Evaluating the lining of anechoic room

Abstract

This paper reports the evaluation of a method for measuring the pressure reflection factor of an anechoic room lining in situ, proposed by Kuttruff and Bruchmuller. In this method, a microphone and a sound source are mounted at a fixed distance and orientation to each other, on a turntable, which is rotated in the anechoic room. The variations in sound pressure level at the microphone, as the system rotates, are due to the residual reflections from the anaechoic lining. The pressure reflection factor can be calculated from the relative standard deviation of the sound pressure. Considerable difficulty was experienced obtaining stable and repeatable results. This was mainly due to low-frequency fluctuations in sound pressure which previous workers had filtered out without being able to explain the cause of the fluctuations. This paper shows that these low-frequency fluctuations are due to the asymmetrical vibration transmission of the turntable as it rotates. The fluctuations can be reduced by vibration isolating the sound source and the microphone.