Standardized methods for measuring sound absorption such as the impedance tube and reverberation chamber methods are limited to normal or diffuse incidence, respectively. Two research axes have been generally followed in the literature to develop alternative techniques, the first one focusing on the measurement part, that is from the two-microphone technique to the use of microphone arrays or pressure-velocity sensors. The second axis focuses on the excitation part with for instance the use of sound field synthesis techniques. Since acoustic impedance and sound absorption coefficient of materials are classically defined under normal and oblique plane wave excitation, synthesizing an “ideal” plane wave using a loudspeaker array would allow measuring these acoustics quantities using a simple microphone pair. In this article, the effect of the different parameters of a loudspeaker array on acoustic plane waves reproduction on a material’s surface is first numerically studied. Then, numerical and experimental results for the estimation of both impedance and absorption coefficients are reported. These results show that sound field synthesis allows to characterize a material for arbitrary incidence angles over a wide frequency range, thus offering an alternative method to standard techniques and an improvement over existing works.
A method for measuring the diffuse field sound absorption coefficient of a material using sound field synthesis is proposed. A planar loudspeaker array is first used to generate acoustic plane waves with variable incidence angle on the surface of a material under test. Using a two-microphone probe positioned closely to the sample’s surface, the angle-dependent sound absorption coefficients are then estimated. Finally, the diffuse field absorption coefficient is computed following Paris formula. Numerical simulations are used to evaluate the respective effects of the maximum incidence angle value and the number of individual incidence angles that are required for a robust calculation of the diffuse sound field absorption. Measurements are conducted on three different materials and compared with either simulation results obtained using the Johnson-Champoux-Allard theory, or with measurement results obtained using the standard reverberation chamber method. For all considered materials and over a wide frequency range, the proposed method leads to results that are in better agreement with theoretical predictions than those obtained using standardized methods.
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