A finite element study of absorption coefficient measurement at low frequencies

The frequency-dependent absorption coefficients of an acoustically absorbent material sample can be measured using the reverberation chamber method, defined by the ISO standard 354:2003. The lower frequency bound for the measurement defined in the standard is 100 Hz, since measurement data below this frequency may be unreliable. However, the ability to measure absorption coefficients at low frequencies is becoming increasingly more important, to facilitate the enhanced control of low frequency sound fields. This paper presents a finite element method study of the reverberation chamber measurement method at frequencies below 100 Hz, and the predicted performance of the ISO standard method is reported. Additionally, a recently proposed method that makes use of finite element-based eigenvalue analyses to estimate the locally reacting complex-valued impedance of a sample at modal frequencies is applied to the problem. It is shown that the proposed method can be used to estimate frequency-dependent absorption coefficients at modal frequencies below 100 Hz.