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• Measured sound absorption using standard methods inherently contains influence of acoustic resonances within the sample. • A measurement method for sound absorption with calculation to eliminate the influence of acoustic resonances is proposed. • By adjusting the delay between the reflected and transmitted sound it is possible to identify impulses and remove resonances. • Removing resonances enables the determination of the optimal granule size for viscous sound energy dissipation. The sound absorption coefficient of a material represents the fraction of acoustic energy not reflected from its surface. The absorption is influenced by viscous losses in porous structures and sample resonances. This study proposes a novel method for measuring absorption coefficients in extended impedance tubes, specifically designed to distinguish between viscous losses and resonances, but can also be used to estimate the sound absorption coefficient in a standard impedance tube. The method is based on a time-domain wave decomposition for its simplicity of use and ability to visualize sound impulse reflections, revealing reflections of sound within the sample. A long vertical impedance tube was used to achieve sufficient separation of the sound impulses. Random noise was used as the excitation signal and spectral division deconvolution was used to calculate the sound wave impulses in the impedance tube. The proposed method was validated against the transfer function method using samples of rock wool, reconstituted foam, and polyether foam. The advantage of the proposed method was demonstrated on measurements of several granular fractions of recycled silica sand to determine the granule sizes where the main mechanism of sound absorption is viscous losses as opposed to resonances. It was found that the sound absorption caused by viscous losses is greatest for granule diameters around 500 μm.
Železnik et al. (Thu,) studied this question.