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1 MEASUREMENT OF sound INTENSITYAND sound POWERVINH TRINHqt, MRL-TR-93-32ON FEBRUARY 1994 Min<- DTICFT hdomenOt huc ( u puli c o .lozs* =d saJo Itdstubut is UnmiimledAPPROVED ( Commonwealth of Auso-FOR PUBLIC RELEASV .1 MATLRIALS Rt'EtSEARCH LA( )RAlT()RYDSTOljm:;'rlD STAT -.!'TECHN!CAL IiNFOFiMATJD~rJ LVIIS AUTHOPASEO 1' AND SELL T141S REporTAcce--jo. rarxNTiS ..C ..A&I-DTI1C TAllU;-,3,)v0 Lou:iced -Jlsq icfhonBy .Dist; ibution IAvailability CcoeES Avail ai';djorgisSt ocialMeasurement of sound Intensityand sound PowerV. TrinhMRL Technical ReportMRL-TR-93-32 AbstractIn this report the concept of a measurement technique for calculating soundintensity in the frequency domain is discussed and also how such a measurementsystem can be implemented in practice by using a frequency domain technique described employs a dual-channel FFT analyser to obtain a crosspower spectrum from the two microphones from which sound intensity iscalculated.)

2 This approach enables a general-purpose FFT analyser together with amicro-computer to perform the function of a dedicated sound intensity application of sound intensity measurement technique in sound powerdetermination of a reference source is OF DEFENCEDSTO MATERIALS RESEARCH LABORATORY94 5 16 078 Published byDSTO Materials Research LaboratoryCordite Avenue, MaribyrnongVictoria, 3032 AustraliaTelephone: (03) 246 8111 Fax: (03) 246 8999 Commonwealth of Australia 1993AR No. 008-561 APPROVED FOR PUBLIC RELEASEA uthorVinh TrinhVinh Trinh obtained his BAppSc (Applied Physics) degreeat the Royal Melbourne Institute of Technology in 1987,and Grad.

3 Diploma in Computing at the MonashUniversity in 1992. He joined MRL in 1989 working inthe Noise and Vibration group of the Ship Structures andMaterials Division. He has worked on noise measurementusing the sound intensity technique and is currentlyinvestigating the airborne noise of the V4-275R INTRODUCTION 72. sound intensity AND ITS APPLICATIONS sound Power and sound Pressure Applications of sound intensity Measurements sound Power Determination intensity Mapping sound Transmission Loss Source Ranking 103. CALCULATION OF sound intensity 104.

4 THE TWO MICROPHONE METHOD FOR PRACTICAL SOUNDINTENSITY MEASUREMENTS The Estimator for Particle Velocity The Estimator for sound Pressure The Estimator for sound intensity 135. sound intensity MEASUREMENT SYSTEM 136. COMPUTER PROGRAM FOR CALCULATION OF SOUNDINTENSITY FROM CF-350 FFI' ANALYSER DATA 157. ERRORS IN THE ESTIMATION OF sound intensity WITH P-PTYPE intensity PROBE Finite Difference Approximation Error Probe Diffraction Error System Phase Mismatch Error System Phase Mismatch Phase Mismatch and Error at Low Frequency Phase Mismatch and The Pressure-Residual intensity Index Air Flow Disturbance Random Error Other Errors 238.

5 PRESSURE- intensity INDEX AND THE SYSTEM DYNAMICCAPABILITY Pressure- intensity Index System Dynamic Capability 249. EVALUATION OF SYSTEM PERFORMANCE 2510. PROCEDURES FOR CALIBRATION OF THE SYSTEM Pressure Calibration intensity Calibration Measurement of Pressure-Residual intensity Index 2911. sound Power Determination Of A Reference Source 3012. Discussion 3613. Conclusions 3714. References 3815. List of Symbols 40 Appendix 1 42 Appendix 2 43 Measurement of sound intensity and SoundPower1. IntroductionThis report describes a new and practical method for measuring and analysingthe noise signature caused by air-borne and structure-bone noise.

6 If the soundintensity of noise signatures can be measured, it is then possible to identify andminimize the average acoustic spectrum and emission of the Navy's surface shipsand techniques for measuring sound power [ ,6], or thetransmission loss of sound -absorbing materials, require special facilities such asanechoic or reverberant chambers. This report deals with a measurementtechnique for sound intensity that is an attractive alternative to these sound power by means of sound intensity has a number ofadvantages including the ability to locate the sources and sinks of sound , todetermine sound power from a source in situ even in a noisy machinery room,and the ability to map the intensity of the sound source.

7 These cannot be done bythe traditional techniques except under controlled acoustic Olson laid down the theoretical background for intensity measurements in1932 [ but it was not until the 1970s that the electronic instruments requiredfor a reliable measurement of sound intensity became available [ ]. A soundintensity measurement system comprises:1. An intensity probe for sensing the sound signalCurrently there are two categories of probe in widespread use. The "p-u" probecombines a pressure transducer with a particle velocity transducer.]

8 The "p-p"probe is made up of two matched microphones (pressure transducers) separatedby a spacer for measuring sound pressure and particle velocity [ This reportdiscusses the use of the "p-p" type of probe, upon which the measurement systemis An analyser for processing the sound signalThe analyser may be one of two types. It may have either digital or analog filters,and operate in the time domain (as do Type 4437 and 2134 Bruel & Kjaer soundintensity analysers); or it may be a dual channel FFT analyser which operates inthe frequency domain of the sound signals [ ].]

9 The former are faster, have real-time processing, and produce output in theoctave or fractional octave frequency bands that are frequently used in acousticmeasurements. However, if the analysis function can be performed on a generalpurpose FFT analyser, it has the important advantage of eliminating the need foran expensive and limited piece of equipment. This rwport describes how a dualchannel FFT analyser can be used to determine sound intensity , by taking thecross power spectrum from two sound intensity and its applicationsSound intensity describes the rate of energy flow (ie.)

10 The power flow) per unitarea at a point in space. In the SI unit system, the unit for sound intensity is wattsper square contrast to sound pressure which is a scalar quantity, sound intensity is avector quantity as it has both direction and magnitude. Usually, sound intensity ismeasured in the direction normal to the surface through which there is a net flowof sound sound Power and sound PressureSound power can be considered as the strengh of the source which gives rise to asound pressure in space. The cause of noise is sound power; what we hear is itseffect, sound power is virtually independent of the environment.