AS1081.1 Acoustics—Measurement of Airborne Noise Emitted by Rotating Electrical Machinery

8/11/2019 AS1081.1 AcousticsMeasurement of Airborne Noise Emitted by Rotating Electrical Machinery

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AS 1081.11990ISO 1680/1: 1986

Australian Standard

AcousticsMeasurement ofairborne noise emitted by rotatingelectrical machinery

Part 1: Engineering method forfreefield conditions over areflective plane

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AS 1081.11990

Australian Standard

AcousticsMeasurement ofairborne noise emitted by rotatingelectrical machinery

Part 1: Engineering method forfreefield conditions over areflective plane

First published in part as AS 10811975.Revised and redesignated AS 1081.11990.

PUBLISHED BY STANDARDS AUSTRALIA(STANDARDS ASSOCIATION OF AUSTRALIA)1 THE CRESCENT, HOMEBUSH, NSW 2140

ISBN 0 7262 6186 4A c c e s s edb yDOWNEREDILIMITED o n21N o v2011


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PREFACE

This Standard was prepared by the Standards Australia Committee on AcousticsMachinery Noise tosupersede, in part, AS 10811975, Measurement of airborne noise emitted by rotating electrical machinery . It is identical with and has been reproduced from ISO 1680/11986, AcousticsTest code for the measurement of airborne noise emitted by rotating electrical machinery Part 1: Engineering method for freefield conditions over a reflective plane .

This Standard is one of the series which deals with rotating electrical machinery noise, the seriesbeing arranged as follows:

Part 1: Engineering method for freefield conditions over a reflective plane (this Standard).

Part 2: Survey method.

For the purpose of this Australian Standard, the ISO text should be modified as follows:

(a) Clause 3 : The terms which have their equivalents in AS 1633, AcousticsGlossary of terms and related symbols should follow the definitions of the Australian Standard. The terms which do not haveequivalents in the AS 1633 should be as in this Standard.

(b) References : The reference to other publications should be modified as follows:

ISO AS266 AcousticsPreferred frequencies

for measurements2533 AcousticsPreferred frequencies for

measurements354 AcousticsMeasurement of sound

absorption in a reverberation room1045 AcousticsMeasurement of sound

absorption in a reverberation room1680/2 AcousticsTest code for the

measurement of airborne noiseemitted by rotating electricalmachineryPart 2: Survey method

1081.2 AcousticsMeasurement of airbornenoise emitted by rotating electricalmachinery Part 2: Survey method

3740 AcousticsDetermination of soundpower levels of noisesourcesGuidelines for the use of

basic standards and for thepreparation of noise test codes

1271.1 Determination of sound power levels ofnoise sources,Part 1: Guidelines for the use of basic

standards for the preparation of noisetest codes

3741 AcousticsDetermination of soundpower levels of noisesourcesPrecision methods forbroadband sources inreverberation rooms

1217.2 AcousticsDetermination of soundpower levels of noise sources,Part 2: Precision methods for broad band sources in reverberation rooms

3742 AcousticsDetermination of soundpower levels of noisesourcesPrecision methods fordiscretefrequency andnarrowband sources inreverberation room

1217.3 AcousticsDetermination of soundpower levels of noise sources,P a r t 3 : P r e c i s i o n m e t h o d s f o rdiscretefrequency and narrowbandsources in reverberation room

3744 AcousticsDetermination of soundpower levels of noisesourcesEngineering methods forfreefield conditions over areflecting plane

1217.5 AcousticsDetermination of soundpower levels of noise sources,Part 5 : Engineering methods forfreefield conditions over a reflectingplane

3745 AcousticsDetermination of soundpower levels of noisesourcesPrecision methods foranechoic and semianechoicrooms

1217.6 AcousticsDetermination of soundpower levels of noise sources,Part 6: Precision methods for anechoicand semianechoic rooms

IEC34/1 Rotating electrical machines

Part 1: Rating and performance1359 R o ta t in g e l ec t r ica l m ach in es

General requirements

225 Octave, halfoctave andthirdoctave band filters intendedfor the analysis of sounds andvibrations

Z41 Octave, halfoctave and onethirdoctave band pass filters intended foranalysis of sound and vibrations

651 Sound level meters 1259 Sound level metersA c c e s s edb yDOWNEREDILIMITED o n21N o v2011


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CONTENTS

Page 0 INTRODUCTION 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 SCOPE 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 REFERENCES 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 DEFINITIONS 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4 ACOUSTIC ENVIRONMENT 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 INSTRUMENTATION 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 INSTALLATION AND OPERA TION OF THE MACHINE 7. . . . . . . . . . . . .7 SOUND PRESSURE LEVELS ON THE MEASUREMENT

SURFACE 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 CALCULATION OF SURF ACE SOUND PRESSURE LEVEL AND

SOUND POWER LEVEL 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 INFORMATION T O BE RECORDED 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10 INFORMATION T O BE REPOR TED 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ANNEXES

A QUALIFICATION PROCEDURES FOR THE ACOUSTICENVIRONMENT 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

B COMPUTATIONAL PROCEDURES FOR CALCULA TINGAWEIGHTED SOUND POWER LEVEL FROM OCTA VE ORONETHIRD OCTA VE BAND POWER LEVELS 16. . . . . . . . . . . . . . . . . . . . .

C METHOD FOR ESTIMATING THE DIFFERENCE IN NOISE LEVELBETWEEN NOLOAD AND ONLOAD OPERA TIONS 17. . . . . . . . . . . . . .

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AUSTRALIAN STANDARD 4

AcousticsMeasurement of airborne noise emittedby rotating electrical machineryPart 1: Engineering method for free-field conditionsover a reflecting plane

0 Introduction

This part of ISO 1680 is based on ISO 3744 and hasbeen drafted in accordance with ISO 3740.

The main purpose of this part of ISO 1680 is to specifya clearly defined measurement method for rotating

electrical machines operating under steady-stateconditions, the results of which can be expressed insound power levels so that all machines tested using thiscode can be directly compared. Other methods, such asthe precision methods of ISO 3741, 3742 and 3745, mayalso be used for determining sound power levels if theinstallation and operating conditions of this part ofISO 1680 are used.

1 Scope and field of application

1.1 General

This part of ISO 1680 specifies, in accordance withISO 2204, an engineering method (grade 2) formeasuring the sound pressure levels on a rectangularparallelepiped surface enveloping the machine and forcalculating the sound power level produced by themachine. It outlines the procedures which may be usedto evaluate the test environment and specifies thecharacteristics of suitable measuring instruments. Amethod is given for determining the A-weighted soundpower level and, if required, octave or one-third octaveband sound power levels of the machine from the meanof the sound pressure levels measured on therectangular parallelepiped surface.

This part of ISO 1680 applies to the measurement ofairborne noise from rotating electrical machines, such asmotors and generators (d.c. and a.c. machines) withoutany limitation on the output or voltage, when fitted withtheir normal auxiliaries. It applies to rotating e lectricalmachines with any linear dimension (length, width orheight) not exceeding 15 m.

This part of ISO 1680 applies to measurements carriedout in environmental conditions that meet the criteriagiven in clause 4 and annex A (environmental correctionK 2 dB, correction for background noise 1 dB). Ifthese criteria are not met, standard deviations of the testresults may be greater than those given in table 1, i.e.the engineering grade of accuracy may not be achieved.

The method given in ISO 1680/2 shall then be used,which will result in A-weighted sound power levels of

lower accuracy. In this case, no reference shall be madeto this part of ISO 1680.

1.2 Measurement uncertainty

Measurements carried out in conformity with this part ofISO 1680 usually result in standard deviations which are

equal to or less than those given in table 1. The standarddeviations given in table 1 reflect the cumulative effectsof all causes of measurement uncertainty, excludingvariations in the sound power level of the machine fromtest to test. For a machine which emits noise with arelatively flat spectrum in the 100 to 10 000 Hzfrequency range, the A-weighted sound power level isdetermined with a standard deviation of approximately2 dB. For outdoor measurements, the standard deviationin the octave band centred on 63 Hz will beapproximately 5 dB.

NOTE The standard deviations in table 1 include the effectsof allowable variations in the positioning of the measurementpositions and in the selection of the stipulated measurement

surface.

Table 1 Uncertainty in determining sound power levelsfor engineering measurements indoors or outdoors

Octave bandcentre

frequencies

One-thirdoctave band

centrefrequencies

Standarddeviationof mean

value

Hz Hz dB

125250 to 500

1 000 to 4 0008 000

100 to 160200 to 630

800 to 5 0006 300 to 10 000

3,02,0

1,52,5

2 References

ISO 266, Acoustics Preferred frequencies for measurements .

ISO 354, Acoustics Measurement of sound absorption in a reverberation room .

ISO 1680/2, Acoustics Test code for the measurement of airborne norse emitted by rotating electrical machinery Part 2: Survey method .

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ISO 2204, Acoustics Guide to International Standards on the measurement of airborne acoustical noise and evaluation of its effects on human beings .

ISO 3740, Acoustics Determination of sound power levels of noise sources Guidelines for the use of basic

standards and for the preparation of noise test codes .ISO 3741, Acoustics Determination of sound power levels of noise sources Precision methods for broad-band sources in reverberation rooms .

ISO 3742, Acoustics Determination of sound power levels of noise sources Precision methods for discrete- frequency and narrow-band sources in reverberation rooms .

ISO 3744, Acoustics Determination of sound power levels of noise sourcesEngineering methods for freefield conditions over a reflecting plane .

ISO 3745, Acoustics Determination of sound power levels ofnoise sourcesPrecision methods for anechoic and semi-anechoic rooms .

ISO 6926, Acoustics Determination of sound power levels of noise sourcesCharacterization and calibration of reference sound sources .1)

IEC Publication 34-1, Rotating electrical machines Part 1: Rating and performance .

IEC Publication 225, Octave, half-octave and third-octave band filters intended for the analysis of sounds and vibrations .

IEC Publication 651, Sound level meters .3 Definitions

For the purposes of this part of ISO 1680, the followingdefinitions apply.

3.1 free field: A sound field in a homogeneous,isotropic medium free of boundaries. In practice, it is afield in which the effects of the boundaries are negligibleover the frequency range of interest.

3.2 free field over a reflecting plane: A sound field inthe presence of a reflecting plane on which the source islocated.

3.3 anechoic room: A test room the surfaces of whichabsorb essentially all the incident sound energy over thefrequency range of interest, thereby affording free-fieldconditions over the measurement surface.

3.4 semi-anechoic room: A test room with a hardreflecting floor the other surfaces of which absorbessentially all the incident sound energy over thefrequency range of interest, thereby affording free-fieldconditions above a reflecting plane.

3.5 sound pressure level, Lp , in decibels: Twenty timesthe logarithm to the base 10 of the ratio of the sound

pressure to the reference sound pressure. The weightingnetwork or the width of the frequency band and its centrefrequency used shall be indicated: for example,A-weighted sound pressure level, Lp A, octave band soundpressure l evel, one-third octave band sound pressurelevel, etc. The reference sound pressure is 20 Pa.

3.6 surface sound pressure: The sound pressureaveraged in time on a mean-square basis and alsoaveraged over the measurement surface using theaveraging procedures specified in 8.1 and corrected forthe effects of background noise and the influence ofreflected sound at the measurement surface.

3.7 surface sound pressure level, , in decibels:Ten times the logarithm to the base 10 of the ratio of thesquare of the surface sound pressure to the square ofthe reference sound pressure.

3.8 sound power level, LW , in decibels: Ten times thelogarithm to the base 10 of the ratio of a given soundpower to the reference sound power. The weightingnetwork or the width of the frequency band used shall beindicated: for example, A-weighted sound power level,LW A, octave band sound power level, one-third octaveband sound power level, etc. The reference sound poweris 1 pW ( = 10 -12 W).

NOTE The surface sound pressure level is numericallydifferent from the sound power level and its use in lieu of thesound power level is not correct because the size of t hemeasurement surface is not covered by this quantity.

3.9 frequency range of interest: For general purposes,the frequency range of interest includes the octavebands with centre frequencies between 125 and 8 000Hz or the one-third octave bands with centre frequenciesbetween 100 and 10 000 Hz. Any band may be excludedin which the level is more than 40 dB below the highestband pressure level. For special purposes, the frequencyrange of interest may be extended at either end,provided that the test environment and instrumentaccuracy are satisfactory for use over the extendedfrequency range. For sources which radiatepredominantly high (or low) frequency sound, thefrequency range of interest m ay be limited in order tooptimize the test facility and procedures.

3.10 measurement surface: A hypothetical surface ofarea S enveloping the source on which the measurementpositions are located and which terminates on thereflecting plane.

3.11 reference box: A hypothetical surface which is thesmallest rectangular parallelepiped that just encloses thesource and terminates on the reflecting plane.

3.12 measurement distance: The minimum distancefrom the reference box to the measurement surface.

3.13 background noise: The sound pressure level ateach microphone position with the source inoperative.

1) At present at the stage of draft.

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4 Acoustic environment

4.1 General

The test environments that are suitable formeasurements in accordance with this part of ISO 1680include the following:

a) a room which provides a free field over a reflectingplane;

b) a flat outdoor area that meets the requirements of4.2 and annex A;

c) a room in which the contributions of the reverberantfield to the sound pressures on the measurement surfaceare small compared with those of the direct field of thesource.

Conditions described under c) above are met in verylarge rooms as well as in smaller rooms with sufficientsound-absorptive materials on their walls and ceilings.

4.2 Criteria for adequacy of the test environment

Annex A describes a procedure for determining whetheror not a test environment is adequate for measurementsin accordance with this part of ISO 1680. Testenvironments which are suitable for engineeringmeasurements permit the sound power level to bedetermined with an uncertainty that does not exceed thevalues given in table 1.

Ideally, the test environments are free from reflectingobjects other than a reflecting plane so that the sourceradiates into a free field over a reflecting plane. Annex Adescribes procedures for determining the magnitude ofthe environmental correction (if any) to account fordepartures of the test environment from the idealcondition.

To comply with this part of ISO 1680, the environmentalcorrection K shall not exceed 2 dB.

If it is necessary to make measurements in spaces whichdo not meet the criteria of annex A, standard deviationsof the test results may be greater than those given intable 1. In those cases, ISO 1680/2 shall be used. (Seeclause 0.)

4.3 Criterion for background noise

At each microphone position, the sound pressure level ofthe background noise shall be at least 6 dB, andpreferably more than 10 dB, below the sound pressurelevel to be measured in each frequency band within thefrequency range of interest.

Background noise less than 6 dB below the soundpressure levels to be measured is too high for thepurposes of this part of ISO 1680. Under suchcircumstances, the survey method of ISO 1680/2 shallbe used. (See clause 0.)

4.4 Wind

The wind velocity existing at the test site or caused bythe machine under test shall be less than 6 m/s. Awindscreen should be used for wind velocities above1 m/s to ensure that the level of the background noise(caused by the cumulative effect of the wind and otherbackground noise sources) is at least 6 dB, andpreferably more than 10 dB, below the level with thesource operating. The appropriate instructions providedby the microphone manufacturer shall be followed.

5 Instrumentation

5.1 General

The instrumentation shall be designed to measure themean-square value of the A-weighted sound pressure

level and the octave or one-third octave band levels,averaged over time and over the measurement surface.Surface averaging is usually carried out by measuringthe time-averaged sound pressure levels with aprescribed time constant for a fixed number ofmicrophone positions (7.2) and computing the averagevalue in accordance with 8.2.

The instrumentation used can perform the required time-averaging in one of two different ways:

a) By continuous averaging of the squared signalusing RC-smoothing with a time constant A. Suchcontinuous averaging provides only an approximationof the true time-average, and it places restrictions onthe settling and observation times (see 7.3.3).

NOTE An example of an instrument using such averagingis a sound level meter fulfilling at least the requirements fora type 1 instrument in accordance with IEC Publication 651with the tim e weighting S.

b) By integrating the squared signal over a fixedtime-interval D. This integration may be performed byeither digital or analogue means.

Examples of suitable instrumentation systems are givenin ISO 3744.

5.2 The microphone and its associated cable

A condenser microphone, or the equivalent in accuracy,stability and frequency response, shall be used. Themicrophone shall have a flat frequency response, overthe frequency range of interest, for the angle ofincidence specified by the manufacturer.

NOTE This requirement i s met by a microphone of astandardized sound level meter fulfilling at least therequirements for a type 1 instrument in accordance withIEC Publication 651 and calibr ated for free-fieldmeasurements.

The microphone and its associated cable shall bechosen so that their sensitivity does not change over thetemperature range encountered in the measurement. Ifthe microphone is moved, care shall be exercised toavoid noise of acoustical origin (for example, noise fromwind, gears, mechanical moving parts) or electrical noise(for example, noise from flexing cables or slidingcontacts) that could interfere with the measurements.

5.3 Frequency response of the instrumentationsystem

The frequency response of the instrumentation systemfor the angle of incidence specified by the manufacturershall be flat over the frequency range of interest withinthe tolerances given for a type 1 instrument in IECPublication 651.

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5.4 Weighting network and frequency analyser

An A-weighting network complying with the tolerancerequirements of IEC Publication 651 and, if required, anoctave band or one-third octave band filter set fulfillingthe requirements of IEC Publication 225 shall be used.

The centre frequencies of the frequency bands shallcorrespond to those of ISO 266.

5.5 Calibration

Before and after each series of measurements, anacoustical calibrator with an accuracy 0,5 dB shall beapplied to the microphone to check the calibration of theentire measuring system at one or more frequenciesover the frequency range of interest. The calibrator shallbe checked annually to verify that its acoustical outputhas not changed. In addition, an acoustical and anelectrical calibration of the instrumentation system overthe entire frequency range of interest shall be carried outat intervals of not more than 2 years.

6 Installation and operation of the machine

6.1 Machine mounting

If practicable, the machine should be mounted in thesame way as it would be for normal usage. Care shouldbe taken to minimize the transmission and the radiationof structure-borne noise from all mounting elementsincluding the foundation. Usually, this minimizing can beachieved by resilient mounting for smaller machines.Larger machines can usually only be tested under rigidmounting conditions.

6.1.1 Resilient mounting

The natural frequency of the support system and themachine under test shall be lower than a quarter of thefrequency corresponding to the lowest rotational speedof the machine.

The effective mass of the resilient support shall not begreater than 1/10 of that of the machine under test.

6.1.2 Rigid mounting

The machines shall be rigidly mounted to a surface withdimensions adequate for the machine type (for exampleby foot or flange fixed in accordance with themanufacturers specifications). The machine shall not besubject to additional m ounting s tresses from incorrectshimming.

The mass of the support shall be at least twice that ofthe machine under test.

6.2 Operation of machine during test

The machine shall operate at no load, at rated voltage(s)and speed(s), and with the corresponding excitation(s)(see IEC P ublication 34-1).

For a.c. machines, the sinusoidality of the supply voltageand the degree of unbalance of the supply voltagesystem shall comply with the same limits that arespecified in IEC P ublication 34-1.

Synchronous machines shall be run with the excitationcurrent which permits the rated voltage at no load.

For machines not suitable for no-load operation, e.g.machines with the behaviour of series-wound motors, theoperating conditions shall be agreed upon and stated in

the test report.A method for estimating the difference in the level of thenoise from a machine between no-load operatingconditions and rated load or any other specified load isgiven in annex C.

6.3 Auxiliary equipment and coupled machines

All auxiliary equipment (loading machines, gears,transformers, external cooling systems) and coupledmachines which are necessary for the operation of themachine under test, but which do not form an integralpart of the machine, shall not significantly affect thenoise measurement (see 8.1). If they do, they should beshielded acoustically or located outside the testenvironment.

7 Sound pressure levels on the measurementsurface

7.1 Reference box and measurement surfaces

In order to facilitate the positioning of the microphonepositions, a hypothetical reference box is defined (see3.11). When defining the dimensions of this referencebox, elements protruding from the machine which areunlikely to be major radiators of sound energy may bedisregarded.

The microphone positions lie on the measurement

surface (see 3.10).For rotating electrical machines, regardless of their size,the measurement surface shape is a rectangularparallelepiped (see figures 2 to 4) the sides of which areparallel to the sides of the reference box and spaced outat a distance d (measurement distance) from thereference box.

The measurement distance, d , shall be at least 0,25 m.Distances larger than 1 m may be excluded by theenvironmental requirements given in this part ofISO 1680 (see 4.2, 4.3 and annex A). The preferredmeasurement distance is 1 m.

The area S of the measurement surface is given by theequation

S = 4 ( ab + bc + ca )

where, in accordance with figures 2, 3 or 4,

a = 0,5 l 1 + d ;

b = 0,5 l 2 + d ;

c = l 3 + d ;

l 1 , l 2 and l 3 are the dimensions of the reference box;

d is the measurement distance, normally 1 m.

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7.2 Microphone array

7.2.1 Complete measurement position array

From figure 1, the principle of how to construct themeasurement array for different sizes of reference boxcan be derived.

Each side of the measurement surface shall be treatedseparately. If the length or width of the side of themeasurement surface under consideration exceeds 3d ,this side is divided into a m inimum number of partialareas so that their lengths and widths do not exceed 3d (see figure 1).

To comply with the engineering method of this part ofISO 1680, measurement positions shall be placed at themiddle and the corners of each partial area, except atthose corners which lie in the reflecting plane. Thecorner positions of a partial area are identical with thecorner positions of the neighbouring partial areas. 1)

The resulting complete measurement array is shown infigures 2 to 4 for different sizes of the reference box.

Neighbouring measurement positions may be connectedto achieve continuous paths along which the microphoneis carried continuously with constant velocity (see figures2 to 4).

NOTE For the survey method complying with ISO 1680/2,only the positions in the middle of the partial areas (or therelevant paths through these positions) are used.

7.2.2 Simplified measurement position array

The arrangement of the measurement positions given infigures 1 to 4 may, especially for large machines, besimplified, if, for that type of machine, it can be shown,with the help of preliminary investigations on somemachines of that type, that the sound field is adequatelyuniform and that measurements lead to values of soundpower level deviating by no more than 1 dB from thosedetermined with a complete arrangement ofmeasurement positions.

For sources that produce a symmetrical radiationpattern, it may be sufficient to distribute themeasurement positions over only a portion of themeasurement surface. This is acceptable if, for that typeof machine, it can be shown, with the help of preliminaryinvestigations on some machines of that type, that themeasurements lead to values of sound power leveldeviating by no more than 1 dB from those determinedwith a complete arrangement of measurement positions.

7.3 Conditions of measurement

7.3.1 General

Environmental conditions may have an adverse effect onthe microphone used for the measurements. Suchconditions (for example, due to strong electric ormagnetic fields, wind, impingement of air discharged

from the machine under test, high or low temperatures)shall be minimized by proper selection or positioning ofthe microphone. The microphone shall always bedirected in such a way that the angle of incidence of thesound waves is that for which the microphone iscalibrated.

The observer shall not stand between the microphoneand the source under test.

The measurements shall be carried out once themachine under test is operating under steady-stateconditions.

The sound pressure level shall be observed over atypical period of operation of the source. Readings of thesound pressure level (corresponding to the level of themean-square sound pressure) shall be taken at eachmeasurement point with A-weighting and, if required, foreach frequency band within the frequency range ofinterest.

The following data shall be obtained:

a) the A-weighted sound pressure levels and, if required,the band pressure levels during operation of the machineunder test;

b) the A-weighted sound pressure levels and, if required,the band pressure levels produced by the backgroundnoise.

For the frequency bands centred on or below 160 Hz,the observation period shall be at least 30 s. ForA-weighted sound pressure levels and for the frequencybands centred on or above 200 Hz, the observationperiod shall be at least 10 s.

7.3.2 Measurements with a sound level meter

If the indicating meter of a sound level meter is used, thetime weighting S shall be used. If the fluctuations of theindicating pointer on the sound level meter are less than 3 dB using the time weighting S, the noise isconsidered to be steady for the purposes of this part ofISO 1680 and the level is taken to be the average of themaximum and minimum levels during the period ofobservation. If the meter fluctuations during the period ofobservation are greater than 3 dB, the noise isconsidered to be non-steady and one of theinstrumentation systems described in ISO 3744 shall beused.

7.3.3 Measurements with RC-smoothing orintegration systems

If RC-smoothing is used, the time-constant A should belong enough to obtain an estimate of the r.m.s. levelduring the period of observation with an accuracy of 0,5 dB.

If true integration is used, it is necessary for theintegration time to be equal to the period of observation.

1) The array is in complete accordance with ISO 3744 for small machines (see figure 3) and, in pri nciple, in accordance w ithISO 3744 for large machines, taking into account the sound field structure of rotating electrical machinery.

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Figure 1 Procedure for fixing the measurement positions where a side of the measurement surface exceeds 3 d

NOTE For the survey method of ISO 1680/2, only paths 1 and 3 (positions 1, 2, 3, 4, 9) are used.

Figure 2 Example of a measurement surface and measurement positions (paths) for a small machine(l 1 d , l 2 d , l 3 2 d , where d is the measurement distance, normally 1 m)

NOTE For the survey method of ISO 1680/2, only paths 1 and 3 with their positions are used.

Figure 3 Example of a measurement surface and measurement positio ns (paths) for a long machine(4d < l 1 7 d , l 2 d , l 3 2 d )

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NOTE For the survey method of ISO 1680/2, only paths 1, 3 and 5 w ith their positions are used.

Figure 4 Example of a measurement surface and measurement positions (paths) for a large machine(4d < l 1 7d , d < l 2 4 d, 2 d < l 3 5 d )

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8 Calculation of surface sound pressure level andsound power level

8.1 Corrections for background noise

The measured sound pressure levels shall be correctedfor background noise in accordance with table 2.

Table 2 Corrections for background soundpressure levels

Difference betweensound pressure level

measured with machineoperating and background

sound pressurelevel alone

Corrections t o besubtracted from sound

pressure level measuredwith machine operating to

obtain sound pressurelevel due to machine alone

dB dB

< 6678

910> 10

Measurements invalid1,01,01,0

0,50,50

8.2 Calculation of sound pressure level averagedover the measurement surface

For the A-weighted sound pressure level and the level ineach frequency band of interest, an average soundpressure level over the measurement surface, , iscalculated from the relevant measured sound pressurelevels Lpi (after corrections for background noise areapplied in accordance with 8.1) by using the followingequation:

... (1)

where

is the sound pressure level averaged over themeasurement surface, in decibels; reference: 20 Pa;

Lpi is the A-weighted or band pressure level resultingfrom the i th measurement, in decibels; reference:20 Pa;

N is the total number of measurement positions.

8.3 Calculation of surface sound pressure level

The surface sound pressure level, , shall be obtainedby correcting the value of for reflected sound toapproximate the average value of the sound pressurelevel which would be obtained under free-field conditions,by using the following equation:

... (2)

where

is the surface sound pressure level, in decibels;reference: 20 Pa;

K is the mean value of the environmental correction

over the measurement surface, in decibels.

For the purposes of this part of ISO 1680, the maximumacceptable range of the environmental correction, K , is2 dB to +2dB.

NOTE The environmental correction, K , accounts for t heinfluence of a non-ideal environment (for example, the presenceof reflected sound). It ranges typically from 2 dB (formeasurements outdoors wi th absorbing ground) to +10 dB (formeasurements indoors in highly reverberant rooms). Theprocedures given in annex A are used to calculate th e value ofthe environmental correction.

8.4 Calculation of sound power level

The sound power level characterizing the noise emittedby the source shall be calculated from the followingequation:

... (3)

whereLW is the A-weighted or band sound power level of thesource, in decibels; reference: 1 pW;

is the surface sound pressure level determined inaccordance with 8.3, in decibels; reference: 20 Pa;

S is the area of the measurement surface, in squaremetres (see 7.1);

S 0 = 1 m2.

If only band sound power levels are determined, theA-weighted sound power level may be determined in

accordance with annex B.9 Information to be recorded

The following information shall be compiled and recordedfor all measurements carried out in accordance with therequirements of this part of ISO 1680.

9.1 Machine under test

a) Description of the machine under test (including itsdimensions).

b) Operating conditions.

c) Mounting conditions.

d) If the m achine has multiple noise sources, adescription of source(s) in operation during themeasurements.

9.2 Acoustic environment

a) Description of the test environment;

1) if indoors, description of physical treatment ofwalls, ceiling and floor, sketch showing the location ofmachine under test and room contents;

2) if outdoors, sketch showing the location ofmachine under test with respect to surroundingterrain, including physical description of testenvironment.

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b) Acoustical qualification of the test environment inaccordance with annex A.

c) Air t emperature in degrees Celsius, barometricpressure in pascals, and relative humidity.

d) Wind velocity and direction.e) Sound power level of the reference sound source,if used.

9.3 Instrumentation

a) Equipment used for the measurements, includingname, type, serial number and manufacturer.

b) Bandwidth of frequency analyser.

c) Frequency response of instrumentation system.

d) Method used for checking the calibration of themicrophones and other system components; the dateand place of calibration shall be given.

e) Characteristics of windscreen (if used).

9.4 Acoustical data

a) The measurement distance, the location anddirection of microphone positions.

b) The area S of the measurement surface.

c) The corrections, in decibels, if any, applied in eachfrequency band for the frequency response of themicrophone, frequency response of the filter in thepassband, background noise, etc.

d) The environmental correction, K , calculated inaccordance with one of the procedures given inannex A.

e) The surface sound pressure level, , in decibels,calculated from the measured A-weighted soundpressure levels or from the sound pressure levels ineach frequency band of interest; reference: 20 Pa.

f) The sound power level, L W , in decibels, calculatedfrom the A-weighted surface sound pressure leveland, if required, from the surface sound pressurelevels for all frequency bands used; reference: 1 pW.

g) If required, difference of the levels of the noisebetween no-load and on-load operation, A-weightedand, if required, in frequency bands.

h) Remarks on subjective impression of noise(audible discrete tones, impulsive character, spectralcontent, temporal characteristics, etc.).

i) The date when the measurements were carried out.

10 Information to be reported

The test report shall contain the statement that thesound power levels have been obtained in full conformitywith the procedures of this part of ISO 1680.

The following information shall be reported:

a) a description of the machine under test;

b) the operating conditions;

c) the A-weighted sound power level, LW A, indecibels, and, if required, sound power levels infrequency bands; reference: 1 pW;

d) if required, difference of the levels of the noisebetween no-load and on-load operation, A-weightedand, if required, in frequency bands;

e) the date when the measurements were carried out.

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If a windscreen is used to shield the microphone fromthe effects of wind, proper corrections of the measuredsound pressure levels shall be made.

A.3 Absolute comparison test to determine K

A.3.1 Procedure

A reference sound source with characteristics that meetthe requirements of ISO 6926 shall be mounted in thetest environment in essentially the same position as thatof the source under test. The sound power level of thereference sound source is determined in accordance withthe procedures of clauses 7 and 8 without theenvironmental correction K (i.e. K is initially assumedequal to zero). The same measurement surface is usedas during the measurements of the source under test.The environmental correction K is given by

K = LW LW r

where

LW is the calculated A-w eighted or band power levelof the reference sound source using procedures ofclauses 7 and 8 [with K = 0 in equation (2)], indecibels; reference: 1 pW;

LW r is the name plate A-weighted or band power levelof the reference sound source, in decibels; reference:1 pW.

A.3.2 Locations of reference sound source in testenvironment

A.3.2.1 Source can be removed from test site

The reference sound source shall be located on the

reflecting plane, independent of the height of themachine. One single location is sufficient, even whenvery large machines are to be tested, provided the ratioof the length of the machine under test to its width is notgreater than 2. If the ratio is greater than 2, thereference sound shall be operated on the floor at fourpoints. Assuming the projection of the machine undertest on the floor to be approximately rectangular inshape, the four points are located at the middle points ofthe sides of the rectangle. To obtain LW , the surfacesound pressure level, shall be calculated with thereference sound source located at each of the fourpoints on the floor. At each point on the measurementsurface, the sound pressure level shall be averaged for

the four source locations on a mean-square basis, i.e.using equation (1) in 8.2.

A.3.2.2 Source cannot be removed from test site

The reference sound source shall be located on theupper surface of the machine which should preferably beacoustically reflective. This method should not be appliedif the machine has highly absorptive surfaces (forexample, textile machines), In this case, the procedureof clause A.4 should be followed.

A.3.3 Qualification requirements

For the measurement surface in a given testenvironment to be satisfactory for m easurements in

accordance with the requirements of this part of

ISO 1680, the environmental correction, K , shall benumerically less than or equal to 2 dB. If theenvironmental correction, K , exceeds 2 dB, either asmaller measurement surface or a better testenvironment is required.

When the necessary changes have been made, the testprocedure to determine K shall be repeated.

A.4 Reverberation test to determine K

A.4.1 Test procedure

This test procedure is applicable to rooms the maximumdimension of which does not exceed significantly itsminimum dimension.

The environmental correction, K , is obtained from theexpression

The value of K may be obtained from figure 5 byentering the abscissa with the appropriate value of A / S .The area S of the measurement surface is calculated inaccordance with the requirements given in 7.1.

The total sound absorption area, A , of the test room isdetermined from measurements of the reverberation timeof the test room in octave bands for the entire frequencyrange of interest (see ISO 354).

The value of A is then

A = 0,16 ( V / T )

where

V is the volume of the test room, in cubic metres;

T is the reverberation time of the test room in octaveband, in seconds.

A.4.2 Qualification requirements

For the measurement surface in a test room to besatisfactory for measurements in accordance with therequirements of this part of ISO 1680, the ratio A to S should exceed 6, that is A / S > 6.

If this requirement cannot be satisfied, a newmeasurement surface shall be chosen. The newmeasurement surface should have a smaller total area,

but still lie outside the near field. Alternatively, the ratioA / S may be increased by introducing additionalsound-absorptive materials into the test room and thenredetermining the value of the ratio A / S under the newconditions.

If the requirements of this clause cannot be satisfied forany measurement surface which lies outside the nearfield of the source under test, the particular testenvironment chosen cannot be used for measurementson the source under test in accordance with therequirements of this part of ISO 1680. A new testenvironment shall be selected or it shall be assumed thatthe uncertainties may exceed the values given in table 1.

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Figure 5 Environmental correction, K , in decibels

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Annex B

Procedures for calculating A-weighted sound power levelfrom octave or one-third octave band power levels

(This annex forms an integral part of the standard.)

B.1 Calculate the A-weighted sound power level, LW A, indecibels (reference: 1 pW) from the formula:

where ( LW )J is the level in the J th octave or third-octave

band.

B.2 For calculations with octave band data, J max = 7 andC J is given in table 3.

Table 3 Parameters for calculating A-weightedsound power level from octave band power levels

J Octave band centrefrequency C J

Hz dB

1234567

125250500

1 0002 0004 0008 000

16,1 8,6 3,2

0+ 1, 2+ 1, 0 1,1

B.3 For calculations with one-third octave band data,J max = 21 and C J is given in table 4.

Table 4 Parameters for calculating A-weightedsound power level from one-third octave band

power levels

J One-third octave bandcentre frequency C J

Hz dB

1

23

100

125160

19,1

16,1 13,4

456

200250315

10,9 8,6 6,6

789

400500630

4,8 3,2 1,9

101112

8001 0001 250

0,800,6

131415

1 6002 0002 500

1,01,21,3

161718

3 1504 0005 000

1,21,00,5

192021

6 3008 000

10 000

0,1 1,1 2,5

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Annex C

Method for estimating the difference in noise levelbetween no-load and on-load operations

NOTE This annex does not form an integral part of the standard insofar as this estimation method cannot be classified as being ofengineering grade accuracy.

C.1 General

This annex describes a method which can be used to obtain an estimate of the difference in the levels of the noisebetween no-load and on-load operations.

This method should be used by agreement between the manufacturer and the purchaser if it is expected that there willbe a significant difference in the noise emission of the machine between no load and the rated or any other specifiedload.

C.2 Installation and operation of machines

C.2.1 Machine mounting

See 6.1.

C.2.2 Operation of machine during test

The machine shall be operated, coupled to its load machine, at its rated voltage(s), speed(s) and correspondingexcitation(s) with no load applied to the load machine and then with the rated or other specified load applied.

If possible, the machine under test should be mechanically uncoupled from an electrical load machine so thatbackground noise measurements can be made with the load machine operating as a motor at rated voltage(s), speed(s)and corresponding excitation(s). There will be many occasions where it is neither possible to uncouple the load machinemechanically nor to operate the load machine as a motor. Under these conditions, measurements should be m ade atno load and at the rated or other specified load only.

C.3 Measurement array

The measurement array used shall be the same as that for the no-load test (see clause 7), but the distance of themicrophone from the reference surface of the machine shall be 0,15 m. If it is not possible to have a particularmeasurement location, due to the presence of, for example, a bearing pedestal, the measurement position should bemoved just enough to overcome the obstruction.

NOTE As the method described in this annex aims at the determination of a level difference ( see clause C.5), the measurementarray is treated as being sufficient also for the small measurement distance of 0.15 m, which is taken to minimize the environmentalinfluences.

C.4 Measurement of sound pressure levels

At each microphone position on the measurement surface, the A-weighted sound pressure level shall be measured withthe machine operating at no load and then at the rated or other specified load. The measurements shall be carried outafter the machine has reached a steady temperature.

If possible, measurements should be made at each microphone position, after the machine has been switched off andmechanically uncoupled from the load machine and w ith the load machine operating as a motor at its rated voltage(s),speed(s) and corresponding excitation(s).

The sound pressure levels measured with the test machine operating at no load and then on full load are corrected forbackground noise (see 8.1) and are then used to determine the average sound pressure levels over the measurementsurface in accordance with 8.2. The background noise will include the no-load noise of the loading machine if it waspossible to run it disconnected from the machine under test.

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C.5 Estimation of differences in level of the noise between no-load and on-load operations

The difference between the on-load and no-load sound pressure levels is obtained as follows:

where

is the A-weighted sound pressure level over the measurement surface according to clause C.3 in on-loadconditions (for calculation, see 8.1 and 8.2);

is the A-weighted sound pressure level over the measurement surface according to clause C.3 at no load.

NOTE If it has not been possible to m easure the level of the background noise contributed by the loading machine, the difference has to be calculated from the mean values of the A-weighted sound pressure levels as measured, without correction for thatbackground noise; this case should be indicated in the test report.

An estimate of the on-load A-weighted sound power level of the machine may be obtained by adding the difference to the value of A-weighted sound power level obtained from no-load measurements in accordance with clauses 7 and8. This estimate being derived f rom measurements at 0,15 m distance may lead to an uncertainty greater than indicatedin table 1. Therefore, this estimate of the on-load A-weighted sound power level cannot be classified as being ofengineering grade accuracy.

The difference may also be determined in frequency bands, if required.

C.6 Information to be recorded

The information listed in clause 9 should be recorded for all tests carried out in accordance with this annex.

C.7 Information to be reported

The information listed in clause 10 should be reported together with the calculated difference A.

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AS1081.1 Acoustics—Measurement of Airborne Noise Emitted by Rotating Electrical Machinery

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