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Sound Power Measurements

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Three different measurement methods to determine sound power level of a sound source are compared. Two methods are sound pressure methods. The first one is a comparison method, using a reference sound source and the second one is a direct method without using a reference sound source. Third, the sound power level is measured with a sound intensity method, using scanning.
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    Sound power measurements CHRISTER HEED SD2165 Stockholm October 2008    Marcus Wallenberg Laboratoriet för Ljud- och Vibrationsforskning Sound power measurements Christer Heed  Approved Date: Signature: SD2165 Acoustical measurements   1 ABSTRACT Three different measurement methods to determine sound power level of a sound source are compared. Two methods are sound pressure methods. The first one is a comparison method, using a reference sound source and the second one is a direct method without using a reference sound source. Third, the sound power level is measured with a sound intensity method, using scanning. The comparison method yields octave band with the centre frequency range 125 Hz - 8 kHz and A-weighted sound power level. The direct method yields only A-weighted sound power level. The intensity method yields octave  band with the centre frequency range 125 Hz - 4 kHz and A-weighted sound power level. Standards for all methods are used. The comparison method and the intensity method attain engineering accuracy (grade 2). A few minor derogations are made from them. The standard deviation should still be about 1.5 dB. The discrepancy of the results of these two methods is less than 1 dB. The discrepancy is most significant at the 125 Hz and the 500 Hz octave bands. The direct method attain survey accuracy (grade 3) and thus the standard deviation of the direct method is 4 dB and its determined sound power level is 1 dB higher than for the intensity method and 2 dB higher than for the comparison method. SAMMANFATTNING (the abstract in Swedish) Tre olika mätmetoder för att bestämma ljudeffektnivån från en ljudkälla jämförs. De två första är ljudtrycksmätmetoder, med jämförandet av en referensljudkälla respektive utan. Medan den tredje är en intensitetsmetod med scanning. Intensitetsmetoden ger ljudeffektnivån i oktavband med centerfrekvenser 125 Hz – 4 kHz och den A-vägda. För  jämförandemetoden fås ljudeffektnivån i oktavband med centerfrekvenser 125 Hz – 8 kHz och den A-vägda. För den direkta ljudtrycksmetoden fås bara A-vägd ljudeffektnivå. Standarder för alla metoder används. Mest noggrann är jämförandemetoden och intensitetsmetoden. Några mindre avsteg från dessa bådas standarder görs, men i princip är standardavvikelsen 1,5 dB. Skillnaden i resultatet mellan de båda noggrannare metoderna är mindre än 1 dB. Skillnaden är störst i 500 Hz oktavbandet och i 125 Hz oktavbandet. Den direkta metoden har standardavvikelsen 4 dB och dess bestämda ljudeffektnivå är 1 dB högre än den för intensitetsmetoden och 2 dB högre än den för  jämförandemetoden.   2 TABLE OF CONTENTS 1 INTRODUCTION.......................................................................................................3 1-1 Task.....................................................................................................................3 1-2 Test object and its physical description...............................................................3 2 STANDARDS.............................................................................................................4 2-1 ISO 3747:2000 - Determination of sound power levels of noise sources using sound pressure -- Comparison method in situ.................................................................4 2-2 ISO 3746:1995 - Determination of sound power levels of noise sources using sound pressure -- Survey method using an enveloping measurement surface over a reflecting plane................................................................................................................4 2-3 ISO 9614-2:1994 - Determination of sound power levels of noise sources using sound intensity -- Part 2: Measurement by scanning.......................................................4 3 MEASUREMENTS....................................................................................................4 3-1 Measurement environment..................................................................................4 3-2 Instrumentation....................................................................................................4 3-3 Data acquisition and settings...............................................................................5 3-4 Measurement procedure, using ISO 3747:2000..................................................5 3-4-1 Calibration of the measurement system......................................................5 3-4-2 Measurement of the indicator of accuracy..................................................5 3-4-3 Measurement of the sound pressure levels..................................................6 3-4-4 Measuring the background- sound pressure level and noise correction......6 3-4-5 Measurement result.....................................................................................6 3-5 Measurement procedure, using ISO 3746:1995..................................................6 3-5-1 Reference box, measurement surface and microphone positions................6 3-5-2 Estimation of environment correction.........................................................7 3-5-3 Measurements and result.............................................................................7 3-6 Measurement procedure, using the intensity method, ISO 9614:1994................8 3-6-1 The intensity method by scanning...............................................................8 3-6-2 Calibration...................................................................................................8 3-6-3 Scanning......................................................................................................8 3-6-4 Requirements...............................................................................................9 3-6-5 Calculations, measurement results and accuracy........................................9 3-6-6 Derogation...................................................................................................9 3-6-7 Results.........................................................................................................9 4 DISCUSSION............................................................................................................10 4-1 Comparison of the results..................................................................................10 4-2 Comparison of the results in octave bands........................................................10 4-3 Measurement improvements.............................................................................11 4-4 The importance of calibration............................................................................11 4-5 Advantages........................................................................................................11 5 REFERENCES..........................................................................................................11 6 APPENDIX...............................................................................................................12 6-1 Measurement, results and calculations..............................................................12 6-1-1 The comparison method, ISO 3747...........................................................12 6-1-2 The direct method, ISO 3746....................................................................13 6-1-3 The intensity method, ISO 9614................................................................14 6-2 Important parameters of measurement accuracy [4].........................................14
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