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Acoustic parameters and uncertainties associated with determining sound power level in hemi-anechoic rooms.

Simmons, D; Jobling, B; Payne, R (2004) Acoustic parameters and uncertainties associated with determining sound power level in hemi-anechoic rooms. NPL Report. DQL-AC 007

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Where it is necessary to accurately quantify the sound power output of a noise source, it is common in practice to use hemi-anechoic rooms. In such specialist facilities the acoustical performance is carefully designed around the assumption that no sound power is absorbed or transmitted through the floor, that the effects of ground reflection are neglected where the source is placed on the floor, and that hemispherical sound propagation is uninhibited, such that environmental effects due to the room can largely be ignored. However, prior to accepting that such effects are negligible, the acoustical performance of such facilities must be tested for compliance with the design following construction or prior to making measurements on particular machines. Commonly, reference will be made to the international standard ISO 3745. This describes methods for the assessment of room performance stipulating qualification criteria and specifying precision methods for determining sound power levels. The standard does not however provide information on how measurement uncertainty associated with sound power level determination is related to the room performance criteria. This being the case, users of the standards will not know to what degree their test facility will affect the measured sound power result. The practical implications of having this information are important, where uncertainty values are used in practice, to derive the final guaranteed sound power level, which is used as an absolute indicator on noise labels affixed to the machine.
The research described in this report is designed as a contribution to current efforts aimed at improving existing sound power standards. NPL have conducted an extensive series of measurements, examining the effect of varying the performance of a hemi-anechoic room on sound power determinations for a range of noise sources. A series of conclusions have been drawn and it is proposed that the output of the work be used as part of the improved treatment of measurement uncertainty in the ISO 3740 series of standards.

Item Type: Report/Guide (NPL Report)
NPL Report No.: DQL-AC 007
Subjects: Acoustics
Acoustics > Sound in Air
Last Modified: 02 Feb 2018 13:16
URI: http://eprintspublications.npl.co.uk/id/eprint/3025

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