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Natural Science Forum / Physics / Acoustics / July 2006Sound power level measurement according to ISO 3745 - the problem of origin of coordinates
Dear List Members, I am doing the measurements of sound power level of a small device (personal computer) according to ISO 3745:1977 (in anechoic chamber with reflecting surface). I am in a dilemma with location of the origin of Cartesian coordinates (x, y, z). According to the description (Annex C) "the origin is centered on the projection of the acoustic centre of the source on the reflecting plane". Does it mean that the height of the microphone positions should be measured from the level of reflecting plane or from the centre of the measured object (geometrical centre of computer case)? I think that the right is first one (the height of the microphone positions should be measured from the level of reflecting plane; in other words z=0 means reflecting plane), but please correct me if I am wrong. Regards, Jack > I am doing the measurements of sound power level of a small device > (personal computer) according to ISO 3745:1977 (in anechoic chamber [quoted text clipped - 4 lines] > reflecting plane". Does it mean that the height of the microphone > positions should be measured from the level of reflecting plane Yes. > or from > the centre of the measured object (geometrical centre of computer > case)? No. > I think that the right is first one (the height of the microphone > positions should be measured from the level of reflecting plane; in > other words z=0 means reflecting plane), Correct. The influence of the sound reflectded from the reflecting plne is controlling, and will cause interference maxima and minima, whose field is best sampled precisely as . It is the consequence of not having for easy use a fully anechoic chamber. About the only place that is ever convenient is outdoors over grass, and even then only at higher frequencies since soil is hard and can reflect low frequency sound, whereas most sound emitted by small computers is above 200 Hz. But outdoor background noise is considerable.. Perhaps at 3am, it could be as low as 30 to 40 dBA, but not at other times. So one is obliged to tediously measure sound the carefully measured points in space as declared in 3745, Annex C. Angelo Campanella |
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