AGMA 914-B04

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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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ii

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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AGMA 914--B04AMERICAN GEAR MANUFACTURERS ASSOCIATION

iii© AGMA 2004 ---- All rights reserved

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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iv © AGMA 2004 ---- All rights reserved

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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v© AGMA 2004 ---- All rights reserved

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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vi © AGMA 2004 ---- All rights reserved

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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1© AGMA 2004 ---- All rights reserved

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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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


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

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