100 I

I 4

w rn 90

SENEL Charac ter i s t ics F l igh t Path:

I ' I1 A/C 2 I

100 200 1000 Slan t Range - ft

1. Calculate t h e Daytime Noise Level, L,

2 1 (54000-igl jglNiJAT i j )

. 100 + + 25ant 10 50ant - 102.5

10 $ = l0Log 10

w b s erver SR = 193'

- L 75

(54000-50(15)-25(10)) a n d ( - 4 7 . 3

A 1

LD = 10loglo -47.3

= 124.5-47.3 = 77.2 dBA

2.

1 1 -45.1

+ (32000-15) ant

LN = 10loglo 11.78~10 11 + 1.01x1012} -45.1

5 = 75.6 dBA

Calculate t h e Day-Night Noise Level, LDN 3.

LDN = 10loglo

bN = 10loglo

ant ($-,/lo) + 0.375 an t ($/lo))

ant (77.2/10) .C 0.375 an t (75.6/10)) LDN = 10loglo {3.28xlO 7 -+ 1.36~10

LDN = 76.7 dBA

A2

community acceptance of helicopter noise criteria and application

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