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

Some room acoustic parameters require the sound power of the sound source. The Sound Strength G uses the free field sound pressure level at 10 meters distance as a reference value. Speech intelligibility parameters like the A-weighted Speech Level, L_{p,A,S, 4 m} , and the Speech Transmission Index, STI, can require an absolute source level defined at 1 m distance from the sound source. The Early and Late Support parameters use the sound level at 1 m distance as a reference level. In this paper, all proposed methods to obtain the sound power level for room acoustic applications are investigated, using various omnidirectional sound sources with a dodecahedron 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 a single plane of the sound source, show deviations up to 2 dB per octave band. Different stepwise rotational averages, used in such a single 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) and when using equal-angular rotations with 5 or 7 steps. Furthermore, the uncertainty of in-situ calibration has been investigated. A comparison of results from different researchers shows that a correction factor should be applied to correct the in-situ calibration for its deviation from the laboratory calibration. For each calibration method the uncertainty is presented. Results show that some methods might be sufficiently accurate to be able to measure single number ratings for G, ST, L_{p,A,S, 4 m} and STI with an uncertainty in the order of magnitude of 1 JND, provided that no other measurement errors are introduced in the measurement chain.