The Measurement of Double Decay Rates in Small Enclosures Containing Highly Absorbent Samples

One common method of obtaining the absorption properties of a sample of material is to measure the decay of sound in an enclosure partially lined with the sample. In order to infer absorption coefficients from decay rate measurements, it is assumed that the sound field in the enclosure is diffuse. When measurements are made in enclosures which are small compared with wavelength, and contain non-uniform absorption, the diffuse field may no longer be three-dimensional in nature. Descriptions are given of measurements made in a small rectangular enclosure with one surface lined with highly absorbent material, and the results presented. Predictions are made of the decay rates in this situation and compared with the experimental results. Finally, the different field states present during the decay are explored.