12 VERTIFLITE March/April 2014

The Issue

Helicopter operations around the world, and inparticular in the United States, are giving rise toobjections over noise from the general public; in the

last two or three years there have been major objections tohelicopter use in the New York and Los Angeles areas. Thedevelopment of new heliports, and changes to services atexisting facilities, tend to be controversial and are oftenrejected as a result of public opposition. Such negativeresponse to helicopter operations as a result of noise is alittle difficult to understand because most helicoptersgenerate less noise than the noise certification standardsand, in most cases, meet established community noise ratingcriteria and guidelines. Also, the same rating methods aregenerally considered to be successful in understanding theenvironmental impact of large commercial aircraft and otherforms of transportation, so there appears to be somethingdifferent about the way in which helicopters are perceived.Even so, the issue of helicopter noise in connection withheliport operation is continually being reported in theaviation and general press. This issue has also beenaddressed in a report on non-military helicopter noise issuedto the US Congress by the Federal Aviation Administration(December 2004) and for the UK Department forEnvironment, Food and Rural Affairs (Defra) in a June 2008study (see www.vtol.org/noise for the reports).

Social Surveys

Areview of case histories, press reports and informationcollected by industry associations makes it clear thathelicopters and heliports, in many locations, have only

a low level of public acceptance. This was put intoperspective a number of years ago when the results from anumber of studies connected with the operation ofhelicopters in the United Kingdom was reported in 1993 bythe UK Civil Aviation Authority (CAA). The CAA results aregiven in Figure 1 and shows annoyance as a function of thenoise levels expressed in terms of “LAeq (16 hr)” – effectivelythe average sound level over a specified time.*

Data from the 1982 survey indicated on the figure wereobtained by the CAA along the route of the Gatwick-Heathrow Airlink service (no longer operating) and atAberdeen, Scotland, the major base for offshore oil industryhelicopter operations in the North Sea. This data reveals thathelicopters operating in the London area are considered tobe up to 15 dB(A) more annoying than fixed-wing aircraft.This result contrasts with that obtained in Aberdeen whichshows helicopters to be no more annoying than fixed-wingaircraft. In both cases, the characteristics of the acoustic

environment were influenced by large, acoustically non-impulsive helicopters.

One prominent researcher at the time suggested thisdisparity in reaction between London and Aberdeen couldbe explained in socio-economic terms: “Better off peopletend to be more annoyed.” In addition, residents under theAirlink flight path were less favorably disposed towards thehelicopter shuttle service which was being used by first classpassenger, whilst in the Aberdeen area, North Sea oiloperations contribute significantly to the local economy.

In a small-scale 1992 study in London the results weresimilar to those for the Gatwick-Heathrow Airlink evaluated10 years earlier. The London flights were dominated by thecorporate market using light/medium helicopters, includinga large number of Bell JetRangers and LongRangers plusAerospatiale (now Airbus Helicopters) Dauphins, Sikorsky S-76s and a few larger helicopters. Studies carried out by theGreater London Council in the same timeframe indicated anunderlying concern of the residents about noise and safetyof helicopters, which up until that time was not welldocumented.

The conclusion reached by many observers, who generallyignored the Aberdeen result, was that for a similar level ofannoyance or acceptance, helicopter noise levels need to bemuch lower than those of fixed-wing aircraft and other forms

* LAeq (16 hr) expresses time-varying A-weighted (filtering sounds to mimic the response of the human ear) noise levels occurring dur-ing an observation period as a single constant value having the same acoustic energy. The 16 hour period from 7:00 a.m. to 11:00 p.m.is used in the UK. This metric is similar to the Day-Night Average Level (DNL or LDN) metric used in the United States.

Helicopter Noise: What is the Problem?

By Dr. John W. Leverton

Figure 1: UK CAA Social Survey Results (1993)

of transportation. The author does notsupport this point of view.

Public Acceptance

Community noise rating proceduresare considered to predict relativelywell the impact of fixed-wing

aircraft noise around airports and withinlocal communities during overflight. Thisis not the case for helicopters andheliports, which appear to create a levelof adverse reaction disproportionate tothe measured and predicted noise levels.A partial explanation for the disparitybetween noise assessments andcommunity reaction to helicopteroperations has been identified asdeficiencies in the rating methods. For amore complete analysis of the issues, it isnecessary to examine the way in whichhelicopter operations are perceived.Fixed-wing aircraft operations at airports typically involve alarge number of flights per day and, because the noisecharacteristics of most of the large jets are similar to oneanother, the noise climate is relatively uniform. Away fromairports, aircraft fly at very high altitude so that noise levelson the ground are low. In addition, there is little concern overaircraft safety. Helicopter operations are very different. Ingeneral, the paths, unlike those used by fixed-wing aircraft,vary widely and so at any one location the noise pattern ismuch less consistent. There are also very large differencesin both level and, more importantly, the character ofnoise created by different helicopters with some smallhelicopters sounding noisier than larger ones. Overflightsare generally made at relatively low altitudes so that anyconcerns over safety are heightened.

In the context of public acceptance, it should be notedthat even relatively sophisticated noise rating methodsbased on complex objective measurements fail to accountfor the disturbance caused by helicopters and it has been(and is being) suggested that the noise certification limitsand the criteria associated with community ratingprocedures should be made more stringent. Although minoradjustments to the assessment criteria may be helpful,analysis of the issues indicate that such action will have littleor no direct effect on the level of public acceptance. Forexample, a comprehensive study of helicopter operations at amilitary airbase in the UK in 2000 concluded that there wasno meaningful correlation between helicopter noiselevels and subjective annoyance.

Helicopter Noise Characteristics

Ageneralized A-weighted sound pressure level timehistory of a helicopter flyover is shown in Figure 2 toillustrate the influence of various helicopter noise

sources on overall noise level. The principal sources are mainrotor thickness/high speed impulsive (HSI) noise, main rotorblade/blade vortex interaction (BVI) noise, main rotorwake/tail rotor interaction (TRI) noise and tail rotor (TR)noise. HSI, TRI and TR noise are most pronounced duringflyover. BVI noise is normally the dominant source duringdescent, although TR and TRI noise may also be present. BVIcan also occur on some helicopters during flyover/cruise

flight and is pronounced during banked turns. In the case oftandem rotor aircraft, BVI occurs continuously, regardless offlight condition.

Measured in conventional subjective units, the form of thedB(A) time history will be similar to that indicated in thefigure whichever of the sources dominant. Moreover, becauseall of the principal sources generate similar absolute noiselevels, there will be little change in the time history even ifone or two of the sources is pronounced at the same time.The directional characteristics of HSI and BVI are such thatthese sources have little influence on the maximum noiselevel which normally occurs close to the overhead position.Although TRI and high levels of tail rotor (TR) noise canincrease the maximum level by up to 5 dB(A), the effect ofthese sources is much greater – 10-15 dB(A) – at distance asthe aircraft approaches as shown on Figure 2. Mostimportantly, it can be seen that the greatest effect of theintrusive sources occurs more than 10 dB(A) below themaximum value, so they will have little or no influence ontime integrated units such as Sound Exposure Level (SEL)and Effective Perceived Noise Level (EPNL).

The idealized traces shown on Figure 2 represent flightsduring which the impulsive sources are generatedcontinuously. However, these sources often occurintermittently, in which case the time history will exhibitrelatively rapid increases and decreases in level. Helicoptersoperated close to a BVI or TRI threshold will be particularlysensitive to control inputs and the ambient temperature inwhich the helicopter is operating. These changes in noiselevel will be more marked on higher tip speed rotors simplybecause the sources are naturally more intense. From asubjective point of view, the intermittent generation of theintrusive sources is equally or more annoying than if thesound occurred continuously, and tends to draw immediateattention to the helicopter. This is important whenconsidering annoyance.

Helicopter Noise – Subjective Impact

The subjective impression created by the impulsive noisesources are very important when considering publicacceptance. Also, except in the case of tail rotor noise,

the sources of interest are mainly detected at levels wellbefore the so-called “10 dB down” point – the point on the

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Figure 2: Generalized dB(A) Flyover Time History

sound pressure level time-history at which the level is 10 dBbelow the maximum or peak level.

A study of the various factors involved shows the level ofpublic acceptance can be considered to be a function ofboth acoustic (direct) noise and a non-acoustic element,termed virtual noise, as illustrated in Figure 3. The responseto acoustic noise is a function of maximum noise level asdefined by objective measurements and, more importantly inthe context of public acceptance, the subjectivecharacteristics of the noise as it first becomes audible. Themagnitude of the non-acoustic component (virtual noise) isnot related directly either to the absolute level or to thecharacter of the noise generated by helicopters, but it istriggered by the direct acoustic signal.

The annoyance or level of public acceptance is usuallyquantified using measured noise levels as illustrated in Figure1. Consequently, the virtual noise element is treated, for allpractical purposes, in the same way as the direct acousticenergy (noise) radiated by the helicopter. Virtual noise isdependent on a wide range of inputs but is triggered initiallyby any distinctive feature of the acoustic signature and, to afar lesser extent, the absolute noise level.

It cannot be stressed highly enough that wheneveradverse reaction to helicopter operations results fromvirtual noise, attempts to address the problem byreducing acoustic noise at the source will be largelyineffectual. It is not simply that the level of sound – at longrange as the helicopter approaches or flies towards theobserver – is higher than on helicopter models with little orno noticeable HSI, TR, TRI, or BVI noise. Rather it is that thetonal and impulsive characteristics of these sources are inthemselves more annoying and draw attention to thehelicopter.

Even though the influence of the characteristics ofhelicopter noise below the “10 dB down” point is clear to theauthor, many researchers continue to argue that EPNL – andby implication the SEL, LDN or LAeq metrics – give a realisticmeasure of both the source level and public response,implying that any increase in the sound associated with BVI,HSI, TRI and tail rotor noise is accounted for in full by metricswhich take into account the duration.

The subjective rating of helicopter noise was investigatedthoroughly in the late 1970s and early 1980s. One objective

at the time was to develop an impulsivecorrection that could be added to moreconventional metrics to account for thesubjective effect of BVI and tail rotornoise. Despite the considerable effortexpended, the results of these studies incombination were considered by manyto be largely inconclusive. After anextensive review of all the issues, theInternational Civil Aviation Organization(ICAO) in 1983 adopted the EPNL forhelicopter certification, with the provisothat manufacturers should strive toeliminate intrusive noise sources.

NASA studies on subjective responseto helicopter sounds showed that theaddition of an impulsive correction,which some had suggested, did notimprove the human responsepredictions. While the case presented byNASA is valid, it is apparent that both the

level and character of sound audible at distances greaterthan those involved in EPNL calculations, the author wouldsuggest play a major part in the rating or acceptance ofhelicopter noise by the general public. It would appear thatwhen the degree of BVI, HSI, TRI and/or tail rotor tonal noiseis pronounced, these distinctive sources act as an audiblecue, increasing the negative response to helicopter noise.These low absolute level triggers are not accounted for inEPNL or SEL calculations, which only account for acousticenergy within 10 dB of the maximum value.

The studies based on UK data, supplemented byinformation from other locations, including that associatedwith Airspur who operated in the Los Angeles, California areain the early 1980s, show that the noise characteristics andvirtual noise are of equal or even greater importancethan the maximum noise level observed during aparticular flyover or flyby event. It is difficult to ascertainprecise values for these components because they are partlyinterrelated. For example, a helicopter generating BVI or HSInoise may cause annoyance directly, while at the same timeacting as a trigger to highlight public opposition by someother aspect of the operation. The information available alsosuggests that sounds such as tail rotor whine and/or mainrotor impulsive noise (BVI or HSI) also exacerbate concernsover the safety of the helicopter because the sound may(falsely) suggest mechanical problems or conjure up animage of a helicopter crashing, as often seen in movies,television and online videos. Taking this argument toextremes, a helicopter generating low but perceivable levelsof tonal or impulsive noise flying over an area where thepublic have major concerns on helicopter safety could createthe same negative response as one with high levels of tailrotor, TRI, HSI or BVI noise operating over communities whichare generally more tolerant of helicopters.

Subjective Considerations

Studies have shown that there is a need to consider thecharacter as well as the absolute noise levels of soundheard by an observer. It is extremely difficult to quantify

the effect on individuals of particular sounds in terms of asubjective weighting, but studies undertaken in the period1975 - 1985 suggested values of 4 to 6 dB(A) and 6 to 9 dB(A)

14 VERTIFLITE March/April 2014

Figure 3: Elements of Public Acceptance

should be added to measuredlevels to account for signals withhigh levels of tail rotor noise andhigh levels of impulsive (BVI)noise, respectively.

When the helicopter noiseand complaint informationavailable was examined initially,a number of observations couldnot be explained. Furtheranalysis showed that if anoperation involves a mixture of helicopters with high levelsof BVI, HSI, TRI and/or TR noise and those without suchsources, the least acceptable will tend to dictate the level ofpublic acceptance. Thus a few noisy helicopters can createadverse response which will then affect the publicresponse to all helicopters.

If, however, the number of operations of noisy helicoptersis very low, this may not always be the case. In Aberdeen,Scotland one type of helicopter that generates high levels ofimpulsive noise (HSI and BVI) is known to provoke adversepublic response. However, because of the small number ofdaily flights made by this aircraft and the careful selection ofroutes, it does not appear to detract from a generallyacceptable level of public acceptance.

Concluding Remarks

T he reaction to helicopters and heliports isdependent on several factors, some of which arecompletely unrelated to the absolute level of the

helicopter noise. These non-acoustic phenomenadescribed collectively as virtual noise are usuallytriggered by acoustic noise. The non-acoustic componentcan dictate the level of public response to helicoptersunder certain circumstances

The regulatory authorities, both nationally andinternationally within ICAO, often argue that decreasing theabsolute level of helicopter noise by lowering the noisecertification limits or introducing operational noise limits, willdramatically improve the public acceptance of helicoptersand solve most of today’s objections to the level of noisegenerated by helicopters. The studies conducted by the

author do not support this view.The subjective character of thesound is equally or moreimportant than the maximumnoise level. The sound quality ofthe noise at levels 20 dB or morebelow the maximum levelprovides the initial audible cuesthat alert an individual to thepresence of a helicopter, i.e.provide the trigger for the virtual

noise component. It follows that improvements to the noisesignature by reducing or eliminating the impulsive sourceswill result in greater public acceptance irrespective of theabsolute noise level generated. It also implies that many oftoday’s small and medium size helicopters will need to fly at2,000 – 4000 ft (600 – 1200 m) or more, and that the use ofnoise abatement procedures for normal operations areessential. Also, they will need to fly much slower thananticipated if impulsive noise is not to create a problem.

Designs to achieve a high degree of public acceptanceshould not be based only on achieving compliance with thenoise certification limits. It is also essential to take intoaccount the sound pressure level and the subjectivecharacteristics of noise throughout the period over whichit is detectable, i.e. well outside the “10 dB down” rangeused to calculate EPNL and SEL. This is particularlyimportant if high tip speeds are being considered for themain and/or tail rotor.

Even so, there is certainly a need for more research intothe subjective response to helicopter noise – the mainactivity in this area was over 20 years ago and there has beenlittle examination of these aspects since then. From theindustry point of view (operators and manufacturers) it isessential to establish what really needs to be done toimprove public acceptance.

Analysis of the social survey results also reveals a strongconnection between noise and safety, and that safety, orperceptions about safety, also play a significant part in publicreaction towards helicopters which, of course, has a directbearing on the level of acceptance.

About the Author

John Leverton is the Environmental andInfrastructure Advisor for AHS International, withmore than 45 years of experience in rotorcraft

acoustics, environmental and infrastructureconsiderations. He is the president of LevertonAssociates.

This article is based on papers published jointlyby the author and A.C. (Tony) Pike, AcousticSpecialist, AgustaWestland, Yeovil, England, andadditional papers published by the author duringthe period 1998 to 2009, plus studies by the authorsince that time. Key publications on noise studies –including a longer version of this study and otherpapers by the author – are available atwww.vtol.org/noise, free of charge for AHSInternational members.

Vol. 60, No. 2 15

US Customs and Border Protection AW139s patrol New York City. (CBP photo by JamesP. De Boer)

. . . a few noisy helicopterscan create adverse

response which will thenaffect the public response

to all helicopters.