Effects Of A Finite Size Reflecting Disk On Sound Power Measurements

The sound power is an important parameter of sound sources which characterizes the rate of sound energy radiated by machinery equipment.In practical sound power measurements in an anechoic room based on sound pressure-based methods,a baffle sometimes has to be used to support the sound source under test or the anechoic room are used as a hemi-anechoic room by laying a baffle.The sound field radiated by a constant velocity sound source differs after the introduce of a finite size baffle.This thesis investigates the effects of a disk on sound power measurements conducted in an anechoic room.Firstly,an analytical mode,where a monopole scattered by a disk,is established based on the oblate spheroidal coordinates,to derive the exact solution of the sound power output of the source.The discrete summation form of the sound power radiated to the upper half-space is derived based on the sound intensity travelling out in the upper half-space.Three practical measurement cases are considered,i.e.,a supporting baffle has to be used in measurements in a full anechoic room and the sound pressure on spherical measurement surface is obtainable;a supporting baffle has to be used in measurements in a full anechoic room an only the sound pressure on the hemi-spherical measurement surface above the disk is obtainable;an anechoic room is used to simulate the environment in a hemi-anechoic room by laying a finite size disk on the wire-meshed floor.The correction terms for three cases are then presented.Secondly,the theoretical mode is established and validated by Boundary Element Method(BEM).The distribution of sound field due to a monopole being placed at the axis of a rigid disk is computed and analyzed,and the results show that large deviations of sound pressure occur on the spherical measurement surface which indicates the disk impact the measurement results.The effects of the disk radius and the height of the source above the disk are discussed based on the numerical results.Finally,the experiments in both an anechoic room and a hemi-anechoic room are conducted to validate the theoretical model and the correction terms computed by numerical simulations.The errors between the theoretical solution and the measured correction terms are analyzed based on the factors:the area density of the real disk,the distribution of the sound field,and the directivity of sound source.Results show that the correction terms can be large,which means the effects of the disk should not be neglected in practical measurements.It is found that the measurement error can be up to 7.0 dB without correction.