agma 914-b04
TRANSCRIPT
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AGMA INFORMATION SHEET(This Information Sheet is NOT an AG MA Standa rd)
A G M A 9 1 4 - B 0 4
AGMA 914- B04{Revision of AGMA 299.01
AMERICAN GEAR MANUFACTURERS ASSOCIATION
Gear Sound Manual
Part I - Fundamentals of Sound as Related to Gears
Part II - Sources, Specifications and Levels of Gear Sound
Part III - Gear Noise Control
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Gear Sound ManualPart I -- Fundamentals of Sound as Related to GearsPart II -- Sources, Specifications and Levels of Gear SoundPart III -- Gear Noise ControlAGMA 914--B04[Revision of AGMA 299.01]
CAUTION NOTICE: AGMA technical publications are subject to constant improvement,revision or withdrawal as dictated by experience. Any person who refers to any AGMA
technical publication should be sure that the publication is the latest available from the As-
sociation on the subject matter.
[Tables or other self--supporting sections may be referenced. Citations should read: See
AGMA 914--B04, Gear Sound Manual: Part I -- Fundamentals of Sound as Related to
Gears; Part II -- Sources, Specifications and Levels of Gear Sound; Part III -- Gear Noise
Control, published by the American Gear Manufacturers Association, 500 Montgomery
Street, Suite 350, Alexandria, Virginia 22314, http://www.agma.org.]
Approved March 4, 2004
ABSTRACTNoise measurement and control on gear driven equipment is dependent upon the individual characteristics of
the prime mover, gear unit and driven machine, as well as their combined effects as a system in a particular
acoustical environment.
Because of the wide variation of gear driven systems and acoustical environments, this manual attempts to
indicate certain areas where special considerations might be necessary, and must be agreed upon between
purchaser and the gear manufacturer, when discussing gear sounds.
The information is arranged in three parts. Part I presents the fundamentals necessary to understand sound as
related to gears. Part II describes the sources, specifications and levels of gear sound. Reduction or control of
noise, as addressed in Part III, requires attention to connecting equipment and the acoustical environment, as
well as the gear unit.
Published by
American Gear Manufacturers Association500 Montgomery Street, Suite 350, Alexandria, Virginia 22314
Copyright © 2004 by American Gear Manufacturers Association
All rights reserved.
No part of this publication may be reproduced in any form, in an electronic
retrieval system or otherwise, without prior written permission of the publisher.
Printed in the United States of America
ISBN: 1--55589--820--3
American
GearManufacturers
Association
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Contents
Page
Foreword vi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Part I -- Fundamentals of Sound as Related to Gears
1.1 Scope 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 References 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Symbols and definitions 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.4 What is sound? 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 Description of sound 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.6 Sound or noise? 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.7 Generation of sound in gear units 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.8 Sound transmission 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.9 Noise control 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Part II -- Sources, Specifications and Levels of Gear Sound
2.1 Gear sound sources 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Sound spectrum experience 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Specification and standards 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Gear system sound levels 20. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Part III -- Gear Noise Control
3.1 Source noise control 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Gear design noise control 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Gear housing noise control 29. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 Bearing noise control 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 Shaft and hub design noise control 31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6 Lubrication noise control 31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.7 Noise control with system analysis 31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.8 Noise of gear unit accessories 32. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.9 Noise control in the transmission path 32. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.10 Noise control materials 34. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.11 Total enclosures 35. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.12 Control summary 36. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figures
1--1 Sound wave forms 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1--2 Frequency responses 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1--3 Typical A--weighted sound levels 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1--4 Calculation for expected sound level 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-- 5 Chart for combining levels of uncorrelated noise signals 9. . . . . . . . . . . . . . . . .
2--1 Sound pressure level vs. frequency 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--2 Triple reduction gear motor frequency analysis 3600 rpm input, ratio --45 to 1 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--3 Gear noise analysis by constant--bandwidth, 10 Hz filter 15. . . . . . . . . . . . . . . .
2--4 Unfiltered sound measurement 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--5 Fast Fourier Transform analysis of sound 16. . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--6 Waterfall analysis of gear unit sound 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--7 Sound test microphone position 20. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--8 AGMA typical maximum and average sound pressure level vs. high speedmesh pitch line velocity 21. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--9 AGMA typical maximum and average sound pressure level vs. catalogpower rating 22. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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2--10 Sound pressure level vs. pitch line velocity taken 3 feet from housing 22. . . .
2--11 Change in dBA sound pressure level relative to that at 1750 rpm (! LPA)vs. input speed 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--12 Sound pressure level vs. worm speed 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--13 Change in dBA sound pressure level relative to that at no load (! LPA)vs. P / Pat 24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--14 Change in dBA sound pressure level relative to that at no load (! LPA)vs. P / PR 24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--15 Sound pressure level vs. center distance -- taken 5 feet from housing 25. . . .3--1 Contact of helical gears 28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3--2 Contact of spur gears 28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3--3 Variation of length of contact lines/face ratio with face width 29. . . . . . . . . . . . .
3--4 Tip relief on gear teeth 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3--5 Sound transmission paths for gear unit in typical installation 33. . . . . . . . . . . .
3 -- 6 Noise attenuating devices in gear unit surroundings 33. . . . . . . . . . . . . . . . . . .
3--7 Effect of noise attenuating devices in gear unit surroundings -- octaveband results 34. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3--8 Sound transmission paths for gear unit with vibration isolators andtotal enclosure 36. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tables
1--1 Symbols and definitions 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1--2 Center and approximate cut--off frequencies for standard set ofcontiguous--octave and one--third--octave bands covering audiofrequency range 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--1 Common sources of airborne and structure--borne sounds generated ingear drive systems 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--2 Occupational noise exposure -- OSHA Regulation (Standard 29 CFR) 18. . . .
2--3 ANSI noise specifications 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--4 International standards 19. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2--5 No twist steel rod mills “A” weighted sound levels 25. . . . . . . . . . . . . . . . . . . . . .
3--1 Considerations for noise control 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Foreword
[The foreword, footnotes and annexes, if any, in this document are provided for
informational purposes only and are not to be construed as a part of AGMA Information
Sheet 914--B04, Gear Sound Manual: Part I -- Fundamentals of Sound as Related to Gears;
Part II -- Sources, Specifications and Levels of Gear Sound; Part III -- Gear Noise Control.]
Concern with industrial noise created a need for a sound standard on all types of products.
Noise measurement, control and attenuation on gear driven equipment is dependent uponthe individual characteristics of the prime mover, gear unit, and driven machine -- as well as
their combined effects as a system in a particular acoustical environment.
Proper assessment of these considerations is essential for realistic determination of
acoustic values. The knowledge and judgment required to properly evaluate the various
factors comes primarily from years of accumulated experiencein designing, manufacturing,
and operating gear units. For this reason, the detailed treatment of the testing and resultant
conclusions for specific product applications is best accomplished by experts in the field.
The complexity makes most sound standards difficult to apply or interpret properly. The
AGMA Acoustical Technology Committee developed the Gear Sound Manual 299.01 to
provide improved communication between project engineers, gear manufacturer, and user
in the areas of Fundamentals of Sound as Related To Gears (Part I), Sources,Specifications and Levels of Gear Sound (Part II), and Gear Noise Control (Part III).
This Information Sheet was originally issued as three separate documents: AGMA 299.01,
Section I, Fundamentals of Sound as Related to Gears ; AGMA 299.01, Section II, Sources,
Specifications and Levels of Gear Sound ; and AGMA 299.01 Section III, Gear Noise
Contro l. Section I was approved by the membership in January 1978, Section II was
approved in October 1978, and Section III was approved in October 1978. Combining the
three entitled, AGMA SOUND MANUAL, was approved by the AGMA Technical Division
Executive Committee in October 1987.
The first draft of AGMA 914--B04 was made in November, 2002. It combines all three parts
into one document with three clauses, updates references, and adds a subclause on Fast
Fourier Transform analysis. It was approved by the AGMA membership in March, 2004.
Suggestions for improvement of this document will be welcome. They should be sent to the
American Gear Manufacturers Association, 500 Montgomery Street, Suite 350, Alexandria,
Virginia 22314.
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PERSONNEL of the AGMA Sound and Vibration Committee
Chairman: Darwin D. Behlke Twin Disc, Incorporated. . . . . . . . . . . . . . . . . . . . . . . . . .
Vice Chairman: Richard A. Schunck Falk Corporation. . . . . . . . . . . . . . . . . . .
ACTIVE MEMBERS
J.B. Amendola MAAG Gear AG. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
L. Lloyd Lufkin Industries, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .J.J. Luz General Electric Company. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
J.L. Radovich Davis--Standard. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
J.R. Sears General Motors Corporation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ASSOCIATE MEMBERS
E.J. Bodensieck Bodensieck Engineering Company. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D.L. Borden D.L. Borden, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F. Choy University of Akron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Coffey General Motors Corporation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D.R. Houser Ohio State University. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A.J. Lemanski Penn State University. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
J.V. Lisiecki Falk Corporation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
W.D. Mark Penn State University. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H. Minasian Stoneridge Control Devices, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G.W. Nagorny Nagorny & Associates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D. Palmer Pittsburgh Gear Company. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.I. Rivin Wayne State University. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D.C. Root Otis Elevator Company. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F.A. Thoma F.A. Thoma, Inc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. von Graefe MAAG Gear AG. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Ward Recovery Systems, LLC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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AGMA 914--B04 Part 1AMERICAN GEAR MANUFACTURERS ASSOCIATION
American Gear ManufacturersAssociation --
Gear Sound Manual:
Part I -- Fundamentals of
Sound as Related to
Gears
1.1 Scope
The purpose of this manual is to establish a common
base for communications pertaining to various types
of gear units in differing applications and to encour-
age the maximum practical degree to uniformity and
consistency between sound measurement practices
within the gear industry.
Because of the wide variation of gear driven systemsand acoustical environments, this manual attempts
to indicate certain areas where special consider-
ations might be necessary and must be agreed upon
between purchaser and gear manufacturer when
discussing gear sounds.
1.2 References
The following standards contain provisions which
are referenced in the text of this information sheet.
At the time of publication, the editions indicated were
valid.
AGMA 913--A98, Effect of Lubrication on Gear
Surface Distress
ANSI/AGMA 1012--F90, Gear Nomenclature,
Definitions Of Terms With Symbols
ANSI/AGMA 6025--D98, Sound for Enclosed Heli-
cal, Herringbone and Spiral Bevel Gear Drives
1.3 Symbols and definitions
The terms used, wherever applicable, conform to
ANSI/AGMA 1012--F90.
NOTE: The symbols and definitions used in this stan-
dard may differ from other AGMA standards. The user
should not assume that familiar symbols can be used
without a careful study of their definitions.
The symbols and terms, along with the clause
numbers where they are first discussed, are listed in
alphabetical order by symbol in table 1--1.
Table 1--1 -- Symbols and definitions
Symbol Definition Units First
referenced
ai Sound pressure level from a single source or octave dB Eq 1.5
f Frequency Hz Eq 1.1
Lp Sound pressure level dB 1.5.2.1
Lw Sound power level dB 1.5.2.2
N Number of single levels investigated ---- Eq 1.5
p Sound pressure being measured mN/m2 Eq 1.2
po Sound pressure, reference mN/m2 Eq 1.2
v Velocity -- -- Eq 1.1
W Sound power picowatt 1.5.2.2
W o Sound power reference picowatt 1.5.2.2
" Wavelength -- -- Eq 1.1
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1.4 What is sound?
Sound can be defined as the variations in pressure,
stress, or particle displacement of a medium, or the
sensation of hearing resulting from these variations.
These variations propagate through the medium to a
receiver (ear, pick--up, transducer, etc.). Therefore,
there are three elements of sound: source, transmit-ting medium and receiver.
When an object vibrates, a disturbance is caused in
the surrounding medium. This disturbance causes a
pressure oscillation, which travels through the
medium to the receiver, where it is transformed back
into a vibration. This receiver may either cause an
auditory sensation or excite some type of read--out
instrumentation.
Thetransmission of pressure variations is referred to
as a sound wave. A sound wave has the following
basic characteristics:
-- amplitude;
-- frequency;
-- velocity;
-- wavelength;
-- waveform.
1.4.1 Amplitude
Amplitude is the amount of variation in the pressure
reading of the medium, relative to a standard
reference pressure. Amplitude determines the ener-
gy level or strength of thesound, normally expressed
in terms of a decibel level.
1.4.2 Frequency
Frequency is the number of variations in the
amplitude per a given period of time, normally
expressed in Hertz (cycles per second).
1.4.3 Velocity
Velocity of the sound is the speed of the wave, and is
a function of the elastic modulus and the massdensity of the medium.
1.4.4 Wavelength
Wavelength is the distance between adjacent waves
of the same frequency. The relationship of frequen-
cy, velocity, and wavelength is expressed by:
"! v (1.1)
where
" is wavelength;
v is velocity;
f is frequency.
1.4.5 Waveform
Waveform defines the type of sound wave, i.e.,
whether the wave is simple (sinusoidal), complexdeterministic (periodic), or a complex random wave
consisting of multiple frequencies, harmonics, ran-
dom pulses, etc. See figure 1--1.
1.5 Description of sound
1.5.1 Description
Sound is commonly measured or described by one
or more of the following characteristics:
Level
-- sound pressure level;
-- sound power level.
Frequency content
-- A, B, and C weighing networks;
-- octave and 1/3 octave band filters;
-- narrow band filters.
Descriptive properties
-- sound intensity;
-- loudness;
-- pitch;
-- tone;
-- directivity.
1.5.2 Level
The level of sound is normally described in terms of
either sound pressure level at a given distance from
the source or sound power level. In each of these,
the desired quantity (pressure or power) is ex-
pressed in the numerator of a ratio with thereference
level as the denominator. Because of the extremelywide range of levels measured (very small to
extremely large) in everyday environments, both
pressure and power ratios are expressed by loga-
rithmic scales.
1.5.2.1 Sound pressure level, Lp
Sound pressure level, Lp, expressed in decibels, is
20 times the logarithm to the base 10 of the ratio of
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the sound pressure being measured to the reference
sound pressure.
Lp ! 20 log10 p po
, dB "re 20 mN#m2$ (1.2)
where
p is sound pressure being measured, mN/m2;
po is reference sound pressure, 20 mN/m2
.
The reference sound pressure, po, is internationally
accepted as 20 micro Newtons/meter squared,
which is about the threshold of normal hearing at a
frequency of 1000 Hz. All sound measuring instru-
ments respond to sound pressure.
Example: The sound pressure near a punch press is
measured as being 0.0025 psi. What is the sound
pressure re 20 mN/m2
in dB?
I
k--
I
Single frequencysinusoidal wave form
Example of complexwave form
Sinusoidal wave form “A”when combined with form “B”results in complex form A + B
P r e s s u r e
P r e s s u r e
P r e s s u r e
P
r e s s u r e
P r e s s u r e
P r e s s
u r e
Period(time)
Time
Time
Time
Form “A”
Form “B”
Form A + B
Amplitude
Amplitude
Time
Example of complex -- random wave
A + B + Random pulses
Frequency ! 1Period
Wavelength(distance)
Velocity !Wavelength
Period(speed of sound)
Wavelength ! Velocity % Period
Figure 1 --1 -- Sound wave forms
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Since 1.0 psi = 6890 N/m2, then 0.0025 psi = 17.225
N/m2.
! 118.7 dB "re 20 mN#m2$ (1.3)
Lp ! 20 log10"17.22N#m220 mN#m2 $! 20 log10"8.612% 105$
So we would commonly say the noise of the punch
press is 119 dB.
1.5.2.2 Sound power level, Lw
Sound power level, Lw, is the ratio, expressed in
decibels, of the sound power under consideration to
the reference sound power, one picowatt (10--12
watt).
Lw ! 10 log10W W o
, dB "re 10&12 watt$ (1.4)
where
W is sound power under consideration,
picowatt;
W o is reference sound power, picowatt.
Sound power cannot be measured directly. It can be
obtained only by calculation after having measured
sound pressure levels in a known acoustical environ-
ment (i.e., anechoic chambers, reverberant rooms,
etc.).
1.5.3 Frequency content
The frequency content of a sound is normally
described as a particular frequency or by the level
content in a band of frequencies.
1.5.3.1 A, B and C weighing networks
The frequency response of the human ear is not as
good as a sound level meter. Therefore, various
weighing networks (filters) have been established so
that the objective meter measurement will come
close to indicating what the ear hears. Figure 1--2
shows the attenuation of the A, B and C weighingscales of a sound level meter. The A scale is a
filtering system that roughly matches the human
ear’s response at sound levels below 55 dB. The B
scale roughly matches the ear at levels between 55
dB and 85 dB, and the C scale is to match above 85
dB. However, the A scale (sound pressure level
measured in dBA) has received prominence due to
its use in OSHA, for measuring levels up to 115 dB. It
is interesting to note the tremendous attenuation the
A scale performs on low frequencies. At about 95
Hz, for example, there is about a 20 dB attenuation.
Only 1/10 of the actual sound is indicated on the
meter. Therefore, gears generating low frequency
sound are more likely to pass a dBA specification,
and be less annoying to the ear. AGMA sound
standards use an A weighted sound level (dBA) as a
common indication of performance. See figure 1--3.
1.5.3.2 Octave and 1/3 octave band filter
Another filtering system often used in the measure-
ment of sound is the octave and 1/3 octave bands.
These arediscrete filters which only register a limited
range of frequencies. The octave and 1/3 octave
bands are used for analytical work and are usually
specified by their center frequencies. See table 1--2.
The 63 Hz octave band to the 8000 Hz octave band
are most commonly used in industry specifications.
1.5.3.3 Narrow band filters
A narrow band filter (spectrum analyzer) is similar to
octave band filters, however, the band filter is greatly
reduced in width to allow better resolution of
component frequencies in a noise spectrum. A
narrow band filter may have a bandwidth of only 2
Hz. Real time analyzers are a special form of narrow
band filter that enables the investigator to look at all
bands in an instant, instead of sweeping through
each band slowly.
1.5.4 Descriptive properties
The characteristics described are the ones which
must be investigated properly in order to obtain an
accurate description of a generated sound level and
to be able to prescribe proper corrective measures
for reduction of excessive levels.
1.5.4.1 Sound intensity
Sound intensity is the quotient, expressed in watts
per square meter, obtained when the average rate of
sound energy flowing in a specified direction is
divided by the area, perpendicular to that direction
toward which it flows.
1.5.4.2 Loudness
Loudness is the attribute of sound intensity which
depends primarily on the sound pressure. Loudness
is typically ranked on a scale ranging from soft to
loud. See figure 1--3.
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A, B, and C electrical weighting networks for the sound--level meterThese numbers assume a flat, diffuse--field response for the sound--level meter and microphone
Frequency,Hz
A--weightingrelative re-sponse, dB
B--weightingrelative re-sponse, dB
C--weightingrelative re-sponse, dB
Frequency,Hz
A--weightingrelative re-sponse, dB
B--weightingrelative re-sponse, dB
C--weightingrelative re-sponse, dB
10.0 --70.4 --38.2 --14.3 500 --3.2 --0.3 0.0
12.5 --63.4 --33.2 --11.2 630 --1.9 --0.1 0.0
16.0 --56.7 --28.5 --8.5 800 --0.8 0.0 0.0
20.0 --50.5 --24.2 --6.2 1 000 0.0 0.0 0.0
25.0 --44.7 --20.4 --4.4 1 250 0.6 0.0 0.0
31.5 --39.4 --17.1 --3.0 1 600 1.0 0.0 --0.1
40.0 --34.6 --14.2 --2.0 2 000 1.2 --0.1 --0.2
50.0 --30.2 --11.6 --1.3 2 500 1.3 --0.2 --0.3
63.0 --26.2 --9.3 --0.8 3 150 1.2 --0.4 --0.5
80.0 --22.5 --7.4 --0.5 4 000 1.0 --0.7 --0.8
100.0 --19.1 --5.6 --0.3 5 000 0.5 --1.2 --1.3
125.0 --16.1 --4.2 --0.2 6 300 --0.1 --1.9 --2.0
160.0 --13.4 --3.0 --0.1 8 000 --1.1 --2.9 --3.0
200.0 --10.9 --2.0 0.0 10 000 --2.5 --4.3 --4.4
250.0 --8.6 --1.3 0.0 12 500 --4.3 --6.1 --6.2
315.0 --6.6 --0.8 0.0 16 000 --6.6 --8.4 --8.5
400.0 --4.8 --0.5 0.0 20 000 --9.3 --11.1 --11.2
Frequency, Hz
R e l a t i v e r e s p o n s e ,
d e c i b e l s
Frequency responses forSLM weighting characteristics
Figure 1 --2 -- Frequency responses
1.5.4.3 Pitch
Pitch is the psychophysical attribute of sound
corresponding approximately to frequency by which
sounds may be ordered from low to high. Pitch
depends primarily upon the frequency of the sound,
but it also depends upon the sound pressure and
wave form.
1.5.4.4 Tone
Tone is an auditory sensation of pitch. There are two
types of tones: a pure tone and a complex tone. A
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pure tone is created by a simple sinusoidal function.
A complex tone is created by a combination of simple
sinusoidal functions. Most of the sound which is
investigated around gear units is a combination of
complex tones and random noise.
1.5.4.5 Directivity
Directivity describes the directionality of sound in a
field. Sound does not propagate equally in all
directions except in a textbook free field case. In
measuring sound pressure level, directionality must
be taken into consideration. A gear unit against a
wall radiates a higher level of sound in a given
direction away from the wall than an isolated unit
removed from reflecting surfaces.
Decibelsre 20 mN/m2
At a given distance from noise source Environment
Pain
Deafening
Very loud
Loud
Moderate
Faint
Very faint
Threshold of hearing,youths 1000--4000 Hz
Soft whisper (5’)
Large transformer (200’)
Freight train (100’)Vacuum cleaner (10’)
Speech (1’)
Pneumatic drill (50’)
Textile weaving plantSubway train (20’)
Cut--off saw
Pneumatic peen hammer
Riveting machine
Jet takeoff (200’)
50 HP siren (100’)
Studio for sound pictures
Studio (speech)
Minimum levels ----residential areas in
Chicago at night
Private business officeLight traffic (100’)Average residence
Tabulating room
Inside sport car (50 mph)
Boiler roomPrinting press plant
Electric furnace area
Casting shakeout area
Near freeway (auto traffic)Large storeAccounting office
130
0
10
20
30
40
50
60
70
80
90
100
110
120
140
Typical A --weighted sound levels increase
Increase in levels
"Decibels& 20 log p
po$ Increase in sound
pressure level1 dB 1.12 times
3 dB 1.41 times
6 dB 2.00 times
10 dB 3.16 times
12 dB 4.00 times
20 dB 10.00 times
40 dB 100.00 times
Figure 1 --3 -- Typical A--weighted sound levels
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Table 1 --2 -- Center and approximate cut --off frequencies for standard set of contiguous--octave and
one--third--octave bands covering audio frequency range
Frequency, Hz
Octave One--third octavean
Lower bandlimit
Center Upperband limit
Lower bandlimit
Center Upperband limit
12 11 16 22 14.1 16 17.8
13 17.8 20 22.4
14 22.4 25 28.215 22 31.5 44 28.2 31.5 35.5
16 35.5 40 44.7
17 44.7 50 56.2
18 44 63 88 56.2 63 70.8
19 70.8 80 89.1
20 89.1 100 112
21 88 125 177 112 125 141
22 141 160 178
23 178 200 224
24 177 250 355 224 250 282
25 282 315 355
26 355 400 447
27 355 500 710 447 500 56228 562 630 708
29 708 800 891
30 710 1 000 1 420 891 1 000 1 122
31 1 122 1 250 1 413
32 1 413 1 600 1 778
33 1 420 2 000 2 840 1 778 2 000 2 239
34 2 239 2 500 2 818
35 2 818 3 150 3 548
36 2 840 4 000 5 680 3 548 4 000 4 467
37 4 467 5 000 5 623
38 5 623 6 300 7 079
39 5 680 8 000 11 360 7 079 8 000 8 913
40 8 913 10 000 11 220
41 11 220 12 500 14 130
42 11 360 16 000 22 720 14 130 16 000 17 780
43 17 780 20 000 22 390
1.6 Sound or noise?
The differentiation between sound and noise can be
defined simply: sound is a variation in pressure;
noise is undesired sound. Noise also implies
undesired frequencies which tend to mask useful
information, causing possible misrepresentation of
actual sound characteristics. Examples of noisesextraneous to gear sound measurement are lubri-
cation pump noise, air--drill noise, 60 cycle hum,
instrumentation, electrical noise, etc.
Sound measurement and analysis are required to
determine what sound is typically generated and
what sound is undesired noise. This analysis is
accomplished by the use of a sound analyzer. A
sound analyzer is an instrument which displays
sound waves in the form of rms levels at various
frequencies or frequency bands. Using an analyzer
will help separate undesired frequencies from the
sound spectrum and contribute to an accurate
interpretation of sound data. The bandwidth of the
analyzer governs the amount of useful data dis-
played for analysis. The narrower the bandwidth, the
more discrete frequency information available, theeasier it becomes to identify extraneous noise
frequencies from the other generated sound in a
gear driven system.
In all possible cases, the elimination of unwanted
noise in the area under investigation should be
carried out before proper gear sound analysis is
initiated. This will make the engineer’s job of
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analyzing the data much easier and will enable him
to give better results.
1.7 Generation of sound in gear units
The sounds generated during gear unit operation
can be from one or more of the following majorsources:
-- gear dynamics;
-- bearing dynamics;
-- coupling noises;
-- system resonance or critical speeds;
-- accessories such as fans, lubrication sys-
tems, etc.
Sound generation in gears is related to design
tolerances and operation. The mating accuracy of agear set must be maintained, commensurate with
the desired operation. Gear sound is often gener-
ated by the mesh action of the teeth. Ifthe teeth have
irregularities in their profile or spacing, noise may be
generated at the frequency of the irregularities. One
must understand that a 100% accurate theoretical
tooth profile will still generate sound due to the
dynamics of gear mesh. Improper lubrication may
allow noise to be generatedin the mesh. The sounds
generated will often be at the mesh frequency (i.e.,
the frequency of rotation times the number of teeth
on the rotor), harmonics of mesh frequency, or atsideband frequencies (mesh frequency plus and
minus pinion or gear rotational frequencies).
Sound in ball and roller bearings can be generated
by the irregularities in the bearing elements, friction,
deflections under load, misalignments, loose cages
and races, windage, roller skewing and/or skidding,
etc. Misalignments and deflections under load are
the major causes of antifriction bearing noise.
Couplings may produce noise due to windage.
Exposed bolts, exposed holes and high velocity
surfaces can all add to the ability of a coupling togenerate noise due to windage. The windage shows
up as a rotational frequency and multiples of
rotational frequency, depending on the number of
exposed bolts and exposed holes.
System resonances and critical speed generate
sound in gear units. The structural resonant frequen-
cies of the casing and the baseplate can be excited
by internally generated frequencies (tooth mesh) to
produce noise. Care must be taken to determine the
natural frequencies of support structures to ensure
that the rotational frequency and other generated
frequencies are not coincident to, or a multiple of,
natural frequencies. Likewise, lateral and torsional
natural frequencies in the rotating system may be
excited to produce noise if they are too close to a
generated frequency or its harmonics.
Often, other equipment is required for proper
operation of a gear unit. Accessories such as
cooling fans and lubrication systems (pumps, mo-
tors, relief valves, etc.) can be sources of noise
which may appear to be generated by the gear units.
1.7.1 Overall sound level
All of these sources as well as extraneous noise from
the surrounding environment (background noise)
add up to the overall sound level in the area of the
gear unit. The interrelationship between them helps
to define the sound level. The overall level is
determined by the addition of different generatedlevels by the following expression:
Lp ! 10 log10 ' N
i!110"0.1a
i$ (1.5)
where
Lp is sound pressure level, dB;
ai is sound pressure level from a single source
or octave;
N is number of single levels investigated.
In an octave band analysis, N is the number ofoctaves.
1.7.2 Example 1
The installation in figure 1--4 shows a motor, parallel
shaft double increasing gear unit, and a compressor
in an industrial plant environment. The sound of
each piece of equipment was measured by its
manufacturer to have the listed sound levels at the
operator location shown. Totaling the levels by the
formula gives an expected level at the operator of 94
dBA. Actual measurement after installation indi-
cated 95 dBA at full load.
Therefore, a means of adding or subtracting sound
generated from different sources is also available.
Any school student will tell you that (82 + 88 = 89) is
an invalid equation. However, if we state that in the
same environment 82 dB + 88 dB = 89 dB we would
be correct. Figure 1--5 shows a chart which can be
used to assist in adding and subtracting sound
pressure levels in dB units of measure.
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Figure 1--4 -- Calculation for expected sound level
To add levels
Enter the chart with the numerical difference between two levels being added. Follow the line corre-sponding to this value to its intersection with the curved line, then left to read the numerical difference be-tween total and larger level. Add this value to the larger level to determine the total.
Example: Combine 75 dB and 80 dB. The difference is 5 dB. The 5 dB line intersects the curvedline at 1.2dB on the vertical scale. Thus, the total value is 80 + 1.2 or 81.2 dB.
To subtract levels
Enter the chart with the numerical difference between total and larger levels if this value is less than 3dB. Enter thechart with the numerical difference between total and smallerlevels if this value is between3 and 14 dB. Follow the line corresponding to this value to its intersection with the curved line, then eitherleft or down to read the numerical difference between total andlarger(smaller) levels. Subtract this val-ue from the total level to determine the unknown level.
Example: Subtract 81 dB from 90 dB. The difference is 9 dB. The 9 dB vertical line intersects the curvedline at 0.6 dB on the vertical scale. Thus, the unknown level is 90 -- 0.6 or 89.4 dB.
N u m e
r i c a l d i f f e r e n c e b e t w e e n
t o t a l a
n d l a r g e r l e v e l , d e c i b e l s
Numerical difference between totaland smaller levels, decibels
Figure 1--5 -- Chart for combining levels of uncorrelated noise signals
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1.7.3 Example 2
There are four gearmotors generating equal
amounts of sound energy (power). Together they
produce a level of94 dBA. To cut the sound by3 dBto
91 dBA, two (half) of the gearmotors would have to
beturnedoff. To be below the OSHA limit of 90dBA a
third (one--fourth original number) gearmotor would
have to be shut down, resulting in a level for onegearmotor of about 88 dBA.
Also, the lowering of the level of the major contributor
to a high noise level from a single gear unit will drop
the overall more significantly than lowering any other
level.
1.7.4 Example 3
There are four levels -- 70, 86, 78, 91 at different
frequencies. When added together the overall is
92.4 dB. Lowering the major contributor (91 dB) by 7
dB, lowers the overall to 88.6 dB -- a reduction of 3.8
dB. Lowering the second major contributor (86 dB)
by 7 dB, lowers the overall to 91.5 dB -- a reduction of
only 0.9 dB. This shows it is the major contributor
which must be reduced for effective noise control.
1.8 Sound transmission
There are two types of sound transmitted to the
receiver. These are structure--borne sound and
airborne sound. Structure--borne sound is sound
that reaches the receiver over at least part of its pathby vibrations of a solid structure. Airborne sound is a
sound that reaches the receiver by propagation
through the air.
An extremely important consideration when evaluat-
ing generated sound pressure levels of machinery is
that sound can be “structure--borne” for consider-
able distances without significant attenuation.
Structural steel beams may provide a path for
structure--borne sound (vibrations) to travel signifi-
cant distances and then radiate “airborne” sound
pressure levels at nearly the same level as thesource. A screwdriver often is used to transmit
structure--borne sound from the gear case to the ear.
Furthermore, structure--borne sound may excite
natural resonances of other equipment and struc-
tures, and thus create a sound pressure level louder
than the source under investigation. If the sound
levels of a gas turbine driven--gear compressor
system are being measured to determine the “gear
noise” one could ask the following questions:
-- What is the major noise source: turbine, gear
unit, compressor, piping or structure?-- How much of the noise is traveling through
the support structures and radiating at some point
other than its source?
-- Is the gearunitmesh frequency exciting a nat-
ural resonance in the sheet metal cover of the tur-
bine, or the piping, etc.?
-- Is a blade pass frequency exciting a natural
resonance of the bull web or the gear housing?
-- What are the sound levels at different loads or
speeds?
This list could be continued at great length; however,
one can see that there are many different influences
when trying to determine the sound level of a gear
unit in the middle of a power transmission system.
The exact same gear unit may generate completely
different sound levels in two different systems.
1.9 Noise control
When we discuss noise control (or noise reduction)
two approaches must be considered: either control-ling the source or controlling the transmission path.
Reducing the noise level at its source is accom-
plished by a change in design and/or manufacturing
(quality). Noise control in the transmission path
involves interrupting the transmission of the noise or
changing its direction. The method which is chosen
often depends on the economics involved. A
detailed approach on various methods of noise
control will be covered in Part III.
In order for industry to effectively combat the noise
problems of today and the requirements of thefuture, knowledgeable steps must be taken during all
phases of design, manufacture, assembly, test and
field installations of gear driven systems.
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AGMA 914--B04 Part IIAMERICAN GEAR MANUFACTURERS ASSOCIATION
American Gear ManufacturersAssociation --
Gear Sound Manual:Part II -- Sources,
Specifications and
Levels of Gear Sound
2.1 Gear sound sources
There are a multitude of factors, as discussed in Part
I, which can contribute to the sounds produced in a
gear driven system. The influence of each factor, its
contribution, and what methods can be employed to
economically control the sources of noise must be
analyzed to minimize the generated levels. There-
fore, it becomes important to separate the specific
sources, specifications and levels related to the gear
unit from others related to the drive system.
Most industrial gear driven system sounds can be
generally explained by one of the following relation-ships:
2.1.1 Harmonic frequencies
Those directly related to the frequency or harmonic
frequencies of a mechanical motion.
2.1.2 Resonant frequencies
Those related to the resonance frequencies and/or
critical speeds of the system, part of the system, or
its structure.
2.1.3 Complex frequencies
Complex source frequencies due to waveform
combinations, i.e., amplitude modulation, frequency
modulation, products, sums, differences of mechan-
ical motion or resonant frequencies.
2.1.4 Frequency origins
Generally, the primary sound frequencies generated
by a gear unit are predominantly described in 2.1.1
or 2.1.2. Occasionally, a complex problem may exist
where an analysis of frequencies listed in 2.1.3 is
necessary for a solution. Therefore, knowing the
major frequencies of mechanical motion or reso-
nants will supply the origins ofa majority of the soundsources generated by a gear unit.
2.1.5 Common frequencies
If the frequency components of the overall sound
generated by gear units are reviewed in general,
there will be many similarities. The most common
frequencies will be the rotational speeds, their
multiples, periodic motions (such as tooth mesh),
windage, critical speeds and natural resonances.
Table 2--1 defines some of the common sources of
airborne and structure--borne sounds generated in
gear driven systems.
2.1.6 System frequency range
It is interesting to note that the majority of common
sound frequencies mentioned above for moderate
and high speed industrial gear driven systems lie in
the 250 to 8000 Hz octave bands. These frequen-
cies, when related to mechanical motion, might be
used to detect sources related to antifriction
bearings, hydrodynamic bearings, looseness,
distortion, lube pump systems, etc.
2.1.7 Typical sound investigation
The following are typical investigations of gear
system sound generations.
2.1.7.1 Investigation 1--gear unit
A typical result of a gear unit sound investigation to
determine the major sources is shown in figure 2 --1.
Tabulated are the A, B and C weighted sound
pressure levels as measured with a sound level
meter. These results reveal no information as to the
major sound sources. Octave band results (curve 1)
are plotted versus frequency and reveal the two
major source frequencies to be approximately 250and 1000 Hz. The exciting frequencies for the gear
unit show the 1000 Hz peak to be associated with the
high speed mesh, and the 250 Hz peak to be
associated with either or both the low speed mesh or
high speed fan. The 1/3 octave band results (curve
2) further define the frequency spectrum pointing to
the high speed mesh as a major sound source, but
still not resolving whether either or both the low
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speed mesh or high speed fan are major sources.
Results of a 1/10 octave band analysis (curve 3)
clearly show the major sources to he the high speed
mesh (84 dB @ 1060 Hz) and low speed mesh (81
dB @ 285 Hz). This analysis indicates frequencies
directly related to mechanical motion of the shafts
and tooth meshes.
Table 2--1 -- Common sources of airborne and structure --borne sounds generated in gear drive
systems
Instruments that provide the operator with not only the amplitude of the vibration or noise, but, also thepredominant frequencies can be a tremendous aid in determining sources. These causes normally pres-ent themselves as follows:
1. Balance. Residual unbalance presents itself ata frequency equal to once per shaft revolution andit will increase in amplitude as speed is increased.
2. Alignment. Misalignment will present itself atonce or sometimes twice and three times per shaftrevolution. However, the amplitude will remain fair-ly constant with speed changes.
3. Friction. This is difficult to pinpoint by vibrationand noise frequency. Amplitude may be very highwhen continuous sliding occurs. It may also be ran-dom, high--amplitude, shock--type pulses, as in hy-drodynamic bearing rubbing. It may be irregular andoften violent.
4. Looseness. This may cause unbalance, mis-alignment and friction rubbing at moderate and highspeeds. At low speeds, it may display itself as anirregular rattle. Often it shows up at twice shaft
rotational speed.
5. Distortion. This is often an indirect cause ofvibration and noise, which also leads to unbalance,misalignment, or friction. It will tend to change inamplitude with load or operating temperatures,when speed is held constant.
6. Critical speeds. These occur through any givenspeed range and are points at which a rotating sys-tem likes to vibrate torsionally or laterally at a par-ticular frequency. Rotors characteristically showviolent increase in amplitude at particular critical
speeds, but are fairly stable above and below thesespeeds. A critical speed may change frequencywith load and temperatures.
7. Resonances. These also display themselves asfrequencies at which system members like to vi-brate. The distinction from critical speeds is thatresonances occur in other than rotating members,and affect alignment. Resonances occur at fixedfrequencies and change in amplitude with load,speed and temperature.
8. Tooth mesh, i.e., tooth contact. This will showup at tooth mesh frequency (i.e., rotating speedtimes number of teeth) and multiples of this meshfrequency.
9. Bearing instability. Bad antifriction bearings willcause high--frequency vibration at several timesrotational speed; also, friction vibration will occur.Hydrodynamic bearings, lightly loaded, will tend towhirl at 0.43 to 0.47 times the rotational speed.This so--called “half frequency whirl” will “on--set”violently with speed or temperature changes, and
may continue until the rotor is completely stopped.
10. System pulses. These may occur in manytypes of systems, such as the vane--pass frequencyof a pump or compressor (rotational speed timesthe number of vanes), and the beating of recipro-cating engines which cause frequencies at one--halfand one--quarter rotational speed at various ampli-tudes.
11. Windage. Couplings and other rotating partsgenerally create broadband noise, but can be at abolt pass frequency or fan blade pass frequency.
NOTE:
All of these types of vibrations and noise frequencies can be generated in a gear drive. Major frequencies can interactand cause frequency modulation and phase shifts. Any combination, sum, difference and multiple (harmonics) of theprime frequencies can occur if the forcing magnitude and system freedoms are such that they will cause and allow thegenerated vibration to become predominant. Generally, only the prime frequencies will present themselves as problemmodes. However, sometimes very elusivefrequencies appear, suchas periodic cutting machineerror appearing on oneof the gears.
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ExcitingSource frequency, Hz
HS Mesh 1060LS Mesh 285HS Fan 243HS Shaft 30.4Int. Shaft 20.4LS Shaft 4.84
S o u n d p r e s s u r e l e v e l , d B r e 2 0 m P a
Double reduction gear unit -- fan cooled, 4.8” LScenter distance, 6.26 total ratio, 1820 input rpm, fullload, microphone 5’ from unit side at height of HSS
Curve 1:octave bandresults
Curve 2:1/3 octave
band results
Curve 3:1/10 octaveband results
Frequency, Hz
Sound level meter results89 dBC, 88 dBB, 86 dBA
100
HS MESH
Figure 2--1 -- Sound pressure level vs. frequency
2.1.7.2 Investigation 2--gear motor
Similar analysis of a gear motor shown in figure 2--2
did not clearly indicate the major sources of noise
even after a 500 band real time analyzer was used.
The major frequencies were present at 565 Hz and
1,325 Hz as shown on curve 1, but the sources were
not apparent. Only after further investigation of the
system indicated by the structure--borne noise
curves 2 and 3, did the sources present themselves.
Resonance frequencies of the motor case and
support structure, excited by other frequencies in the
system, were responsible for the major frequencies.
If a narrow band filter had not been used, the major
sources of noise could have been mistaken for the
high speed gear mesh frequency (1,270 Hz) and two
times the intermediate speed gear mesh frequency
(512 Hz).
S o u n d
p r e s s u r e l e v e l , d B r e 2 0 m P a
Curve 1:Area noise 3 feetfrom gear case
Frequency, Hz
Resonant structure1325 Hz
Motor resonance565 Hz
1270Hz
512Hz
Intermediatemesh
256 Hz
Curve 2:Support structurestructure--borne noise
(acceleration)Curve 3:Motor case structure--bornenoise (acceleration, fan cover)
Figure 2 --2 -- Triple reduction gear motor frequency analysis
3600 rpm input, ratio -- 45 to 1
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2.1.7.3 Investigation 3--spectrum analysis
Analysis of a typical industrial sound spectrum
reveals much information about component heredity
and physical makeup. For example, for a set of
involute gears, the contact frequency (meshing rate
of the teeth) is f c = 1,038 Hz, the pinion rotates at
f p = 38.44 Hz, and the wheel rotates at f w = 27.33 Hz.
The pinion was cut using an indexing wheel with 69teeth. A 10--Hz--wide frequency analysis of directly
radiated gear sound is shown in figure 2--3.
2.1.7.3.1 Noise regions
Three major regions in this spectrum exist: one
centered around 885 Hz, another around 2,035 Hz,
and a third around 2,649 Hz. The first region is
centered around a frequency that is not the contact
frequency, as might be expected. Rather, the
maximum level at 885 Hz occurs at an amplitude
modulation sideband caused by some eccentricity in
the pinion during rotation.
2.1.7.3.2 Identifications
This identification can be made because the ampli-
tudemodulation process gives a set of sum--and--dif-
ference terms involving the frequencies in the
modulation process. If pinion eccentricity causes the
teeth to be driven into and away from the wheel
teeth, a load fluctuation results. Thus, amplitude of
tooth contact sound level (1,038 Hz) is increased
and decreased and an amplitude modulation pro-
cess occurs. In its simplest form, a 100 percent
modulation, the 1,038 Hz frequency disappears and
two amplitude modulated sidebands are generated
at ( f c + f p) and ( f c -- f p). In reality, the modulation
process is neither simple nor 100 percent. The
details of amplitude modulation are discussed in
many electronics textbooks. More complex modula-
tion processes allow extended sideband structures
about the primary frequency. In the involute gear
example, the fourth lower sideband is the largest;
that is, ( f c -- 4 f p) = 886 Hz.
2.1.7.3.3 Amplitude modulation
The amplitude modulation sidebands throughout the
whole analysis are dominated by the pinion rotation,
although wheel effects show up occasionally. These
sidebands indicate that there is an eccentric pinion in
the system, as explained above. It is important to
remember that a 10 Hz filter can discern frequencies
only within ( 5 Hz and, during dynamic scanning,within( 8 Hz. The frequency of a peak can then fallwithin ( 8 Hz of the actual value.
2.1.7.3.4 Problem aspects
Improvement to this particular gear sound level may
be achieved by improved concentricity of the pinion.
However, there are other aspects of the problem to
be understood.
2.1.7.3.5 Sidebands
Frequencies around 2,035 Hz are the sidebandsassociated with the second harmonic of tooth
contact frequency. However, 2 f c = 2,076 Hz is not
the predominant frequency. All the high level
sidebands are associated with the pinion, as can be
seen by the 38 Hz spacing. Again, this suggests
pinion eccentricities.
2.1.7.3.6 Ghost noise and index wheel errors
The last major frequency region around 2,649 Hz is
also amplitude modulated by pinion frequency. But,
first, it is important to know why the 69th harmonic
(69% 38.44 = 2,652 Hz) of the pinion rotary speed islarge when no other harmonic is significant. The
answer is that this frequency -- 2,649 Hz -- is not a
rotational speed harmonic. Instead, it is associated
with slight inaccuracies manufactured into the pin-
ion. During manufacture, erroneous table position-
ing relative to the gear cutter resulted in periodic
variations of pinion tooth geometry. In effect, the
cutting machine generated surface undulations
appearing as a ghost gear on top of the actual gear.
Minute errors generated in the pinion’s involute tooth
form corresponding to errors in the indexing wheel
constitute the ghost gear which has the same
number of teeth as does the manufacturing index
wheel. Thus: (number of teeth on indexing wheel)%(rotary speed of pinion) = [first ghost, ( f gi)].
Secondary and tertiary ghosts have also been
informally reported. These ghosts are generated by
a gear that has inaccuracies from machines that are
one and two generations removed from the machine
that manufactured the gear.
2.1.7.3.7 Sound level improvement
Therefore, the generated sound level of this gear setcould be improved by:
-- improving the pinion eccentricity;
-- correcting the machining errors (hone away
undulations or remachine on a different or
improved table).
NOTE: Discussions of noise control methods are
covered in Part III of the Gear Sound Manual .
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k 6 i 6
S o u n d p r e s s u r e l e v e l ( d B r e 2 0 m P a )
S o u n d p r e s s u r e l e v e l ( d B r e 2 0
m P a )
Figure 2--3 -- Gear noise analysis by constant--bandwidth, 10 Hz filter
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2.1.7.4 Fast Fourier Transform (FFT) and
waterfall analysis
There are many ways that a measurement of sound
level can be processed to give useful information
about a gear unit. A digital, Fast Fourier Transform
(FFT) analysis can be used to separate the discrete
frequencies. Figure 2--4 shows an unfiltered ampli-
tude measurement of sound for a short period of agear unit operating at 640 rpm. Very little information
can be ascertained until the measurement signal is
processed. An FFT analysis, as in figure 2--5, shows
the same measurement as a function of its discrete
frequencies in the spectrum from 0 to 400 Hz.
A “waterfall” plot, using FFT analysis, gives a picture
of a multitude of measurements at different operat-
ing speeds. Such an analysis, see figure 2--6, can
show the frequency components that change with
speed and those that do not. This can give anindication of the resonant frequencies and the
excitation frequencies as a function of operating
speed.
Time, msecs
A m p l i t u d e , v
o l t s
Figure 2 --4 -- Unfiltered sound measurement
Frequency, Hz
R
M S a m p l i t u d e , v o l t s
Figure 2 --5 -- Fast Fourier Transform analysis of sound
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Figure 2--6 -- Waterfall analysis of gear unit sound
2.2 Sound spectrum experience
Generally, experience indicates the sound spectrumof a gear unit will contain tooth meshing frequencies,
natural resonances, bearing noises, windage, and
sounds of auxiliary equipment--such as lubrication
systems. When a gear unit is installed, frequencies
related to the total system may be evident at the gear
unit, i.e., prime mover and driven equipment
frequencies, as well as system resonant frequencies
which will be measured in addition to gear
frequencies.
2.3 Specification and standards
Noise specifications are written by governments,
standards organizations, users, manufacturers and
trade associations.
2.3.1 Governmental specifications
The most significant governmental noise specifica-
tion has been the Occupational Safety and Health
Act (OSHA) Regulations (Standards -- 29 CFR,
Occupational noise exposure -- 1926.52). OSHA
placed limitations on the maximum sound level and
exposure times to which an employee may be
subjected at his working station without personal
protective equipment. Protection against the effects
of noise exposure shall be provided when the
A--weighted sound pressure level exceed those
shown in table 2--2.
When employees are subjected to sound levels
exceeding those in table 2--2, feasible administrative
or engineering controls shall be utilized. If such
controls fail to reduce sound levels within the levels
of the table, personal protective equipment shall be
provided and used to reduce sound levels within thelevels of the table.
If the variations in noise level involve maxima at
intervals of 1 second or more, it is to be considered
continuous.
In all cases where the sound levels exceed the
values shown , a continuing, effective hearing
conservation program shall be administered.
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Exposure to impulsive or impact noise should not
exceed 140 dB peak sound pressure level.
Table 2--2 -- Occupational noise exposure1)
OSHA Regulation (Standard 29 CFR)
Duration per day,hours
Sound leveldBA slow response
8 906 92
4 95
3 97
2 100
1.5 102
1 105
0.5 110
0.25 or less 115
NOTE:
1) Whenthe daily noise exposureis composed oftwoor more periods of noise exposure of different levels,
theircombined effect shouldbe considered, ratherthanthe individual effect of each. Exposure to different lev-els for various periods of time shall be computed ac-cording to the following formula:
F (e) !T (1)
L(1))
T (2)
L(2))***)
T (n )
L(n )
F (e) is equivalent noise exposure factor;
T is period of noise exposure at any essentiallyconstant level;
L is duration ofthe permissible noise exposureat theconstant level.
Example: A sample computation showing an applica-tion of the above formulais as follows. An employee isexposedat the following levelsfor the followingperiods:
110 dBA for 0.25 hour100 dBA for 0.5 hour
90 dBA for 2 hours
! 1.000
F (e) ! 0.250.50
) 0.52 ) 2
8
! 0.5) 0.25) 0.25
Sincethe valueof F (e) doesnot exceed unity, theexpo-sure is within permissible limits.
2.3.2 Standards organizations
Standard organizations, both national and interna-
tional, publish standards related to noise terminolo-
gy, instrumentation, testing and analysis. Some
noise specifications, shown in tables 2--3 and 2--4,
are used in writing of user, manufacturer and trade
association noise specifications.
2.3.3 User specifications
User noise specifications include measurement
techniques and required sound levels or octave
band sound pressure levels to be met by equipment
to be purchased. Formalized user noise specifica-
tions are becoming more frequent, and it is the
purpose of this clause to aid in developing effective
user gear unit noise specifications.
Table 2--3 -- ANSI noise specifications
S1.1 –1994 (R1999)* Acoustical Terminology
ANSI S1.4--1983(R2001)
Specification for Sound Level Meters
S1.11--2004 Octave--Band and Frac- tional--Octave--Band Analog and Digital Fil- ters
S1.13--1995 (R1999)* Measurement of Sound
Pressure Levels in Air S3.4--1980(R2003)* Procedure for the Com-
putation of Loudness of Noise
NOTE:
* Reaffirmed
2.3.4 Manufacturer specifications
Manufacturer noise specifications are written to
describe the noise performance of manufactured
products. However, rather than a single manufactur-
er issuing a noise specification, more commonly,
manufacturers’ groups or trade associations issue
noise specifications covering a particular type of
product.
2.3.5 Trade associations
Trade associations involved with electric motors,
hydraulic pumps and motors, machine tools, pneu-
matic equipment, gear units, etc., have published
noise specifications. Of major concern to the users
of gear units is the gear unit sound standard,
ANSI/AGMA 6025--D98.
2.3.6 ANSI/AGMA 6025--D98, sound standard
The overall purpose of the AGMA sound standard is
to improve communication and understanding be-
tween the gear unit manufacturer and purchaser.
ANSI/AGMA 6025--D98 utilizes ANSI standards
where applicable. Clauses 2.3.6.1 through 2.3.6.4
provide an overview of the sound standard.
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Table 2--4 -- International standards
ISO 3743--1:1994 Acoustics – Determination of sound power levels of noise sources – Engineering methods for small, movable sources in reverberant fields – Part 1: Comparison method for hard--wall test rooms
ISO 3744:1994 Acoustics – Determination of sound power levels of noise sources using sound pressure -- Engineering method in an essentially free--field over a reflecting plate
ISO 3745:2003 Acoustics – Determination of sound power levels of noise sources using sound pressure – Precision methods for anechoic and semi --anechoic rooms
ISO 3746:1995 Acoustics – Determination of sound power levels of noise sources using sound pressure – Survey method using an enveloping measurement surface over a reflecting plane
ISO 4871:1996 Acoustics – Declaration and verification of noise emission values of machinery and equipment
ISO/TR 7849:1987 Acoustics – Estimation of airborne noise emitted by machinery using vibration measurements
ISO 8579--1: 2002 Acceptance code for gears -- Part 1: Determination of airborne sound power levels emitted by gear units
ISO 9614--1:1993 Acoustics – Determination of sound power levels of noise sources using sound intensity – Part 1: Measurements at discrete points
ISO 9614--2:1996 Acoustics – Determination of sound power levels of noise sources using sound
intensity – Part 2: Measurements by scanning ISO 11203:1995 Noise emitted by machinery and equipment -- Determination of sound pressure
levels at a work station and at other specified positions from the sound power level
IEC 61260:1995 Electroacoustics -- Octave--band and fractional--octave--band filter
IEC 61672:2002 Electroacoustics -- Sound level meters -- Part 1: Specifications
2.3.6.1 Standards--scope and limitations
The AGMA sound standard is limited to those units
designed and rated in accordance with applicable
AGMA product standards. Also, gear units are to be
lubricated in accordance with manufacturer’s rec-
ommendations and operated in a system free fromserious critical speeds, torsional vibrations and
overloads. Compliance with the conditions of
ANSI/AGMA 6025--D98 does not imply a warranty of
gear unit sound levels under installed field service
conditions, because particular operations and envi-
ronments must be considered in view of subjects
covered in this information sheet.
2.3.6.2 Standard instrumentation
The standard specifies that sound levels are to be
measured with a sound level meter, Type 1 (preci-sion) or Type 2 (general purpose), conforming to
ANSI specifications. Also, octave band sound
pressure levels, when agreed upon, are to be
measured with an octave band analyzer conforming
to ANSI specifications. Instrument acoustic calibra-
tion is to be checked before and after each test, and
slow meter response is preferred when taking sound
measurements.
2.3.6.3 Standard procedure
The gear unit may be rigidly or resiliently mounted in
its normal operating position and either belt or
coupling connected to the driver. The gear unit is to
be run at no load or with a light brake load, and at the
application speed. A light brake load, stabilizes the
rotating elements -- that is, it eliminates the effects of
gear mesh backlash and bearing clearances on the
gear unit sound. The acoustic environment is to be
that of shop testing locations, which are typically
semi--reverberant, and the test machinery may be
acoustically isolated from the gear unit.
2.3.6.4 Microphone position and ambient
correction
The microphone is to be located perpendicular to the
center of a vertical surface, but not less than one foot
above the test floor or plate, see figure 2--7. The
distance between the microphone and gear unitdepends on the unit size. See table 1 in ANSI/AGMA
6025--D98. Both the overall sound level (gear unit
plus ambient) and the ambient level alone are to be
recorded. Corrections for the influence of the
ambient on the gear unit’s sound level are made to
provide a truer indication of the gear unit’s sound
level. The average meter reading is to be recorded
when the sound pressure level fluctuates.
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2.4 Gear system sound levels
2.4.1 Typical levels
The sources of generated sounds in a gear driven
systemare important. However, the levels which are
generated and the methods of measurement be-
come the points of major interest when determining if
a system will meet a specification. The gear industry
has had years of experience measuring sound, both
on the test stand and in field installations. This
experience has indicated the sound levels that may
be expected on qualification spin or load tests. This
sound level can be obtained from test results of
identical or comparable units and/or empirical data
extrapolated from similar equipment. The levels
generally will not include driving or driven equipment
noise and system influences. When a gear unit is
actually installed, the prediction or estimation of its
sound level is difficult, since the gear unit is now part
of a total acoustic system which includes, in addition
to the gear unit, the prime mover, driven equipment,
gear unit mounting and surrounding acoustic envi-
ronment. Some insight into this problem can be
gained by examining the effect of some system
parameters, such as speed and load.
Key:L = Length of gear unitH = Height of gear unit
W = Width of gear unitD = Distance of microphone perpendicular of unit
as specified in standard for sizeh = Height of microphone perpendicular to floor
d = Distance of microphone from corner of unit
*Note: Load is optional for factory testing
"H2$
"L2$ or "W
2$
Figure 2--7 -- Sound test microphone position
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2.4.2 AGMA typical maximum data
The latest AGMA standard includes typical maxi-
mum sound levels for their representative types of
gear units, see figures 2--8 thru 2--15, for information
only. The typical maximum curves of figures 2--8
through 2--10 were established based on the mea-
sured sound levels of gear units given by the points
in the figures. The curves for gearmotors in figure2--9 were obtained by adding estimated electric
motor sound levels (using motor sound power levels
published by the National Electrical Manufacturers
Association, NEMA) to gear unit sound levels. The
typical maximum levels given in the AGMA stan-
dards are for the test condition stated in these
standards.
2.4.2.1 Speed effects
Change in speed can significantly effect the sound
pressure level. Typical data for the effect of speed
(input rpm) on sound levels is presented for informa-
tion in figures 2--11 and 2--12.
2.4.2.2 Load effects
Another operating parameter affecting the level of
gear unit sound is load. Much of the experimental
literature to date indicates an increase in noise due
to an increase in load, see figure 2--13. Some data
has indicated as much as a 20 dB increase for spur
gearing between a load and no load (spin) test.
However, empirical data collected throughout the
gear industry indicates increased noise level does
not always accompany increased loading; in some
cases, even the reverse occurs, i.e., when the tooth
geometry has been modified for loaded deflections
and operating temperatures. Until these design
loads and temperatures have been reached, the
mesh action may be noisy.
The average statistical difference in gear unit sound
between no load spin and full load (AGMA rated load
+ service factor) is an increase of approximately 4
dBA (see figure 2--14 -- helical, herringbone, spiral
bevel and worm gearing).
The maximum increase in sound between no loadand full load observed was 12 dBA. The data
showed about two--thirds of the units increased in
sound with load. About one--fifth did not indicate a
measured difference with load, and the remainder
had reduced sound levels with increased load.
I
I I
I
C A
S o u n d p r e s s u r e l e v e l , d B
A
High speed mesh pitchline velocity, fpm
Enclosed helical, herringbone and spiral bevel gear drivesSingle, double and triple reduction
No load or light brake loadNo cooling fan
ANSI/AGMA 6025--D98
Figure 2--8 -- AGMA typical maximum and average sound pressure level vs. high speed mesh pitch
line velocity
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Therefore, for gears of this type a majority showed
an average increase of 4 dBA. Only in similar gear
drive systems may this be used as a guideline for the
expected difference between a shop spin test and
field installed loaded operation. Table 2--5 shows
additional data on the operation of geared systems
loaded and unloaded.
Catalog power rating, HP
S o u n d p r e s s u r e l e v e l , d B A
110
Gearmotors, in--line reducers and increasersSingle, double, triple and quad reduction
No load or light brake loadNo cooling fan on gear unit
100
ANSI/AGMA 6025--D98
Figure 2--9 -- AGMA typical maximum and average sound pressure level vs. catalog power rating
High speed mesh pitch line velocity, fpm
S o
u n d p r e s s u r e l e v e l , d B A
High speed helical and double helical, single reductiongear units at full speed, light load or spin test
ANSI/AGMA 6025--D98
Figure 2--10 -- Sound pressure level vs. pitch line velocity taken 3 feet from housing (values shown
are for information only)
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S o u n d p r e s s u r e l e v e l , d B
A
Input speed, rpm
Figure 2--11 -- Change in dBA sound pressure level relative to that at 1750 rpm
(! LPA) vs. input speed
S o u n d
p r e s s u r e l e v e l , d B A
Input speed, rpm
Figure 2--12 -- Sound pressure level vs. worm speed
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S o u n d p r e s s u r e l e v e l , d B A
Power ratio
Figure 2--13 -- Change in dBA sound pressure level relative to that at no load (! LPA) vs. P/ Pat
S o u n d p r e s s u r e l e v e l , d B
Power ratio0 .2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0
Figure 2--14 -- Change in dBA sound pressure level relative to that at no load (! LPA) vs. P/ PR
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S o u n d p r e s s u r e l e v
e l , d B A
Unit center distance, inches
Worm gear speed reducers -- single reductionlight or no load, 1750 rpm input, fan cooled
Figure 2--15 -- Sound pressure level vs. center distance -- taken 5 feet from housing
Table 2--5 -- No twist steel rod mills “A” weighted sound levels
(with and without rod in mill)
Basic data of sample rod mills
Mill horsepower Gear speed (rpm)
Mill Load w/rod No load w/o rod min. max.
A 2200 150 950 6600
B 1100 100 570 4000
NOTE:
There are approximately 35 gear meshes in each mill. The meshes transmit anywhere from 100% of horsepower at thelowest RPM to a small fraction of total horsepower. The primaryincrease of soundlevel with rod load appears to be fromthe gear meshes.
Observed sound level test data at sample rod mills
Mill LocationDistance,
ftw/rod
dB “A”w/o roddB “A”
Diff.dB “A”
BackgrounddB “A”
A Between strands 1 & 2a) Near motor endb) Midway along mill
55
9895
9593
32
9090
A Between strands 3 & 4a) Near motor end 5 94 93 1 90
B Between strands 1 & 2a) At 3 high gear incrementsb) Midway along millc) At high speed end
333
939391
908989
342
818181
B Between strands 3 & 4a) At 3 high gear incrementsb) Mid