Sound Absorption at Low Frequencies: Modelling a Test Room

A description is given of sound absorption in small-furnished rooms at low frequencies. The objective of the work was to characterise room absorption and to develop a model of surface absorption appropriate for a modal description of contained sound fields at low frequencies. An empty test room was measured and numerically modelled, for use as a reference condition for later measurements with obstacles and absorbers in the room. A finite element model of the empty room demonstrated the need to account for even small irregularities in the room geometry. It is also confirmed that the vibration behaviour of the room walls contributes to the whole room absorption mechanism at very low frequencies. A parametric survey, involving model updating, yielded a frequency invariant value of absorption coefficient of 0.02, which produced the best agreement between prediction and measurement. The resultant predicted frequency response functions agreed with measurement within ± 5 dB. It is shown that it is possible to obtain agreement between measurement and prediction, after adjustments of wall acoustic impedance and loudspeaker frequency response.