The Sound Power as a Reference for Sound Strength (<I>G</I>), Speech Level (<I>L</I>) and Support (<I>ST</I>): Uncertainty of Laboratory and In-Situ Calibration

Abstract

Some room acoustic parameters require the sound power of the sound source. The Sound Strength G uses the free field sound pressure levelat10meters distance as areference value. Speech intelligibility parameters likethe Aweighted Speech Level, L p,A,S,4m ,and the Speech Transmission Index, STI, can require an absolute source level defined at 1mdistance from the sound source. The Early and Late Support parameters use the sound levelat1m distance as areference level. In this paper,all proposed methods to obtain the sound power levelfor room acoustic applications are investigated, using various omnidirectional sound sources with adodecahedron shape containing 12 loudspeakers. It is shown that, for octave bands 250 to 8,000 Hz, the sound power can be determined with 0.8 dB uncertainty when using precision methods (diffuse field, intensity or free field). Alternative laboratory calibration methods, that only measure in as ingle plane of the sound source, showd eviations up to 2dBp er octave band. Different stepwise rotational averages, used in such as ingle plane free field method, have been investigated. It can be concluded, that the uncertainty is significantly reduced only when using 12.5 degree steps (ISO 3382-1)a nd when using equal-angular rotations with 5o r7s teps. Furthermore, the uncertainty of insitu calibration has been investigated. Acomparison of results from different researchers shows that acorrection factor should be applied to correct the in-situ calibration for its deviation from the laboratory calibration. Foreach calibration method the uncertainty is presented. Results showthat some methods might be sufficiently accurate to be able to measure single number ratings for G, ST, L p,A,S,4m and STI with an uncertainty in the order of magnitude of 1JND, provided that no other measurement errors are introduced in the measurement chain. PACS no. 43.55.Br,43.55.Mc, 43.58.Vb, 43.58.Fm Received07July 2014, accepted 05 August 2015.sured in the free field at ac ertain distance. Also, when determining speech intelligibility parameters, like STI,the signal to noise ratio is determined based on aratio of absolute sound pressures (following ISO 16268-16 [2], only if background noise is taken into account, and following ISO 3382-3 [3], for all measurements in open plan offices). Following ISO 3382, all of these room acoustic parameters should be determined using an omnidirectional sound source. Ac ertain deviation from omnidirectionality is allowed and the deviation limits are giveninthe standard.Lundeby et al.[4] asked 8different teams to determine room acoustic parameters for 10 source-receiverc ombinations in a1,800 m 3 auditorium. He found astandard deviation for the levelp arameter Sound Strength G of only 0.2 to 0.3 dB for the octave bands 125, 1,000 and 4,000 Hz. These results would suggest that G can be measured with very high accuracy, even when using av ariety of loudspeaker types (a single loudspeaker in abox, acube with six loudspeakers and ad odecahedron with twelvel oudspeakers). Unfortunately,intheir...

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