Bending of sound waves in the vicinity of porous materials

The change in speed of sound in adjacent media redirects the sound waves. It is well known that when the temperature of the air changes gradually, the sound waves bend towards colder air with a lower speed of sound. However,
not much is known about how sound waves bend close to a sudden change in speed of sound, e.g., in the vicinity of absorptive materials. Previous research, done in 1937, proposed that sound waves at 200 Hz bend prior to and upon penetrating a porous material. This paper replicates the old measurement with multiple tones at wide frequency range and presents an investigation procedure for the bending of sound waves propagating inside and in
the close vicinity of acoustic absorbers. A set of measurements was carried out in the anechoic chamber using 16 MEMS microphones in two layers, forming eight pairs and positioned at different heights above and inside the horizontally placed absorbing material. At varying elevations, a three-way point source emitted tones in grazing angle in one-third octave bands. As in the original article, the evidence of bending sound waves is shown by delaying the top layer of microphones and estimating the time delay that minimizes the sum signal of microphone pairs when the phase of the top microphones is inverted. The results indicate that the apparent bending of sound waves varies with the adjacent absorbing material and is frequency dependent. In contrast to previous results, the bending seems to be almost unaffected by the grazing angle.