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AS 1217.5—1985

Australian Standard!

ACOUSTICS—DETERMINATION

OF SOUND POWER LEVELS OF

NOISE SOURCES

Part 5—ENGINEERING

METHODS FOR

FREE-FIELD

CONDITIONS OVER A

REFLECTING PLANE

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y U N S W - L I B R A R Y o n 2 6 M a y 2 0 0 5

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This Australian standard was prepared by Committee AK/2, Techniques for

Measurement. It was approved on behalf of the Council of the Standards Association

of Australia on 30 August 1984 and published on 4 April 1985.

The following interests are represented on Committee AK/2:

Australian Acoustical Society

CSIRO, Division of Building Research

CSIRO, National Measurement Laboratory

Department of Aviation

Department of Industrial Relations, N.S.W.

Department of Housing and Construction

Environment Protection Authority of Victoria

Metal Trades Industry Association of Australia

National Acoustic Laboratories

Telecom Australia

University of Adelaide

University of Queensland

Review of Austr alia n Standa rds. To keep abreast of progress in industry, Australian Standards are subject to periodic review and are kept up to date by the i ssue of amendments or new editions as necessary. It isimportant t herefore t hat Standards users ensure that they are in possession of the latest edition, and anyamendments thereto.

Full details of all Australian Standards and related publications wil l be found in t he Standards AustraliaCatalogue of Publications; t his information is supplemented each month by the magazine ‘The AustralianStandard’, which subscribing members receive, and which gives details of new publications, new editionsand amendments, and of withdrawn Standards.

Suggestions for improvements t o Australian Standards, addressed to the head office of Standards Australia,are welcomed. Notification of any inaccuracy or ambiguity found in an Australian Standard should be madewithout delay in order that the matter may be investigated and appropriate action taken.

This Standard was issued in draft form for comment as DR 82157.

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AS 1217.5—1985

Australian Standard!

ACOUSTICS—DETERMINATION

OF SOUND POWER LEVELS OF

NOISE SOURCES

Part 5—ENGINEERING

METHODS FOR

FREE-FIELDCONDITIONS OVER A

REFLECTING PLANE

First published (as AS 1217) . . . . . . . . . . . . . . 1972AS 1217.5 first published . . . . . . . . . . . . . . . . . 1985

Incorporating:Amdt 1— 1985

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

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AS 1217.5—1985 2

PREFACE

This standard was prepared by the Association’s Committee on Techniques forMeasurement. It supersedes AS 1217 — 1972, Methods of Measurement of AirborneSound Emitted by Machines.

This standard is based on ISO 3744 —1981, Acoustics Determination of Sound Power

Levels of Noise Sources Engineering Methods for Free-field Conditions Over aReflecting Plane.

" Copyright STANDARDS AUSTRALIA

Users of Standards are reminded that copyright subsists in all Standards Australia publications and software. Except where theCopyright Act allows and except where provided for below no publications or software produced by Standards Australia may bereproduced, stored in a retrieval system in any form or transmitted by any means without prior permission in writing fromStandards Australia. Permission may be conditional on an appropriate royalty payment. Requests f or permission and information oncommercial software royalties should be directed to the head office of Standards Australia.

Standards Australia will permit up to 10 percent of the technical content pages of a Standard to be copied for useexclusively in-house by purchasers of the Standard without payment of a royalty or advice t o Standards Australia.

Standards Australia will also permit the inclusion of its copyright material in computer software programs for no royaltypayment provided such programs are used exclusively in-house by the creators of the programs.

Care should be taken to ensure that material used is from the current edition of t he Standard and that it i s updated whenever theStandard is amended or revised. The number and date of the Standard should t herefore be clearly i dentified.

The use of material in print form or in computer software programs to be used commercially, with or without payment, or incommercial contracts is subject to the payment of a royalty. This policy may be varied by Standards Australia at any time.

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3 AS 1217.5—1985

CONTENTS

Page

FO REW ORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

SECTION 1. SCOPE AND GENERAL

1.1 Sc ope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71. 2 Field of Ap plicatio n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.3 Referenced Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.4 Measurement Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71. 5 De fin itions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

SECTION 2. ACOUSTIC ENVIRONMENT

2. 1 Ge neral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2 Criteria for Adequacy of the Test Environment . . . . . . . . . . . . . . . 82.3 Criterion for Background Noise . . . . . . . . . . . . . . . . . . . . . . . . . . 8

SECTION 3. INSTRUMENTATION

3. 1 Ge neral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2 The Microphone and Its Associated Cable . . . . . . . . . . . . . . . . . . 93.3 Frequency Response of the Instrumentation System . . . . . . . . . . . . 93.4 Weighting Network, Frequency Analyser . . . . . . . . . . . . . . . . . . . 93. 5 Ca lib ratio n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

SECTION 4. INSTALLATION AND OPERATION OF SOURCE

4. 1 Ge neral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104. 2 So urc e Po sit io n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104. 3 So urc e Mou ntin g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104. 4 Au xiliary E quipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.5 Operation of Source During Test . . . . . . . . . . . . . . . . . . . . . . . . 10

SECTION 5. SOUND PRESSURE LEVELS ON MEASUREMENTSURFACE

5. 1 Mea surement Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115.2 Additional Microphone Positions on Measurement Surface . . . . . . 125.3 Reduction of Number of Microphone Positions . . . . . . . . . . . . . . 125.4 Conditions of Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

SECTION 6. CALCULATION OF SOUND SURFACE PRESSURELEVEL, SOUND POWER LEVEL, AND DIRECTIVITYFACTOR

6.1 Calculation of Sound Pressure Level Averaged Overthe Measurement Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

6.2 Calculation of Surface Sound Pressure Level . . . . . . . . . . . . . . . 146.3 Calculation of Sound Power Level . . . . . . . . . . . . . . . . . . . . . . . 146.4 Calculation of Directivity Index and Directivity Factor . . . . . . . . 14

SECTION 7. INFORMATION TO BE RECORDED AND REPORTED

7. 1 Ge neral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157. 2 So und Sou rc e U nder Te st . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157. 3 Ac oustic E nviro nme nt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157. 4 In strume ntation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157. 5 Ac oustic al Da ta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157. 6 In formatio n to be Re puted . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

APPENDICES

A Qualification Procedures for the Acoustic Environment . . . . . . . . . 16B Microphone Array on the Hemisphere . . . . . . . . . . . . . . . . . . . . . 19C Microphone Array on the Parallelepiped Measuring Surface . . . . . . 21

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AS 1217.5—1985 4

D Microphone Array on the Conformal Measurement Surface . . . . . . . 23E Examples of Suitable Instrumentation Systems . . . . . . . . . . . . . . . 26F Guidelines for the Detection of Impulsive Noise . . . . . . . . . . . . . . 27G Calculation of the Directivity Index and Directivity

Factor Using Hemispherical Microphone Array . . . . . . . . . . . . . . . 28

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5 AS 1217.5—1985

STANDARDS ASSOCIATION OF AUSTRALIA

Australian Standard

for

ACOUSTICS— DETERMINATION OF SOUND POWER LEVELS OF NOISE SOURCES

PART 5—ENGINEERING METHODS FOR FREE-FIELD CONDITIONS OVER

A REFLECTING PLANE

FOREWORD

This standard is one of a series setting out various methods for determining the soundpower levels of machines and equipment. These basic documents specify only theacoustical requirements for measurements appropriate for different test environmentsas shown in Table A.

In the application of these basic documents, it is necessary to decide which one is mostappropriate for the conditions and purposes of the test. The operating and mountingconditions of the machine or equipment to be tested must be i n accordance with thegeneral principles stated in the basic documents.

Guidelines for making these decisions are provided in AS 1217.1. If no sound testmethod is specified for a particular machine, the mounting and operating conditionsshall be fully described in the test report.

SYNOPSIS

Applic ability . This standard sets out engineering methods for measuring the soundpressure levels on a given surface which envelops the noise source under investigation.The sound power level of the source is calculated from the results of thesemeasurements. The method may be applied in laboratory rooms which provide a free-field over a reflecting plane or in field installations whose acoustical characteristicscomply with the requirements of this standard. If the test room does not comply withthe requirements of this standard, measurements made in accordance with AS 1217.7might nevertheless be possible.

Free-field conditions are usually not encountered in t ypical machine rooms wheresources are normally installed. If measurements are made of such installations,corrections may be required to account for background noise or undesired reflections.These corrections reduce the accuracy of the sound power determination.

The methods set out in this standard conform to the general recommendations includedin ISO 2204. The methods permit t he determination of sound power level both infrequency bands and directly as an A-weighted value.

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AS 1217.5—1985 6

TABLE ABASIC S TANDARDS SPECIFYING VARIOUS METHODS FOR DETERMINING THE SOUND

POWER LEVELS OF MACHINES AND EQUIPMENT

ASnumber

Classificationof method

Testenvironment

Volume of source

Character of noise

Sound powerlevels

obtainable

Optionalinformation

available

AS 1217.2

Precision

Reverberationroom complyingwith specifiedrequirements

Preferably lessthan 1 percentof test roomvolume

Steady,broad-band In one-third

octave oroctave bands

A-weighted soundpower level

AS 1217.3Steady, discrete-frequency, ornarrow-band

AS 12 17.4 En gi neer in gSpecialreverberationtest room

Steady, broad-bandnarrow-band, ordiscrete-frequency

A-weighted and inoctave bands

Other weightedsound powerlevels

AS 1 21 7. 5 En gi ne er in g Outdoors or in

large room

Greatestdimension lessthan 15 m

MostA-weighted andin one-thirdoctave or octavebands

Directivityinformation andsound pressurelevels as afunction of time;other weightedsound powerlevels

AS 1 217. 6 Pr ecis io nAnechoic orhemi-anechoicroom

Preferably lessthan 0.5 percentof test roomvolume

Most

AS 1217.7 Surv ey No special test

environment

No restrictions;limited only byavailable testenvironment

Most A- weighted

Sound pressurelevels as afunction of time;other weightedsound powerlevels

Test environment . Free-field over a reflecting plane (indoors or outdoors).Type of source. Device, machine, component, subassembly.Size of noise source. Greatest l inear dimension less than 15 m.Character of noise radiated by the source. All types, steady, non-steady, broad band, discretefrequency, narrow-band as defined in AS 1633 and SAA MP 44, P art 1.

Accuracy. Engineering or grade 2 (as defined in ISO 2204). (Standard deviation for

determining sound power levels for 1 kHz octave band is about 1.5 dB.)Quantities to be measured. Sound pressure l evels (weighted and in frequency bands) atprescribed microphone positions.Quantities to be determined. Weighted sound power levels (A-weighting i s required; otherweightings are optional); sound power levels in frequency band; directivity characteristics of the source (optional).

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7 AS 1217.5—1985

SECTION 1. SCOPE AND GENERAL

1.1 SCOPE. This standard sets out engineering methodsfor measuring the sound pressure levels on a measurementsurface enveloping the source and for calculating the soundpower level produced by the source. It gives requirementsfor the test environment and instrumentation as well as

techniques for obtaining the surface sound pressure levelfrom which the A-weighted sound power level of thesource and octave or one-third octave band sound powerlevels are calculated.

NOTES:

1. This standard is concerned with specification of the basic acousticrequirements for the measurement of noise under free-fieldconditions over a reflecting plane. It is important that specific testcodes for various types of equipment be established and used as thebasis for measurements according to this standard. Such test codeswill, for each type of equipment, give detailed requirements onmounting, loading and operating conditions for the equipment undertest as well as descriptions of the measurement surface, the numberof microphone positions, and the measurement distance to be used.

2. The use of different measurement surfaces may yield differingestimates of the sound power level of a source. Measurements on

a given family of equipment should all be based on the same shapeof measurement surface. The test method for a particular type of equipment should give detailed information on the particularmeasurement surface that is selected.

1.2 FIELD OF APPLICATION.

1.2.1 Types of noise. This standard applies to sourceswhich radiate broad-band noise, narrow-band noise,discrete tones and combinations of such components. Theprocedures given are applicable to steady noise. Theseprocedures may also be applied to non-steady noise withthe exception of an isolated burst of sound energy or aburst train with a repetition rate less than 10 per second.

1.2.2 Size of source. This standard is not applicable tonoise sources with any linear dimension (length, width orheight) which exceeds 15 m.

1.3 REFERENCED DOCUMENTS. The followingdocuments are referred to in this standard:

AS 1045 Method of Measurement of AbsorptionCoefficients in a Reverberation Room

AS 1217.1 Acoustics—Determination of Sound PowerLevels of Noise Sources,Part 1—Guidelines for the Use of Basic

Standards and for the Preparation of Noise Test Codes

Acoustics—Determination of Sound Power

Levels of Noise Sources,Part 2— Precision Methods for Broadband

Sources in Reverberation RoomsAcoustics—Determination of Sound PowerLevels of Noise Sources,Part 3— Precision Methods for Discrete-

frequency and Narrow-band Sourcesin Reverberation Rooms

Acoustics—Determination of Sound PowerLevels of Noise Sources,Part 4— Engineering Methods for Special

R ev er be r at io n T es t R o omAcoustics—Determination of Sound PowerLevels of Noise Sources,

Part 6— Precision Methods for Anechoicand Hemi-anechoic Rooms

Acoustics— Determination of Sound PowerLevels of Noise Sources,Part 7—Survey Method

AS 1259 Sound Level MetersAS 1633 Glossary of Acoustic Terms

AS 2533 Acoustics —Preferred Frequencies forMeasurements

AS Z41 Octave, Half Octave and One-third OctaveBand Pass Filters Intended for the Analysis of Sound and Vibrations

ISO 2204 Acoustics— Guide to International Standardson the Measurement of Airborne AcousticalNoise and Evaluation of Its Effects on HumanBeings

ISO 6926 Acoustics— Determination of Sound PowerLevels of Noise Sources — Characterizationand Calibration of Reference Sound Sources

1.4 MEASUREMENTUNCERTAINTY. Measurementsmade in conformity with this standard should result in

standard deviations which are equal to or less than thosegiven in Table 1.1. The uncertainties in Table 1.1 dependnot only on the accuracies with which sound pressurelevels and measurement surface areas are determined butalso on the ‘near-field error’ which increases for smallermeasurement distances and lower frequencies, i.e. thosebelow 250 Hz. The contribution to the uncertainty of theerror caused by small measurement distances may bechecked by repeating the measurements at a largermeasurement distance and comparing the resulting soundpower levels. The near-field error always leads to soundpower levels which are higher than those determined withlarger measurement distances.

NOTES:

1. If the methods specified in this standard are used to compare thesound power levels of similar machines that are omnidirectional andradiate broad-band noise, the uncertainty in this comparison tendsto result in standard deviations which are less than that given inTable 1.1, provided that the measurements are performed in thesame environment with the same shape of measurement surface.

2. The standard deviations given in Table 1.1 reflect the cumulativeeffects of all causes of measurement uncertainty, excludingvariations in the sound power level from test to test which may becaused, for example, by changes in the mounting or operatingconditions of the source. The reproducibility and repeatability of thetest results may be considerably better, i.e. smaller standarddeviations, than the uncertainties given in Table 1.1 would indicate.

3. For a source which emits noise with a relatively ‘flat’ spectrum inthe frequency range 100 Hz to 10 000 Hz, the A-weighted soundpower level is determined with a standard deviation of approximately 2 dB. For outdoor measurements, the standard

deviations in the octave band centred on 63 Hz will beapproximately 5 dB.

TABLE 1.1

UNCERTAINTY IN DETERMINATIONS OFSOUND POWER LEVELS FOR ENGINEERINGMEASUREMENTS INDOORS OR OUTDOORS

Octave bandcentre frequencies

One-third octaveband centrefrequencies

Standarddeviation

Hz Hz dB

125250 to 500

1 000 to 4 0008 000

100 t o 160200 t o 630800 to 5 000

6 300 to 10 000

3.02.01.52.5

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9 AS 1217.5—1985

SECTION 3. INSTRUMENTATION

3.1 GENERAL. The instrumentation shall be designed tomeasure the mean-square value of the weighted soundpressure level and the octave or one-third octave bandlevels, averaged over time and over the measurementsurface. Surface averaging is usually carried out by

measuring the time-averaged sound pressure levels with aprescribed time constant for a fixed number of microphonepositions (see Clause 5.1) and computing the average valueaccording to Clause 6.1.

The instrumentation used can perform the requiredtime-averaging in two different ways as follows:

(a) By continuous averaging of the squared signal usingRC-smoothing with a time-constant !A. Suchcontinuous averaging provides only an approximationof the true time-average, and it places restrictions onthe ‘setting’ time and observation time (see Clause5.4.3).

NOTE: An example of an instrument employing such averaging isa sound level meter fulfilling at least the requirements for a Type 2

instrument in accordance with AS 1259, with the time-weightingcharacteristic ‘S’. For noise which is impulsive in character, seeAppendix F.

(b) By integrating the squared signal over a fixedtime-interval !D. This integration may be performedby either digital or analogue means.

Examples of suitable instrumentation systems are given inAppendix E.

3.2 THE MICROPHONE AND ITS ASSOCIATEDCABLE. A capacitor microphone, or the equivalent inaccuracy, stability and frequency response, shall be used.The microphone shall have a flat frequency response overthe frequency range of interest for the angle of incidencespecified by the manufacturer.

NOTE: This requirement is complied with by a microphone of astandardized sound level meter complying with the requirements for aType 2 instrument in accordance with AS 1259, and calibrated forfree-field measurements.

The microphone and its associated cable shall be chosen sothat their sensitivity does not change over the temperaturerange encountered during the measurement. If themicrophone is moved, care shall be exercised to avoidintroducing acoustical noise (e.g. wind noise) or electrical

noise (e.g. noise from gears, flexing cables, or slidingcontacts) that could interfere with the measurements.

3.3 F RE QUE NCY RE S PONS E O F T HEINSTRUMENTATION SYSTEM. The frequencyresponse of the instrumentation system for the angle of incidence specified by the manufacturer shall be flat overthe frequency range of interest within the tolerances givenfor a Type 2 instrument in AS 1259.

3.4 WEIGHTING NETWORK, FREQUENCYANALYSER. An A-weighting network complying with therequirements for a Type 2 sound level meter in accordancewith AS 1259, and an octave band or one-third octave

band filter set complying with AS Z41 shall be used. Thecentre frequencies of the frequency bands shall correspondto those of AS 2533.

NOTE: If other weighting networks are used in addition toA-weighting, the characteristics of such networks shall be reported.

3.5 CALIBRATION. During each series of measurements,an acoustical calibrator with an accuracy of ± 0.5 dB shallbe applied to the microphone for checking the calibrationof the entire measuring system at one or more frequenciesover the frequency range of interest. The calibrator shall bechecked at least once every year to verify that its outputhas not changed. In addition, an acoustical and an electricalcalibration of the instrumentation system over the entirefrequency range of interest shall be carried out at leastevery 2 years.

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AS 1217.5—1985 10

SECTION 4. INSTALLATION AND OPERATIONOF SOURCE

4.1 GENERAL. In many cases, the sound power emittedby a source depends upon its support or mountingconditions as well as the manner in which the source isoperated. This Section gives general recommendationsconcerning the installation and operation of sources.

Reference should be made to specific test methods formore detailed information concerning installation andoperation of specific classes of sources, e.g. rotatingelectrical machines.

Particularly for large machines, it is necessary to make adecision as to which components, subassemblies, auxiliaryequipment, power sources, etc belong to the source undertest.

4.2 SOURCE POSITION. The source to be tested shallbe installed and mounted with respect to the reflectingplane in one or more positions that are typical of normalusage, if practicable. If several possibilities exist, or if typical installation and mounting conditions are unknown,

special arrangements are to be made and described in thetest report. In positioning the source within the testenvironment, it is important that sufficient space beallowed so that the measurement surface can envelop themachine according to the requirements of Clause 5.1.

The source shall be placed at a sufficient distance from anyreflecting wall or ceiling or any reflecting object so thatthe requirements given in Appendix A are complied withon the measurement surface.

NOTE: The typical installation conditions for some sources involve tworeflecting surfaces, e.g. an appliance installed against a wall, or freespace, e.g. a hoist, or an opening in an otherwise reflecting plane (sothat radiation may occur on both sides of the vertical plane). Detailedinformation on installation conditions and the configuration of microphone arrays should be based on the general requirements of thisstandard and specified in specific test codes for such sources.

4.3 SOURCE MOUNTING. Many small sound sources,e.g. ballasts for fluorescent lamps, electrical clocks,although themselves poorradiators of low frequency sound,

may, as a result of the method of mounting, radiate morelow-frequency sound when their vibrational energy istransmitted to surfaces large enough to be efficientradiators. Resilient mounting should be interposed if possible between the device to be measured and the

supporting surfaces so that the transmission of vibration tothe support and the reaction of the source are bothminimized. In this case the mounting base should have asufficiently high mechanical impedance to prevent it fromvibrating and radiating sound excessively. However, suchresilient mounts shall not be used if the device under testis not resiliently mounted in typical field installations.

4.4 AUXILIARY EQUIPMENT. If practicable allauxiliary equipment necessary for the operation of thedevice under test that is not a part of the source (seeClause 4.1) shall be placed outside the test environment.

4.5 OPERATION OF SOURCE DURING TEST.During the acoustical measurements, the source shall be

operated in a specified manner typical of normal use. Oneor more of the following operational conditions may beappropriate:

(a) Device under specified load and operating conditions.

(b) Device under full load (if different from (a) above).

(c) Device under no load (idling).

(d) Device under operating condition corresponding tomaximum sound generation representative of normaluse.

(e) Device with simulated load operating under carefullydefined conditions.

The sound power levels of sources may be determined for

any desired set of operating conditions, e.g. temperature,humidity, device speed. These test conditions shall beselected beforehand and shall be held constant during thetest. The source shall be in a stable operating conditionbefore any noise measurements are made.

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11 AS 1217.5—1985

SECTION 5. SOUND PRESSURE LEVELS O NMEASUREMENT SURFACE

5.1 MEASUREMENT SURFACE.

5.1.1 Reference surface and measurement surface. Tofacilitate the positioning of the microphones, a hypotheticalreference surface is defined. This reference surface is thesmallest possible rectangular box, i.e. rectangularparallelepiped, that just encloses the source and terminateson the reflecting plane. In the defining of the dimensionsof this reference box, elements protruding from the sourcewhich are not significant radiators of sound energy may bedisregarded. These protruding elements should be identifiedin specific test methods for different types of equipment.The microphone positions lie on the measurement surface,a hypothetical surface of area S which envelops the sourceas well as the reference box and terminates on thereflecting plane.

5.1.2 Coordinate system and definition of distance d o.The position of the source under test, the measurement

surface and the microphone positions are defined by acoordinate system with the horizontal axes x and y in theground plane parallel to the length and width of thereference box and with the vertical axis z passing throughthe geometric centre of the box. The characteristic distanced o is the distance from the origin of the coordinate systemto one of the upper corners of the reference box:

d o = [(0.5l1)2 + (0.5l2)

2 + l32]1/2

where l1, l 2 and l3 are the length, width and height of thereference box.

5.1.3 Shape of measurement surface. Any one of thefollowing three shapes may be used for the measurement

surface:

(a) A hemispherical surface of radius r .

(b) A rectangular parallelepiped whose sides are parallelto those of the reference box, in which case themeasurement distance d is the distance between themeasurement surface and the reference box.

(c) A conformal surface which is the same as therectangular parallelepiped except that the corners arerounded, the corners being formed by portions of cylinders and spheres. The measurement distance d isthe distance from the measurement surface to thereference box. Each point on the conformal surface isthe same distance d from the closest point on thereference box.

For measurements on a series of similar sources, e.g.machines of the same type, the use of the same shape of measurement surface is recommended. The specific testmethod pertinent to the particular source underinvestigation should be consulted for detailed information.The construction of the reference box, the size and shapeof the measurement surface as well as the measurementdistance d , or the radius of the hemisphere r, shall bedescribed in the test report.

5.1.4 Selection of measurement surface.

5.1.4.1 General. Where environmental conditions permit,

use of the hemispherical measurement surface is usuallypreferred. Where measurements are to be made close to thesource because of adverse environmental conditions orother constraints, either a rectangular parallelepiped or

conformal surface may be used for the measurementsurface. In general, the conformal surface is expected toyield a more accurate value of the sound power level thanthe rectangular parallelepiped, but use of the conformal

surface requires more effort to position the microphones.The microphone positions on the conformal surface are ata uniform distance from the reference box which may bean advantage under some environmental conditions.

5.1.4.2 Hemispherical measurement surface. Thehemisphere shall be centred on the projection on thereflecting plane of the geometric centre of the referencebox. The radius r of the hemispherical measurementsurface shall be equal to or greater than twice thecharacteristic distance d o, or 4 times the average distanceof the geometric centre of the source from the reflectingplane, whichever is the larger, and not less than 1 m. Theradius of the hemisphere is preferably in the series 1, 2, 4,6, 8, 10, 12, 14, or 16 m. Some of these radii may be so

large that the environmental requirements given inAppendix A of this standard cannot be complied with; suchlarge values of the radii may not be used.

5.1.4.3 Parallelepiped and conformal measurement surfaces. If a hemispherical measurement surface is notselected, either a rectangular parallelepiped or conformalsurface may be selected as the shape of the measurementsurface. The measurement distance d is the perpendiculardistance between the reference box and the measurementsurface. The preferred value of d is 1 m and it must be atleast 0.25 m. The value of d shall preferably be one of thefollowing: 0.25, 0.5, 1, 2, 4, or 8 m. Measurementdistances larger than 1 m may be used only if the

environmental requirements given in Appendix A of thisstandard are complied with.

5.1.4.4 Additional considerations concerning choice of measurement surface. For machines usually mountedand/or to be measured in rooms or spaces underunfavourable acoustical conditions, e.g. many reflectingobjects and high levels of background noise, the selectionof a small measurement distance is appropriate and usuallydictates the selection of a parallelepiped or conformalmeasurement surface. For machines usually mounted and/orto be measured in large open areas under satisfactoryacoustical conditions, a large measurement distance isusually selected and in this case the hemispherical

measurement surface is preferred.If the dimensions of the reference box, l1, l2 and l 3, are allless than 1.0 m, the hemispherical surface is preferred. If one or more dimensions of the reference box exceeds1.0 m, but the reference box is ‘approximately cubical’ inshape, the hemisphere is still preferred. ‘Approximatelycubical’ means that the largest dimension is less than twicethe smallest dimension. If any dimension of the referencebox exceeds 1.0 m and the reference box is notapproximately cubical in shape, then the parallelepiped (orconformal) measurement surface is preferred.

5.1.5 Key microphone positions on the hemisphericalmeasurement surface. The microphone positions lie on

the hypothetical hemispherical surface of area S = 2"r2

,enveloping the source and terminating on the reflectingplane. The centre of the hemisphere is the projection of thegeometric centre of the reference box on the reflecting

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AS 1217.5—1985 12

plane. The key microphone positions of the hemisphericalsurface are shown in Figs B1 and B2 in Appendix B.Fig. B2 prescribes the locations of ten key microphonepositions each associated with equal areas on the surface of the hemisphere of radius r. The hemispherical array of Figs B1 and B2 has been selected to minimize the errorswhich can be caused by interference between the soundwave reaching the microphone directly and the wavereflected by the reflecting plane.

5.1.6 Key microphone positions on the parallelepipedmeasurement surface. The microphone positions lie onthe measurement surface, a hypothetical surface of area S enveloping the source whose sides are parallel to the sidesof the reference box and spaced out a distance d (measurement distance) from the box. The key microphonepositions on the parallelepiped measurement surface areshown in Fig. C1 in Appendix C. The area S of themeasurement surface is given by the following equation:

S = 4 (ab + bc + ca)

where

a = 0.5l1 + d b = 0.5l2 + d

c = l3, + d

where l1, l 2 and l3 are the length, width and height of thereference box.

Fig. C1 prescribes the sites of nine key microphonepositions.

5.1.7 Key microphone positions on the conformalmeasurement surface. The microphone positions lie onthe measurement surface, a hypothetical surface of area S enveloping the source which consists of an enclosureformed by a rectangular parallelepiped with roundedcorners, the corners being formed by portions of cylinders

and spheres. The measurement distance d is the distancefrom the perpendicular side of the measurement surface tothe reference box. The conformal surface is that surfacewhich is defined as being located everywhere a distance d from the nearest point on the envelope of the reference box(see Fig. D1 in Appendix D) . The eight key microphonepositions are shown in Fig. D2 in Appendix D.

The area S of the measurement surface is givenapproximately by the following equation:

S =

where a, b, c and d are the same as for the rectangularparallelepiped.

5.2 ADDITIONAL MICROPHONE POSITIONS ONMEASUREMENT SURFACE.

5.2.1 General. In Clause 5.1, the key microphone positionson the three alternative measurement surfaces aredescribed. Sound pressure level measurements shall alwaysbe obtained at all of the key microphone positions unlessthe criterion of Clause 5.3 is satisfied. Sound pressure levelmeasurements may be required at additional microphonepositions if—

(a) the range of sound pressure level values measured atthe key microphone positions, i.e. the difference indecibels between the highest and lowest soundpressure levels, exceeds the number of key measure-

ment points;

(b) if any dimension of the reference box is larger than2d ;

(c) if the source radiates noise with a high directivity; or

(d) if the noise from a large source is radiated only froma small portion of the source, e.g. the openings of anotherwise enclosed machine.

If condition (a) or condition (b) exists, additional

microphone positions defined in Clause 5.2.2 shall be used.If condition (c) or condition (d) exists, additionalmeasurement positions on the measurement surface in theregion of high noise radiation shall be used (see Clause5.2.4).

5.2.2 Positions of additional microphones.

5.2.2.1 Hemispherical measurement surface. For themicrophone arrayon the hemisphere, an additional 10-pointarray is defined by rotating the original array of Fig. B2through 180 degrees about the z-axis. Note that the toppoint on the z-axis of the new array is coincident with thetop point of the original array. The total number of microphone positions is increased from 10 to 20.

5.2.2.2 Rectangular parallelepiped. For the rectangularparallelepiped array, the additional microphone positionsare shown in Fig. C1 in Appendix C. The total number of microphone positions is increased from 9 to 17.

5.2.2.3 Conformal surface. For the conformal surface, theadditional microphone positions are shown in Fig. D2 inAppendix D. The total number of microphone positions isincreased from 8 to 16.

5.2.3 Sufficiency of number of microphone positions. Ingeneral, the key microphone positions defined in Clauses5.2.2.1, 5.2.2.2 and 5.2.2.3 are sufficient for the purposesof measurements obtained according to this standard. The

only exception is for very large sources, i.e. those forwhich the characteristic distance d o exceeds 5.0 m. In thiscase, additional microphone positions are required. Theyshould be uniformly distributed over the measurementsurface and preferably not more than 2d apart.

5.2.4 Additional localized microphone positions onmeasurement surface. If condition (c) or condition (d) of Clause 5.2.1 exists, a detailed investigation may benecessary of the sound pressure levels over a restrictedportion of the measurement surface. The purpose of thisdetailed investigation is to determine the highest andlowestvalues of the sound pressure level in the frequency bandsof interest. The additional microphone positions will

usually not be associated with equal areas on themeasurement surface. In this case, the calculationprocedure of AS 1217.6, Clause 5.7.1.2 (unequal areas), forthe determination of LW shall be used.

5.3 REDUCTION OF NUMBER OF MICROPHONEPOSITIONS. For sources that produce a symmetricalradiation pattern, it may be sufficient to distribute themeasurement points over only a portion of themeasurement surface. This is permissible if preliminaryinvestigation shows that the surface sound pressure levelsdetermined according to the calculation procedures of Section 6 do not deviate by more than 1 dB from thosedetermined from measurements over the entire measure-ment surface.

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5.4 CONDITIONS OF MEASUREMENT.

5.4.1 General. Environmental conditions may have anadverse effect on the microphone used for themeasurements. Such conditions, e.g. strong electric ormagnetic fields, wind, impingement of air discharge fromthe equipment being tested, high or low temperatures, mustbe avoided by proper selection or positioning of the

microphone. The microphone shall always be oriented sothat the angle of incidence of the sound waves is that forwhich the microphone is calibrated.

The sound pressure level shall be observed over a typicalperiod of operation of the source. Readings of the soundpressure level (corresponding to the level of themean-square sound pressure) shall be taken at eachmeasurement point with A-weighting and for eachfrequency band within the frequency range of interest. Theinstrumentation used must comply with the requirements of Section 3.

The following data shall be obtained:

(a) The A-weighted sound pressure levels and band

pressure levels during operation of the source undertest.

(b) The A-weighted sound pressure levels and bandpressure levels produced by the background noise.

For the frequency bands centred on or below 160 Hz, theperiod of observation shall be at least 30 s. ForA-weighting and for the frequency bands centred on orabove 200 Hz, the period of observation shall be at least10 s.

5.4.2 Measurements with sound level meter. If theindicating meter of a sound level meter is used, thetime-weighting ‘S’ characteristic shall be used. Where thefluctuations of the indicating pointer on the sound levelmeter are less than ± 3 dB using the time-weighting ‘S’characteristic, the noise is considered to be steady for thepurposes of this standard, and the level is taken to be theaverage of the maximum and minimum levels during theperiod of observation. If the meter fluctuations during theperiod of observation are greater than ± 3 dB, the noise is

considered non-steady and one of the instrumentationsystems described in Appendix E shall be used.

For the detection of impulsive noise an additionalmeasurement with the time-weighting characteristic ‘I’ of the sound level meter shall be performed. The procedure of Appendix F shall be used.

5.4.3 Measurements with RC-smoothing or integrating

systems. If RC-smoothing is used, the time constant !Ashould be long enough to obtain an estimate of the r.m.s.level during the period of observation with a precision of ±0.5 dB.

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

For time-varying noise, it is important to specify carefullythe period of observation, and this will usually depend onthe purpose of the measurements. For example, if themachine has two particular modes of operation havingdifferent noise levels, it may be necessary to select aperiod of observation for each mode.

5.4.4 Corrections for background noise. The measured

sound pressure levels shall be corrected for backgroundnoise according to Table 5.1.

TABLE 5.1CORRECTIONS FOR BACKGROUND NOISE

Difference between soundpressure level measured

with sound sourceoperating and backgroundsound pressure level alone

dB

Corrections to besubtracted from sound

pressure level measuredwith sound source

operating to obtain soundpressure level due to sound

source alonedB

10

Measurements invalid1.01.0

1.00.50.50.0

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AS 1217.5—1985 14

SECTION 6. CALCULATION OF SURFACE SOUNDPRESSURE LEVEL, SOUND POWER LEVEL, AND

DIRECTIVITY FACTOR

6.1 CALCULATION OF SOUND PRESSURE LEVELA VER AG ED OV ER T HE M EA SU RE ME NTSURFACE. For the A-weighted sound pressure level and

the level in each frequency band of interest, an averagesound pressure level over the measurement surface Lpm iscalculated from the measured sound pressure levels Lpi(after corrections for background noise are appliedaccording to Clause 5.4.4, if necessary) by using thefollowing equation:

.... (1)

where

Lpm = sound pressure level averaged over themeasurement surface, in decibels(Reference: 20 µPa)

Lpi = A-weighted or band pressure levelresulting from the ith measurement, indecibels (Reference: 20 µPa)

N = total number of measurements.

Note: Where the range of values of Lpi does not exceed 5 dB, a simplearithmetic average will differ by not more than 0.7 dB from the valuecalculated using the energy average, formula (1).

6.2 CALCULATION OF SURFACE SOUNDPRESSURE LEVEL. The surface sound pressure level Lpf shall be obtained by correcting the value of Lpm forreflected sound to approximate the average value of thesound pressure level which would be obtained underfree-field conditions, by using the following equation:

Lpf = Lpm # K .... (2)

where Lpm = surface sound pressure level, in decibels

(Reference: 20 µPa)

K = mean value of the environmental correction overthe measurement surface, in decibels.

For the purposes of this standard, the maximum allowablerange of the environmental correction, K , is -2 d B to+2 dB.

NOTE: The environmental correction K accounts for the influence of a non-ideal environment, e.g. the presence of sound absorption orreflected sound. It typically ranges from -2 dB (for measurementsoutdoors with absorbing ground) to + 10 dB (for measurements indoorsin highly reverberant rooms). The procedures given in Appendix A areused to calculate the value of the environmental correction.

6.3 CALCULATION OF SOUND POWER LEVEL.The sound power level characterizing the noise emitted bythe source shall be calculated from the following equation:

LW = Lpf + 10 log (S /So) .... (3)

where LW = A-weighted or band power level of the

source, in decibels (Reference: 1 pW) LPf = surface sound pressure level determined

according to Clause 6.2, in decibels(Reference: 20 µPa)

S = area of the measurement surface, in squaremetres

So = 1 m2

6.4 CALCULATION OF DIRECTIVITY INDEX ANDDIRECTIVITY FACTOR. If required, values of the

directivity index and directivity factor may be calculatedusing the procedures of Appendix G.

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15 AS 1217.5—1985

SECTION 7. INFORMATION TO BE RECORDED

7.1 GENERAL. The following information, whereapplicable, shall be compiled and recorded for allmeasurements made according to this standard.

7.2 SOUND SOURCE UNDER TEST. The followingshall be recorded:

(a) Description of the sound source under test (includingits dimensions and the dimensions of the referencebox).

(b) Operating conditions.

(c) Mounting conditions.

(d) Position of sound source in test environment.

(e) If the test object has multiple noise sources, adescription of the source(s) in operation during themeasurements.

7.3 ACOUSTIC ENVIRONMENT. The following shallbe recorded:

(a) Description of the test environment; if indoors,description of physical treatment of walls, ceiling andfloor; sketch showing the position of source and roomcontents; if outdoors, sketch showing the position of source with respect to surrounding terrain, includingphysical description of test environment.

(b) Acoustical qualification of test environment accordingto Appendix A.

(c) Air temperature in degrees Celsius, barometricpressure in millibars, and relative humidity.

(d) Wind speed and direction.

(e) Sound power output of the reference sound source, if

used.

7.4 INSTRUMENTATION. The following shall berecorded:

(a) Equipment used for measurements, including name,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 any).

7.5 ACOUSTICAL DATA. The following shall berecorded:

(a) The shape of the measurement surface, the

measurement distance, the site and orientation of microphone positions or paths.

(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 accordingto one of the procedures of Appendix A.

(e) The surface sound pressure level Lpf , in decibels,expressed in terms of a weighted level (with A, orother weighting) and in terms of a level in each

frequency band of interest; reference 20 µPa.

(f) The sound power level L W, in decibels, calculatedfrom the surface sound pressure level for A-weighting(or other weighting) and for all frequency bands used;reference 1 pW.

(g) Remarks on subjective impression of noise (audiblediscrete tones, impulsive character, spectral content,temporal characteristics, etc).

(h) If measured, the difference between time-weighting ‘ I ’and time-weighting ‘S ’ readings of the sound pressurelevel at the selected microphone positions (seeAppendix F).

(j) The date and time when the measurements wereperformed.

7.6 INFORMATION TO BE REPORTED. The reportshall contain the statement that the sound power levelshave been obtained in full compliance with this standard.The report shall state that these sound power levels aregiven in decibels re 1 pW.

Only those data recorded according to the requirements of Section 7 and which are required for the purpose of themeasurements shall be reported.

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AS 1217.5—1985 16

APPENDIX A

QUALIFICATION PROCEDURES FOR THE ACOUSTICENVIRONMENT

A1 GENERAL. An environment providing a free field over a reflecting plane shall be usedfor measurements made according to this standard. This environment may be ahemi-anechoic room, an outdoor space or an ordinary room if the requirements given below

are complied with.The test room shall be large enough and, if possible, free from reflecting objects with theexception of the reflecting plane. The test room shall provide a measurement surface thatlies—

(a) inside a sound field that is free of undesired sound reflection from the roomboundaries; and

(b) outside the near field of the sound source under test.

For open test sites which consist of a hard, flat ground surface such as asphalt or concreteand with no sound-reflecting obstacles within a distance from the source equal to 3 timesthe greatest distance from the source centre to the lower measurement points, it may beassumed that the environmental correction K is less than or equal to 0.5 dB and is,therefore, negligible.

NOTE: An obstacle in the proximity of the source may be considered to be sound reflecting if its width, e.g.diameter of a pole or supporting member, exceeds 10 percent of its distance from the reference box.

The environmental correction K may also be assumed to be negligible for indoorenvironments which are laboratory anechoic rooms complying with the requirements of Appendix A of AS 1217.6.

The evaluation of environmental influences is performed by selecting one of two alternativeprocedures used to determine the magnitude of the environmental correction K . Theseprocedures are used to determine if any undesired environmental influences are present andto qualify a given measurement surface for an actual source under test according to thisstandard.

The first qualification test (absolute comparison test, see Paragraph A3) is carried out witha reference sound source. The second qualification test (reverberation test, seeParagraph A4) may be used if the source under test cannot be moved or if its dimensionsare large. This test requires measurements of reverberation time.

The free-field qualification on a given measurement surface is satisfied by a given test roomif the ratio of the sound absorption A of the room to the area S of the measurement surfaceis sufficiently large. In general, ratios A/S > 10 require no environmental corrections. Forratios A/S between 10 and 6, an environmental correction K can be determined accordingto the procedures given in this Appendix. In this case, K is usually smaller than 2 dB. Forratios A/S < 6, the room correction factor K may exceed 2 dB and cause greater uncertaintyfor the sound power level determinations than those given in Table 1.1. In such cases, asmaller measurement surface, a better environment or a measurement according to thesurvey method (see AS 1217.7) should be chosen. The reflecting plane should comply withthe requirements given in Paragraph A2.1.For outdoor measurements, additional precautionsgiven in Paragraph A2.2 should be considered.

A2 ENVIRONMENTAL CONDITIONS.

A2.1 Properties of reflecting plane.A2.1.1 General. Measurements may be made in one of the following environments:

(a) Over a reflecting plane outdoors.

(b) In a test room with one reflecting surface.

(c) In a test room with sound absorptive surfaces in which a reflecting plane is present.

NOTE: Particularly where the reflecting surface is not a ground plane or is not an integral part of a test roomsurface, care should be exercised to ensure that the plane does not radiate any appreciable sound due tovibrations.

A2.1.2 Shape and size. The reflecting plane should extend at least $/2 beyond the projectionof the measurement surface on the plane for the lowest frequency of the frequency rangeof interest.

A2.1.3 Absorption coefficient. The reference plane should have acoustical characteristics

approximating those of a perfect acoustical reflector over the frequency range of interest.For indoor measurements, a concrete floor is usually satisfactory. For outdoormeasurements, a concrete or a smooth sealed asphalt surface should be satisfactory.

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A2.2 Precautions for outdoor measurements. Care should be taken to minimize the effectsof adverse meteorological conditions, e.g. temperature, humidity, wind, precipitation, on thesound propagation over the frequency range of interest or on the background noise duringthe course of the measurements.

If a device is used to shield the microphone from the effects of wind, proper corrections of the measured sound pressure levels shall be made.

A3 ABSOLUTE COMPARISON TEST.

A3.1 Procedure. A reference sound source with characteristics that comply with ISO 6926shall be mounted in the test environment in essentially the same position as that of thesource under test. The sound power level of the reference sound source is determinedaccording to the procedures of Sections 5 and 6 without the environmental correction K , i.e.K is initially assumed equal to zero. The same measurement surface is used as during themeasurements of the source under test. The environmental correction K is given by thefollowing equation:

K = L W # L Wr

where

LW = calculated A-weighted or band power level of the reference sound source usingprocedures of Sections 5 and 6 (with K = 0 in equation (2)), in decibels(Reference: 1 pW)

LWr = nameplate A-weighted or band power level of the reference sound source, indecibels (Reference: 1 pW)

A3.2 Positions of reference sound source in test environment.

A3.2.1 Source can be removed from test site. The reference sound source is to be positionedon the reflecting plane, independent of the height of the machine. One single position issufficient, even when very large machines are to be tested, provided that the ratio of thelength of the machine under test to its width is not greater than 2. If the ratio is greaterthan 2, the reference sound source shall be operated on the floor at four points. Assumingthe projection of the machine under test on the floor to be approximately rectangular inshape, the four points are positioned at the middle point of the sides of the rectangle. Toobtain LW the surface sound pressure level L pf shall be calculated with the reference soundsource positioned at each of the four points on the floor. At each point on the measurementsurface, the sound pressure level shall be averaged for the four source positions on amean-square basis, i.e. using equation (1) in Clause 6.1.

A3.2.2 Source cannot be removed from test site. The reference sound source is to bepositioned on the upper surface of the machine which should preferably be acousticallyreflective. This method should not be applied if the machine has highly absorptive surfaces,e.g. textile machines. In this case, the procedure of Paragraph A4 should be followed.

A3.3 Qualification requirements. For the measurement surface in a given test environmentto be satisfactory for measurements according to the requirements of this standard, theenvironmental correction K must be numerically less than or equal to 2 dB. If theenvironmental correction K exceeds 2 dB, either a smaller measuring surface or a better testenvironment is required. The procedure shall be repeated. Alternatively, procedures usinga reference sound source may be used or the guidelines given in AS 1217.7 may befollowed to determine the sound power levels of the source under test.

NOTE: Measurement procedures using a reference sound source, may yield sound power levels within the limitsfor the measurement uncertainty specified in Clause 1.4 of this standard, even for values K > 2.

A4 REVERBERATION TEST.A4.1 Test procedure. This test procedure is applicable to rooms that are approximatelycubical in shape.

The environmental correction K is obtained from the following equation:

The value of K may be obtained from Fig. A1 by entering the abscissa with the appropriatevalue of A/S . The area S of the measurement surface is calculated according to therequirements given in Clause 5.1.

The total sound absorption A of the test room is determined from measurements of thereverberation time of the test room in octave bands for the entire frequency range of interest(see AS 1045).

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AS 1217.5—1985 18

The value of A is then—

A = 0.16(V /T )

where

V = volume of the test room, in cubic metres

T = reverberation time of the test room in octave band, in seconds.

A4.2 Qualification requirements. For the measurement surface in a test room to be

satisfactory for measurements according to this standard, the ratio A to S should exceed 6,i.e. A/S > 6.

If the above requirement cannot be complied with, a new measurement surface shall bechosen. The new measurement surface should have a smaller total area, but still lie outsidethe near field (see Clause 1.4). Alternatively, the ratio A/S may be increased by introducingadditional sound-absorptive materials into the test room and then redetermining the valueof the ratio A/S under the new conditions.

If the requirements of this Paragraph cannot be complied with for any measurement surfacewhich lies outside the near field of the source under test, the particular environment chosencannot be used for measurements on the source under test according to the requirements of this standard. A new test environment must be selected or it must be assumed that theuncertainties may exceed the values given in Table 1.1.

Fig. A1. ENVIRONMENTAL CORRECTION

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APPENDIX B

MICROPHONE ARRAY ON THE HEMISPHERE

The positions of ten points associated with equal areas on the surface of a hemisphere of radius r are numbered from 1 to 10 in Figs B1 and B2. The Cartesian coordinates ( x, y, z)with the origin centred on the vertical projection of the centre of the reference box on thereflecting plane are also given in tabular form in Table B2. The additional points on the

hemisphere are numbered from 11 to 20 in the same figure.

NOTES:

1. The overhead position (No 10) may be deleted for safety reasons, if it can be shown by preliminaryinvestigation that the exclusion of the overhead position does not influence the calculated sound power levelof the source by more than 1.0 dB. If this position is deleted, positions 1 to 9 must remain.

2. If the source emits predominant pure tones, strong interference effects may occur if several microphonepositions are placed at the same height above the reflecting plane. In such cases, the use of a microphonearray with the coordinates given in Table B1 is recommended.

TABLE B1COORDINATES OF MICROPHONE ARRAY FOR PURE SOUNDS

No x

r yr

z r

12

3456789

10

0.160.78

0.780.16

-0.83-0.83-0.26

0.74-0.26

0.10

-0.96-0.60

0.550.900.32

-0.40-0.65-0.07

0.50-0.10

0.220.20

0.310.410.450.380.710.670.830.99

Fig. B1. HEMISPHERICAL MEASUREMENT SURFACE — KEY MEASUREMENT POINTS

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AS 1217.5—1985 20

TABLE B2COORDINATES OF KEY MEASUREMENT POINTS

No x

r yr

z r

123

456789

10

-0.990.500.50

-0.45-0.45

0.890.33

-0.660.330

0-0.86

0.86

0.77-0.77

0-0.57

00.57

-0

0.150.150.15

0.450.450.450.750.750.751.0

Fig. B2. MICROPHONE ARRAY ON THE HEMISPHERE

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APPENDIX C

MICROPHONE ARRAY ON THE PARALLELEPIPEDMEASURING SURFACE

The coordinates of the key measurement points given in Table C1 and Fig. C1 aredetermined by the following:

a = 0.5l1 + d

b = 0.5l2 + d

c = l3 + d

h = C # c

where

l1, l 2 and l 3 = dimensions of the reference box

d = measurement distance, normally 1 m.

The height h shall be chosen uniformly for a series of similar sources. The minimum heightof the microphone above the reflecting plane shall be 0.15 m.

For specific kinds of sound sources, the constant C or the height h may be specified in theappropriate test method as one of the following values:

C = 0.5 (In the absence of specifications, C = 0.5 shall be used.)C = 0.5l3/c (the centre of the source)

h = shaft height for rotating machines

h = 1.20 m

NOTES:

1. The overhead position (No 9) may be deleted for safety reasons, if it can be shown by preliminaryinvestigation that the exclusion of the overhead position does not influence the calculated sound power levelof the source by more than 1.0 dB. If this position is deleted, positions 1 to 8 must remain.

2. The number of measurement points may be reduced to five (Nos 1, 2, 3, 4, 9 of Fig. C1 ) if the surfacesound pressure level so determined does not deviate by more than 1.0 dB from that determined frommeasurements at the nine key measurement points of Fig. C1 (see Clause 5.3).

TABLE C1COORDINATES OF KEY MEASUREMENT POINTS

No x y z

123456789

a0

-a0a

-a-aa0

0b0

-bbb

-b-b0

hhhhccccc

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AS 1217.5—1985 22

Fig. C1. MICROPHONE ARRAY ON THE PARALLELEPIPED

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APPENDIX D

MICROPHONE ARRAY ON THE CONFORMALMEASUREMENT SURFACE

The coordinates of the microphone positions given in Table D1 and Fig. D2 are determinedby the following:

a = 0.5l1 + d b = 0.5l2 + d

c = l3 + d

h1 = 0.25(b + c - d )

h2 = 0.75(b + c - d ) $ c

b1 = 0.5(b + c - d ) $ b

where

d = measurement distance, normally 1 m.

The minimum height of the microphone above the ground shall be 0.15 m.

The additional points on the conformal surface are numbered from 9 to 16 on Fig. D2.

TABLE D1COORDINATES OF KEY MEASUREMENT POINTS

No x y z

12345678

a0

-a0

a/2-a/2-a/2a/2

0b0

-bb1b1

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AS 1217.5—1985 24

Fig. D1 CONFORMAL SURFACE AT A DISTANCE d FROM REFERENCE BOX

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Fig. D2. MICROPHONE ARRAY ON THE CONFORMAL SURFACE

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APPENDIX E

EXAMPLES OF SUITABLE INSTRUMENTATION SYSTEMS

E1 GENERAL. Basically, the instrumentation system consists of a microphone, anamplifier with filters, a squaring and averaging circuit and an indicating device. There areseveral methods of processing or conditioning the filter outputs that may be used to obtainan estimate of the mean-square value of the output. These include use of detection

equivalent to RC-smoothing, integration of the squared value of the filter outputs and digitalmethods. Some general aspects are described below.

E2 RC-SMOOTHING, SOUND LEVEL METER. Many analogue devices, including thesound level meter according to AS 1259, employ RC-smoothing.

For the sound level meter set on time-weighting characteristic ‘S ’, the average value of themeter deflection approximates the mean-square sound pressure level if the fluctuations areless than 5 dB.

The microphone on the sound level meter shall have a uniform frequency response at theangle of incidence specified by the manufacturer. A capacitor microphone with a diameterof 13 mm will be suitable for this purpose. The microphone and its associated pre-amplifier(if any) shall be placed in the test room and connected with the sound level meter by acable that complies with the requirements of Clause 3.2. The system shall be calibrated withthe cable inserted between pre-amplifier and sound level meter.

The sound level meter and the observer shall be placed in a room adjacent to the test room.The meter shall be set on time-weighting characteristic ‘S ’ and the readings taken asdescribed in Clause 5.4.2.

Other analogue devices can provide smoothing with longer time-constants and shall be usedif the fluctuations exceed 5 dB.

E3 ANALOGUE INTEGRATORS. Another approach to r.m.s. detection is the ‘true’analogue integrator that computes (approximately) the integral.

where e o(t) is the filter output. The square and square roots are usually accomplished bynon-linear analogue elements. The integral may be computed either by conversion of e o(t)

to a current and accumulation of charge on a capacitor, or by counting the number of cyclesin a signal whose frequency is proportional to e o(t).

E4 DIGITAL SYSTEMS. The r.m.s. value of the filter outputs may be determined bysampling, conversion to digital values, squaring and accumulating the results. The samplingrate can be either —

(a) high compared with highest frequency present in the filter output; or

(b) relatively low compared with the highest frequency present so that the resulting samplesare (approximately) statistically independent.

In either case, the output of the detector after a specified time-interval should be within3 percent of the true r.m.s. value of the time function for all frequencies within thefrequency range of interest.

E5 LEVEL RECORDERS. A level recorder may be used either as a squaring, averaging

and indicating device or exclusively as an indicating device.In the first case, the time-constant of the instrumentation system is determined by thewriting speed of the level recorder. Since the level recorder is a complicatedelectromechanical system, a simple rule for the determination of the resulting time-constantcannot be given. It is advisable to consult the manufacturer in this matter.

If the level recorder is used for indication only, the recorder will normally be set forrecording of the d.c. output of a preceding squaring and averaging device, the time-constantof which will determine the resulting time-constant of the instrumentationsystem.

In both cases, the average value obtained will be an acceptable approximation to the r.m.s.value only if the pen fluctuations are less than 5 dB.

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APPENDIX F

GUIDELINES FOR THE DETECTION OF IMPULSIVE NOISE

In many cases, comparison of the readings of a sound level meter on time-weightingcharacteristic ‘S ’ with that of the sound level meter on time-weighting characteristic ‘ I ’(according to AS 1259) may be helpful in determining whether or not the noise containsimportant impulsive components. For this purpose, the comparison is made at one or moreof the microphone positions, the ‘A’ frequency weighting is applied and the period of observation at each position shall be at least 3 s with both time-weighting characteristics.At least the mean value of the differences between the readings at the chosen microphoneposition is to be given in the test report.

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APPENDIX G

CALCULATION OF DIRECTIVITY INDEX AND DIRECTIVITYFACTOR USING HEMISPHERICAL MICROPHONE ARRAY

The presence of a hard reflecting plane modifies the directivity pattern of a source. Thedirectivity index and directivity factor may be obtained by considering the reflecting planeto be part of the source. The directivity index DI of the source may then be calculated, in

decibels, from measurements in a free field above a reflecting plane by using the followingequation:

DI = Lpi - Lpf + 3 .... (3)

where

Lpi = sound pressure level measured in the particular direction in which DI isdesired, at a distance r metres from the centre of the hemisphericalmeasurement surface, in decibels (Reference: 20 µPa)

Lpf = surface sound pressure level over the hemispherical measurement surface of radius r, in decibels (Reference: 20 µPa)

NOTE: In the report of the measurements, it may be sufficient to include only the highest valueof DI and the direction in which it occurs. The directivity factor Q of the source, in a givendirection, may be determined from the following equation:

Q = 100.1 DI .... (4)

where

DI = directivity index in the same direction obtained from equation (3) in decibels.

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