Sound absorption uncertainty in small reverberation chambers

Manufacturers in the mobility industry typically measure and specify the sound absorption coefficient of materials using a small reverberation chamber according to SAE J2883. These chambers have a 6 to 25 cubic meter volume, use a 1.0 meter x 1.2 meter sample size and report sound absorption in the one-third octave bands from about 400 Hz to 10,000 Hz. This is in contrast to a full-size chamber built to satisfy the requirements of ASTM C423 or ISO 354, which have about a 200 cubic meter volume, use a 2.4 x 2.7 or 3.2 x 3.2 square meter sample size and report sound absorption in one-third octave bands from 100 to 5,000 Hz. Although the room and sample size dimensions scale, many of the factors that contribute to uncertainty in the measurement do not scale. These include relative humidity, temperature, and other physical measurements and if the same instrumentation is used, microphone calibration and measurement uncertainty in the higher frequency range. This paper examines the potential measurement uncertainty in these chambers over the 400 to 10,000 Hz frequency range and compares this to past uncertainty analyses of full-size chambers over a similar frequency range.