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The effect of enveloping surface shape and size on the determination of sound power.

Payne, R C; Simmons, D (1999) The effect of enveloping surface shape and size on the determination of sound power. NPL Report. CMAM 30

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This report describes an experimental programme designed to assess the effect of enveloping surface shape and size on the determination of sound power using procedures based on those described in ISO 3744. The programme focused on hemispherical and parallelepiped measurement surfaces since these underpin all of the sound-pressure based standards, but a conformal surface shape was also considered. All measurements were carried out in hemi-anechoic conditions. Three locations were used: two were outdoor sites that consisted of large flat surfaces covered with either concrete or steel plates, and one a hemi-anechoic room that satisfies the requirements of ISO 3745, for hemispherical surfaces with radii up to 1.5 m.
The sound power levels of four machines were determined using three hemispherical and three parallelepiped enveloping surfaces with radii and measurement distances that conform to the requirements of ISO 3744. The machines were reasonably acoustically complex noise sources and typical of the type that will be subject to compliance with noise labelling and noise limits set out in numerous EC Directives. In addition, a more detailed series of measurements were made using a reference sound source with enveloping surface areas ranging from 3 m² to 56.5 m².
For parallelepiped and hemispherical surfaces sound power level determinations were dependent on the surface area, becoming approximately constant when the radius of the hemisphere was at least three-times the characteristic dimension and the parallelepiped measurement distance was at least 0.5 m. For the conformal surface there was no systematic variation.
Sound power levels determined using a parallelepiped were higher than those for a hemisphere and those for a conformal surface were lower. To reduce measurement uncertainty, the use of a bias correction to account for variations resulting from the use of different enveloping surfaces should be considered.
Significant differences were observed between sound power levels determined in the three hemi-anechoic environments, indicating that the environmental correction factor should be experimentally determined for all measurement locations.

Item Type: Report/Guide (NPL Report)
NPL Report No.: CMAM 30
Subjects: Acoustics
Acoustics > Sound in Air
Last Modified: 02 Feb 2018 13:18
URI: http://eprintspublications.npl.co.uk/id/eprint/1100

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