In-situ impedance and absorption coefficient measurements using a double-layer microphone array

Abstract

Acoustic impedance is typically measured using an impedance tube, which requires a material sample physically fitted to the tube. However, the impedance can vary greatly between the material mounted in the tube and the material located in a real environment, where the mounting conditions are likely to be different. Also, oblique incidence cannot be measured in an impedance tube. In this paper, we investigate the use of a double-layer microphone array for in-situ measurement of surface impedance and absorption coefficient. With the array positioned near the material surface, a source emits broad-band sound towards the array and the material. A measurement is taken, and the sound pressure and the surface-normal particle velocity at the material surface are calculated using Statistically Optimized Near-field Acoustical Holography (SONAH). From the surface pressure and velocity, the impedance across a selected area is calculated, and finally the absorption coefficient is calculated from the impedance. A set of tests has been performed on porous material samples in an anechoic chamber as well as in a fitted room. Different sample sizes and different sound incidence angles have been considered. The results show consistency between the measurements in the anechoic room and the ordinary room as well as good agreement with Miki's model up to large oblique incidence angles.

Angle-Dependent Absorption of Sound on Porous Materials

“…However, the difference at the lower frequencies increases with increasing off-normal incidence angle. Similar results have been reported in [29]. Another relevant difference in the angle-dependent measurements is the change in the angle of sound incidence in the dip appearing at 2000 Hz for the bioboard sample.…”

Development of Adjustable Parallel Helmholtz Acoustic Metamaterial for Broad Low-Frequency Sound Absorption Band

“…Limited by the utilized transfer function tube detector of AWA6290T, only the sound absorption performance of APH-AM with normal incidence was investigated in the experiment. The sound absorption performance with oblique incidence required the modified detector [ 50 ] or the reverberation chamber method [ 51 ]. For the classical transfer function tube detector, measurement of sound absorption performance with oblique incidence required the incident tube, the sample, reflected tube with rear perfect absorbing material, and the intersection angle between incident tube and reflected tube should be adjusted to be equal to the oblique incidence angle [ 50 ].…”

Laboratory and in situ sound absorption measurement under a synthetized diffuse acoustic field

“…17 Others have examined the use of ambient noise as the only source of excitation. 18 A double-layer microphone array was proposed in Hald et al 19 to estimate sound impedance and absorption of porous material samples in an anechoic room and in an ordinary room, with consistent results considering different samples sizes and incidence angles. However, most in situ measurement systems for material absorption remain, however, limited to normal incidence excitation.…”