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An investigation into the measurement of the environmental correction factor used in the determination of the sound power level and emmision sound pressure level of machines.

Payne, R C; Simmons, D (1996) An investigation into the measurement of the environmental correction factor used in the determination of the sound power level and emmision sound pressure level of machines. NPL Report. CIRA(EXT)009

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Abstract

The introduction of a number of EC Directives has made it necessary for machinery manufacturers to provide information on the airborne noise emitted by machinery under defined operating conditions and in defined acoustical environments. Machinery noise emission is evaluated using the procedures described in several ISO standards. The most commonly used methods involve measurements of sound pressure level over a measurement surface (hemispherical or parallelepiped) enveloping the source. The acoustics of the test site are taken into account by means of an environmental correction factor, K2, which is a correction term to account for the influence of reflected or absorbed sound on the measured sound pressure level.
The ISO 3740 and ISO 11200 series of standards describe a number of methods for determining K2 which range from measurements using a reference sound source to subjective assessments of the sound absorption properties of the surroundings. In this investigation, measurements of the environmental correction factor (A-weighted) K2A have been made by different methods in five test rooms with different acoustical properties, and the results compared.
It is concluded that the absolute method using a reference sound source is the only method that will consistently provide an accurate assessment of K2A. The method involving measurement of reverberation time generally over-estimates K2A and will result in values of sound power level that are too low. It performs best in rooms where the reverberation time (for A-weighted broad-band noise) is high (> 1 s). The two surface method generally under-estimates K2A and will result in values of sound power level which are too high. It should not be used where the larger enveloping surface is affected by reflections from nearby objects. The estimated room absorption method provides only a range of possible values of K2A when the descriptors in the ISO standards do not match well the room characteristics, and performs badly in rooms where the reverberation time (for A-weighted broad-band noise) is high (> 1 s).

Item Type: Report/Guide (NPL Report)
NPL Report No.: CIRA(EXT)009
Subjects: Acoustics
Acoustics > Sound in Air
Last Modified: 02 Feb 2018 13:18
URI: http://eprintspublications.npl.co.uk/id/eprint/440

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