Bruel & Kjaer Taiwan Co. Ltd. - resheji.com · Page 2 MIRL_SPW, 7 Sound Power and Sound intensity...

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產品聲功率測試方法與法規說明

必凱科技股份有限公司

Bruel & Kj aer Taiwan Co. Ltd.汪龍飛

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(1) Basi c Parameters of Sound(2) Sound Power Measurement Method(3) Pressure-B ased IS O STD(4) Diffuse Field Methods(5) Free Field Methods(6) Int ensity-B ased IS O STD(7) Sound Pressure Measurement Method(8) Information Technology ISO Noise Test Code(9) The Current Practice(10) The Instrument Solution

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Basic Parameters of Sound

Sound Power, P [Watts]: -Lw(Bel) The rate per unit time at which airborne sound energy is radiated by asource.

Sound Intensity, I [W/m2]: -Li(dB) The rate of acoustic energy flow per unit area.

Sound Pressure, p [Pa]: -Lp(dB)A fluctuating pressure superimposed on the static pressure by the presence of sound.

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Basic Parameters of Sound

Under free-field conditions:

Power: P [W] = J/s

Intensity: I [J/s/m2] = W/m2

Pressure: p [Pa = N/m2]

The Sound Intensity vector, I,describes the amount and di rectionof flow of acoustic energy at agiven position

I = =P

rp

c4 2

2

p rP

P2

P2

I1

r1

r2

I2

Æ

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Sound Source :

Power P [W] Pressure p [N/m2]Electrical Heater :

Power P [W] Temperature t [°C]

Sound Pressure vs. Sound Power

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Pressure vs. Power

l Sound Pressure

- Is dependent on the acoustic environment

- Is the product of the sound source(s) and the acoustic environment

l Sound Power

- Is independent of the acoustic env ironment

- Is therefore a good parameter for making comparisons

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Sound Power and Sound intensity

Sound Power, W (Watts): The rate per unit time at which airborne sound energy is radiated by a source

Sound Intensity, I (W/m2): The rate of acoustic energy flow per unit area

Sound Power Level LW (Bel):Lw = log ,p0 = 1pW

Sound intensity Level L i (dB) =Sound Pressure Level L p (dB)(in Free field and about 20oC, 1000mbar)

dSWS nIÚ=

pp

0

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Why measure Sound Power?

l Giv en a knowledge of a sound source and the acoustic environment, to predict the sound pressure lev el due to the source at a givendistance in that environment

l To compare the noise emissions of sound sources of the same and different types

l To determine whether a sound source complies with noise specifications (legislation, standards)

l In engineering work, to assist in dev eloping quiet machinery andequipment

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Sound power measurement

l It can be determined either based on sound pressure or sound intensity

l Precision and engineering methods of sound power determination based on sound pressure can take place in a diffuse or f ree sound field.

l It is possible to reproduce a diffuse sound field in a reverberation room, and a free sound f ield in an anechoic or semi-anechoic chamber, (and an almost f ree field outdoors or in a large room)

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Why Have Standards?

l Acoustic measurements are highly standardised

- To make sure people use the same methods (GIGO)

- To simplify comparison of results

l Three grades of standards for sound power determination

Precision (most accurate)Engineering (medium accurate)Survey (least accurate)

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Measurement Standards and Test Codes

l Measurement Standards (B standards) :

- are generic measurement standards- define how measurements should be made- Covering a broad range of products- Prescribing the instrumentation to be used

l Noise Test Codes (C standards) :

- are specific to a particular type of equipment- define things like mounting, operating conditions, which measurements

are to be made- refer to measurement standards to define how measurements are to be

made- Describing possible special adaptation (additions and exceptions to the basic standards)

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Sound Power ISO Measurement Standards

· Sound power determination

-Sound pressure based

» Free sound f ield

- Anechoic or Semi-Anechoic chamber (ISO 3745,precision)- Almost free-field (ISO 3744,engineering)

» Diffuse sound field- Reverberation room (ISO 3741/42,precision)- Special reverberation room (ISO 3743,engineering)

-Sound intensity based» Point measurements (ISO 9614-1,precision ,engineering & Survey )

» Scanned Measurements (ISO 9614-2,engineering & Survey)

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How to Measure Sound Power

l Calculate it by measuring the sound pressure in a known acoustic env ironment in time and space averaged (pressure-based)

- Free sound field (anechoic or semi-anechoic chamber)

- Almost free field, (outdoors or in a large room)

- Diffuse sound field, (reverberation room)

l Measure it directly using the sound intensity techniques (Intensity-based) P = I * S

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Pressure-based vs. Intensity-based Methods

l Pressure-based

- Simple to f ollow

- Requires special test environment» (Reverberation ,Anechoic or Semi-anechoic chamber)

l Intensity-based

- Requires more engineering knowledge and skill

- No special test environment involved

- Limitation in frequency and low noise level

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(1) Basi c Parameters of Sound(2) Sound Power Measurement Method(3) Pressure-Based ISO STD(4) Diffuse Field Methods(5) Free Field Methods(6) Int ensity-B ased IS O STD(7) Sound Pressure Measurement Method(8) Information Technology ISO Noise Test Code(9) The Current Practice(10) The Instrument Solution

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Overview of ISO Pressure-based Sound Power Standards

In ternationalStandard

Classi fication ofM ethod

Type of Method 1) T est Environment

ISO 3741 Precision(Grade 1)

Direct or comparison Reverberati on roomm eet ing specifiedrequirements

ISO 3742ISO 3743-1 Engineeri ng

(Grade 2)Comparison Hard-walled test room

ISO 3743-2 Direct or comparison Special reverberat iontest room

ISO 3744 Direct Outdoors or in largeroom

ISO 3745 Precision(Grade 1)

Direct Anechoic or semi-anechoic room

ISO 3746 Survey(Grade 3)

Direct Outdoors or indoors(i.e. no special testenvironment)

ISO 3747 Direct or comparison No special testenvironment; sourceunder test notm oveabl e

1) Direc t: The sound power l eve l i s cal culat ed solely from the measure d sound pressurelevel s produced by the source

Comparison: The sound power l eve l i s cal culat ed by compa ring the me asured sound pressurelevel s produced by the source in the te st room with the sound pressure l evelsproduced in the same room by a referenc e source of known sound powe r output .

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Pressure Methods of Sound Power Determination l Measuring

- Put the dev ice under test in a special test environment reproducing a diffuse or free sound field.- Under the specified operating condition , get the time & space averaged sound pressure lev el.- Correcting the averaged sound pressure level from the environment , back ground noise or temperature & humidity .- Calculate the sound power by the sound pressure & sound power relation formula

l Why it can be measured

- We know the mathematics of a diffuse or free sound field.

- From sound pressure measurements, we can derive the sound intensity, and thereafter the sound power



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Free-field vs. Diffuse-field methods

l Free-field methods

- Almost free-field method easy to implement

- Test environment can be used for other purposes, for example, noise source location, directivity measurements

- Longer measurement time when using multiplexed microphone systems

l Diffuse-field methods

- Shortened measurement time when using rotating microphone boom

- other uses of test environment are limited

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Operating Condition for testing

l With test code , follow the relevant test codel No test code , One or more condition as

– Under specified load (normal)– Under full load– Under no load(idling) – Under maximum sound generation mode– …..

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Diffuse-field Methods (ISO 3741/2/3)l Adv antages

- Comparison method (using reference sound source) very simple to follow

- Fast, when rotating microphone boom is used

- ISO 3741/42 give precision measurements, (ISO 3743 gives engineering)

l Disadvantages

- Require use of a reverberation room meeting specified requirements

- Measurement of noise sources having narrow band content places additional requirements on the room

- Other uses of reverberation room are v ery limited

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Measurement Environments - Diffuse Fieldl Reverberation room -Precision method (ISO 3741/3742)

- Room with very hard surf aces- Surfaces reflect as much of the sound back into the room as possible- The room size : >200 m3 corresponds to 100Hz ,or >100 times the source volume . <300 m3 above 3000Hz.- For Discrete frequency/Narrow-band noise , need more microphone traverse (more than 3 m path)or microphone array, or measuring at more source locations,diffusers (ISO 3742)

l Special reverberation room-Engineering method (ISO 3743) - less rigorous requirements on the room than for the precision method.

NOTE - Precision methods, the measurement environment has to be qualif ied according to the measurement standard ISO 3741 Annex A(Broadband) ,ISO 3742 Annex A(Discrete Frequency & Narrow-band)

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LW = Lp + 10 log (V/V0) - 10 log (T/T0) + 10 log(1+S*l/8*V) - 10 log (B/1000) - 14

Lw : The sound power level(A-weighted or in frequency bands)

Lp : The corrected sound pressure levelV : The Room Volume Size ,[m3]V0 : Reference Volume ,[1m3 ]T : Reverberation Time ,[sec]T0 : Reference Reverberation Time ,[1 sec]S : Total surface of the room ,[m2]l : Wavelength in octave or one-third octave band centre frequency, [m]

B : Barometric pressure , [mbar]

Sound Power Level by ISO 3741/42 - Direct Method

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Diffuse-field Methods - Direct Method

l Measure the reverberation time in the room

l Measure the average sound pressure in the room with the noise source under test operating. The sound pressure can be averaged using a microphone array or a rotating microphone boom

l If necessary, make a background noise correction

l Calculate the sound power based on the measured data, the physical parameters of the room, and (for precision method) ambient temperature and barometric pressure

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LW = Lp - Lpr + Lwr

L w : The sound power level(A-weighted or in frequency bands)

Lp : The corrected sound pressure levelLpr : The band pressure level of the ref erence sound source .Lwr : The band power level of the reference sound source .

Sound Power Level by ISO 3741/42 - Comparison Method

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Diffuse-field Methods - Comparison Method

l Measure the average sound pressure in the reverberation room with the noise source under test operating. The sound pressure can beav eraged using a microphone array or a rotating microphone boom .

l Measure the average sound pressure in the same room with ref erence sound source operating. A ref erence sound source is a device having a calibrated sound power.

l Calculate the sound power of the noise source under test, from the calibration chart of the reference sound source, and the difference between the measured sound pressures

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The Instrumentation for Diffuse-field Measurement

l Microphone : Condenser microphone with flat frequency response f or randomly incident sound ,ty pe 1 of IEC 651.

l Instrumentation : Ty pe 1 of IEC 651 with flat frequency response for randomly incident sound .

l Band pass filter : Octave or one - third octave filter meet IEC 225 .

l Calibrator : With 0.2 dB accuracy.

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Equipment for Diffuse Field Measurements

Reference Sound Source Type 4204

Rotating Microp hone Boo m Type 3923 Multip lexer Type 2822

Plus one or more pressure field microph ones

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System for Diffuse Field Sound Power Determination

Reference Sound Source Type 4204

Rotating Microp hone Boo m Type 3923

PULSE Mu ltianalyzer System Type 3560

Use comparison method and calculate Sound Power as PULSE PL function

Pressure field microphone Type 4192 with preamp lifer Type 2669

Sound Calibrator

Type 4231



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Acoustic Field

Nearfield

Far field

Free field Reverberant field

Lp

r1 2

6 dB

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Free-field Methods (ISO 3745)l Adv antages

- Easy to follow- Give precision measurements- Can give additional information to sound power, f or example, directivity, tonality- Room can be used for other purposes, for instance, noise sourcelocation- Just as quick as diffuse-field methods when parallel microphone measurements are used

l Disadvantages

- Require (costly) acoustic test facility- Slow when using multiplexed microphone arrays

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Almost Free-field Methods (ISO 3744)

l Adv antages

- As for free-field methods, except for accuracy- Do not require a special acoustic test facility, allow sound power determination outdoors or in a large room- Require less initial capital investment than diffuse- or f ree-field methods

l Disadvantages

- Can only give engineering grade information- Background noise a problem for low-level measurements

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Measurement Environments - Free Field (ISO 3745)l Precision method (ISO 3745)

– Anechoic Chamber - Walls absorb as much of the incident sound energy as possible

– Semi-anechoic Chamber - Precision method (ISO 3745)- Similar to anechoic chamber but with a reflecting rather than an absorbing f loor- More practical than anechoic chamber f or measurements on largerobjects

– The room size : > 200 times the source volume. – the measurement environment has to be qualified according to the

measurement standard ISO 3745 Annex A.– Back ground noise < 12 dB of the source level

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Measurement Environments - Almost Free Field

l Engineering method (ISO 3744) – Almost free-f ield (Essentially free field) - outdoors or in a large room– the greatest dimension of the source is less than 15 m .– The back ground noise level should be 6dB below the source sound

pressure level(K1 =1.3 dB),preferably 15 dB(K1 = 0.1 dB).– The measurement environment has to determine the environmental

correction K2 by comparison method or room absorption method according to the measurement standard ISO 3744 Annex A.

– The environmental correction K2Ashould be equal or less 2 dB, K2 for each frequency band NOT exceed 2 dB.

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l Measure the sound pressure at a series of pre-def ined points, on a surface enclosing the noise source under test, with the noise source operating

l Calculate the av eraged sound pressure over the surface

l If necessary, make a background noise correction -K1

l If necessary, make a correction for deviation from true free-f ield conditions --K2 (engineering method), or for temperature and barometric pressure(precision method)

l Calculate the sound powerS : the area of measurement surf ace

Lw = Lp - K1 - K2 + 10 log (S/S0) K1 : background noise correctionK2 : env ironmental correction (3744)

Sound Power Level by Free-field methods

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Measurement Surfaces for ISO 3744/3745

l r : the radius of the measurement surface should be twice or greater to the characteristic dimension do (ISO 3744)/the major source dimension (ISO 3745) ,and not less than 1 m .in ISO 3744 the radius should be 1 ,2 ,4 ,6 ,8 ,12 ,14 or 16 m.

l do : the characteristic dimensiondo = (L12+L22+L32)0.5

l Two microphone positioning :– microphone array– microphone traversing

l microphone array :– 10 key positions

Hemisphere

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Measurement Surfaces for ISO 3744 only

l Parallelepiped is only for ISO 3744l d : the measurement distance

1 m is pref erred value.0.25 m shall be at least.shall be 0.25 ,0.5 ,1 ,2 ,4 or 8 m larger than 1 m f or large source

l Microphone array : 9 key positions

Parall elepiped

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Adding Microphone Positions on Measurement Surfaces

l For ISO 3744 only ,including hemisphere and parallelepiped surface methods– the range of SPL at the key microphone positions (SPLhighest - SPLlowest)

exceeds key microphone positions– the source radiates with high directivity– the noise f rom large source is radiated only from small portion of the

source

l For ISO 3745 only , including hemisphere and sphere surface methods– the range of SPL at the key microphone positions (SPLhighest - SPLlowest)

exceeds half of key microphone positions

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The Instrumentation for Free-field Measurement

l Microphone : 13 mm Free field condenser microphone

l Instrumentation : Ty pe 1 of IEC 651,Type 1 of IEC 804


l Calibrator : 0.2 dB accuracy in ISO 3745.0.3 dB accuracy (IEC 942 class 1) in ISO 3744.

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Equipment for Free Field Measurements

Sound Calibrator Type 4231

Free field micro phone Type 4190 with preamp lifer Type 2669

PULSE Mu ltianalyzer System Type 3560 FFT & 1/3 Octave analysis

sound power sw Type 7748



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Pressure-based Sound Power Measurement used for

l Production audits

l High v olume testing

l Determination of low-level sound powers

l Where the user already has an acoustic test facility

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(1) Basi c Parameters of Sound(2) Sound Power Measurement Method(3) Pressure-B ased IS O STD(4) Diffuse Field Methods(5) Free Field Methods(6) Intensity-Based ISO STD(7) Sound Pressure Measurement Method(8) Information Technology ISO Noise Test Code(9) The Current Practice(10) The Instrument Solution

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Intensity-based Sound Power Determination

l Measurement Procedure

- Measure the average sound intensity on an enveloping surfaceenclosing the device-under-test

- Determine the quality and grade of precision of the measurementsby calculating a number of indicators

- If necessary, take action to improve the quality (and grade of precision) of the measurements

- Calculate the sound power of the device-under-test

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Intensity-based Measurement Systems

System based on portable analyzer(2144)

Sound Level Meter based system(2260E)

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Intensity, Point-based, (ISO9614-1)

l Adv antages

- Can give precision, engineering, or survey grade measurements

- can be used in-situ, no special acoustic test facility required

l Disadvantages

- Method is very difficult to follow

- Precision sound power determination can require that hundreds oreven thousands of points be measured

- Will take much longer than precision pressure-based methods

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Intensity, scanning, (ISO 9614-2)

l Adv antages

- Easier to follow than point-based methods

- can be used in-situ, no special acoustic test facility required

l Disadvantages

- Only gives engineering or survey grade measurements, no precisiongrade

- Experience required to acquire good scanning technique

- Will (usually) take longer then the pressure-based methods

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Intensity Method- ISO 9614-3

l Precision Sound Power Determination using Scanned Intensitymethod

- under development for the above

- However, it looks as if it may be just as difficult to implement as ISO9614-1

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Intensity-based used for

l Intensity-based Sound Power Determination tends to be used for…..

- Engineering measurements in development of new products

- In-situ and survey measurements

- Measurement where it is not conv enient or not possible to put thedev ice-under-test in an acoustic test facility

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Which Method to Use?

l Each of the methods have their own advantages and disadvantages

l Pressure methods may be best-suited to production testing and audits

l Pressure methods best suited to use by non-qualif ied personnel

l Intensity methods maybe best-suited to research and dev elopment,and engineering

l Intensity methods require more engineering skill

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Sound Pressure Measurement Standards- ISO 11201

l Sound pressure level at a work station and at other specified positions – Essentially Free field over a reflecting plane (Engineering method)– The test env ironment : indoor or outdoor provide essentially free-field over a reflecting plane .– A - weighting and in band background noise shall be 6 dB below the noise level machine under test ,Preferably more than 15dB .– The environmental indicator K2 shall not exceed 2 dB.– Sister standard ISO 11202 with survey grade

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The Instrumentation for Sound Pressure Measurement

l Instrumentation : Class 1 of IEC 651, IEC 804


l Calibrator : 0.3 dB accuracy (IEC 942 class 1).



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The Measurement Time Interval and Its Points

l Time interval :– Steady Noise : 15 sec .– Non-steady Noise : Carefully defined – Measurement in Octave / One-third Octave : 30 sec (on or below 160Hz)

15 sec (on or above 200Hz)l Measurement Points :

– Seated Operator : 0.80 m +_ 0.05 m abov e the middle seat plane .– Standing Operator : 1.55 m +_ 0.075 m height at reference point.– Bystander : Four or more microphone positions ,1 m away from each side

of reference box and 1.55 m +_ 0.075 m height (No operator)

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Noise Test Codes: ISO Standards

l Rotating electrical machineryl Construction equipment for outdoor

usel Air-terminal devices, dampers,

valvesl Reciprocating internal combustion

enginesl Information technology and

telecommunications equipmentl Industrial plants (f or evaluation of

sound pressure lev els in the env ironment)

l Acceptance code f or gear unitsl Refrigerators and freezersl Portable chain saws with internal

combustion enginesl Portable brush cutters and grass

trimmers with internal combustion engines

l Power lawn mowers, lawn tractors, etc.

l Textile machines

l Gas turbines

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Information technology and Telecommunication Equipment - Noise Test Code

l ISO 7779 : Measurement of airborne noise emitted by informationtechnology and telecommunication equipment(1999,Aug)

l ISO 9295 : Measurement of high-frequency noise emitted by computerand business equipment(1988, Nov)

l ISO 9296 : Declared noise emission values of computer and businessequipment(1988 , Apr)

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Noise Test Codes for Information Technology

l The method to determine sound power level(ISO 7779 , ISO 9295)

l The method to determine the emission sound pressure level at theoperator position(s) and at the bystander positions(ISO 7779)

l The positions of the work stations (ISO 7779)

l The Noise Character (ISO 7779 Annex D ,E)

l The mounting and operating conditions of the machine under test (ISO 7779 Annex C)

l The method to declare noise emission values as sound power and work station sound pressure ,noise character…(ISO 9296)

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About ISO7779l ISO7779-1999 “Acoustics- Measurement of airborne noise emitted by

information technology and telecommunication equipment”– Is a noise test code providing detailed instructions concerning the

installation and operation of equipment during noise tests. (Based on ECMA-74 ,European Computer Manufacturer Association Standard )

– The acoustical conditions, methods of measurement, and the instrumentation to be used are prescribed by the basic sound power measurement standards ISO 3741, 3744, and 3745, and the standard for sound pressure level measurement at operator and bystander positions by ISO11201

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ISO 7779 Measurement Frequency

l Typical frequency range :

- 100 Hz - 10 KHz (One-third octave band)

- 125 Hz - 8 KHz (Octave Band)

- Include the 16 KHz octave band if it affects the A-weighted Lp or Lw

- Not included 16 KHz octave if in 16KHz with discrete tones

- For IT equipment , A- weighted Lw in 250 Hz - 4000 Hz

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ISO 7779 Sound Power Measurement

l Measurement Standards :

- ISO 3741 : In reverberation chamber comparison method only.

- ISO 3744/3745 : Essentially free- field condition over a reflecting plane

(a) The preferred surfaces are hemispherical with d= 1 m .

(b) For low Lw level case , a parallelepiped may be used to

obtain d as small as 0.25 m .

- If with discrete tones ,Using one-third octave band A - weighted lev el.

- Measure for idling and operating modes

- If several operating modes, measure f or each mode

- For repetitive cycles, measurement should include at least three cycles

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ISO 7779 Sound Pressure Measurement

l Measurement standard : ISO 11201 -microphone positions :

(a) one or more operator positions : - Standing :1.50 m +_0.03 m . - Seated : 1.20 +_ 0.03 m .

(b) no operator required, - Four bystander positions at least : 1.0 m +_ 0.03 m

away from reference box l Annexes call for the measurement of impulsive sound

pressure levels and prominent discrete tones at operator and bystander positions

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Prominent Discrete Tonesl For operator-attended equipmentl Microphone positioning

– one operator position : at the operator position , – more than one operator positions : the highest A- weighted sound

pressure position .– no operator position : the highest A-weighted sound pressure lev el

position and all other positions within 0.5 dB lower A-weighted sound pressure lev el .

l Measuring : (all the sound pressure level should be no A-weighting applied)

(a) the tone sound pressure level Lt. (b) the secondary tones sound pressure level(c) the noise sound pressure level in the critical band Ln. (d) calculate (Lt - Ln ). (e) it is as prominent IF and ONLY IF (Lt - Ln ) >= 6 dB.

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Tone Sound Pressure and Critical Band

l Tone sound pressure measurement :– the f requency resolution shall be less than 1% .– the FFT analyzer with Hanning weighting is pref erred .– Lt is computed the power in tone band that defined by delta cursor. – the tone band should not exceed 15% of its critical band.

l Critical bandwidth dfdf = 25.0 + 75.0(1.0 + 1.4(f t/1000)2)0.69 .

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Impulsive Sound Pressure Levels

l For operator-attended equipmentl Microphone positioning

– one operator position : at the operator position , – more than one operator positions : the highest A- weighted sound

pressure position .– no operator position : the highest A-weighted sound pressure lev el

position and all other positions within 0.5 dB lower A-weighted sound pressure level .

l Measuring : – under same condition ( operation , measurement conditions ) to obtain

the time averaged A-weighted impulse sound pressure level LpAI and the time average A-weighted sound pressure level LpA

– calculate (LpAI - LpA). – it is impulsive IF (LpAI - LpA ) >=3 dB.



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ISO 7779 Sound Pressure Measurement Table

l Test table

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ISO 7779 Operator Positions

l Standingl Seated

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The Instrumentation for ISO 7779 Measurement

l Instrumentation : Class 1 of IEC 60651, IEC 60804


l Calibrator : 0.3 dB accuracy (IEC 60942 class 1).

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16 kHz Octave Band Measurements

l ISO7779 states that noise in the 16 kHz band should be included if it effects the A-weighted level unless the noise in that band includes discrete tones

l When the 16 kHz band does contain discrete tones, then the soundpower measurement should be done according to ISO9295 instead ofISO7779

l This clearly requires a discrete tone detection capability based on FFT analysis.

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ISO 9295 Measurement Methods

l Four methods for sound power measurement.– Three methods are in reverberant f ield (reverberation chamber) .

» two are di rect method one is comparison method» use boom for microphone installation

– One is in f ree f ield ov er a reflecting plane (semi-anechoic chamber) .» Three microphone installations : boom , rotating table and fixed microphone array

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Calculation Sound Power

Direct method :Lw = Lp - 10 lg (4/R)

Comparison methodLw =Lw(RSS) - Lp(RSS) + Lp + 10 lg dF

Free fieldLw = Lpf + 10 lg ( S/So)

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ISO 9295 Measurement Conditionsl Broadband only, 1/3 octave bands can be usedl For discrete tones, use narrow band analy zer

– no wider than 1/12 octave– preferably FFT analysis

l No discrete tones in 16 kHz octave band(11.4 KHz ~ 22.8 KHz)– prov ide A-weighted Lwfor 125 Hz - 16 kHz

l One discrete tone in 16 kHz octave band– provide A-weighted Lw for 125 Hz - 8 kHz – with level/frequency of tone in 16 kHz band

l Multiple discrete tones in 16 kHz octave band– provide A-weighted Lw for 125 Hz - 8 kHz – with level/frequency of all discrete tones whose levels are within

10 dB of the level of the highest tone

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Declaring Noise Emissions

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Noise Declaration Procedures - ISO 9296

l Noise declarations should include allowances f or the standard dev iation across a batch of dev ices(SP) and the standard deviation of the measurement method(SR)

– Theref ore pays to use the most accurate measurement method which is practicable

l Suggests Sound Powers be declared in Bels rather than Decibels, (I Bel equals 10 Decibels), to avoid confusion with Sound Pressures

Batches of machines

LWAd =1/10(LWAm+ 1.5 St + 0.564(2.0 - St))St : total standard deviationSt = (SR2 + SP2 )0.5

Individual machineLWAd =1/10(LWAm + 2.5)

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Declaration Method

l Idling and operating modes– LWAd (1B = 10 dB)– LPAm : operator position or bystander positions– may describe the character of the noise as :

(a) no impulsive noise , no prominent discrete tones .(b) impulsive noise , no prominent discrete tones .(c) prominent discrete tones , no impulsive noise.(d) impulsive noise and prominent discrete tones .

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How is ISO7779 used

l Used in the information technology and telecommunications industries to define noise measurement procedures

l Used f or measurements on both components and completed assemblies

l Single defining standard stating how noise measurements in the industry should be made

l Ensures that noise measurements made at different sites are comparable

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ISO 3743ISO3744ISO 3745

ISO 9614-1 ISO 9614-2

Sound Pressure Meas.

The Structure of Measurement StandardsSound

IntensitySound Pressure

B standards : Sound

Pressure measurement

C standards : Information Tech & Telecommunication

ISO 7779 (General)ISO 9295(High freq 16 KHz)ISO 9296(Declaration)

B standards : Sound power Measurement

Diffusion Field Free Field

ISO 11201 ISO 11202

ISO 3742ISO 3741



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Special Problems in PC industry

l Very low noise emission values place stringent requirements on background noise levels

l Each new generation has lower noise emissions

l OEM customers are placing limits on noise emission values

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The Practice

l Current Trends and Practice

- Test in a semi-anechoic chamber to minimize background noise

- Use pressure methods, (ISO 7779/ECMA 74) - intensity methods are not sensitive enough, and require more engineering skills

- Where necessary, use high sensitiv ity microphones

- To increase throughput, use parallel instead of serial measurements

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Current Practice

l Impulsive Noise Test

- Usually not required

l Pure Tone Determination

- Often required by OEM customers

- Carried out at one of the microphone positions used for the Sound Power Determination

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The Other International and National Standards

l ECMA standards (Standardizing Information and Communication System)– ECMA 74 : Measurement of airborne noise emitted by information

technology and telecommunications equipment .(ISO 7779)– ECMA 108 : Measurement of high-f requency noise emitted by

inf ormation technology and telecommunications equipment .(ISO 9295)– ECMA 109 : Declared noise emission values of information technology

and telecommunications equipment .(ISO 9296)l ANSI standards

– ANSI S12.10 - 1985 : Methods for the measurement and designation of noise emitted by computer and business equipment .

» The discrete tone calculation is same as in ISO 7779.

– ANSI S1.13-1995 : Measurement of sound pressure levels in air .» The discrete tone calculation is with two ways ,one is similar as in ECMA 74/

ISO 7779, The other is referred by ISO 7779 this version and may be as the standard in the next version of ISO 7779 .

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SI International Standards for TI

l ECMA standard– ECMA 160 : Determination of sound power levels of computer and

business equipment (1992)

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Brüel & Kjær Systems for Noise Measurementson PC-dev ice and its assemblies

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Brüel & Kjær Systems

l PULSE/Multichannel Sound Power Software 7748

- Follows ISO 3744 for Sound Power Determination

- Capable of all measurements required by ISO 7779/ECMA 74

- Capable of making all required measurements in parallel

- Can use high-sensitiv ity microphones for very low lev el measurements

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System Configuration (12 Channels)

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Brüel & Kjær Systems

l Sound Level Analyzer 2260E

- Intensity-based Sound Power Determination according to ECMA 160

- Sound Pressure measurements



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