A correction of random incidence absorption coefficients for the angular distribution of acoustic energy under measurement conditions

Abstract

Most acoustic measurements are based on an assumption of ideal conditions. One such ideal condition is a diffuse and reverberant field. In practice, a perfectly diffuse sound field cannot be achieved in a reverberation chamber. Uneven incident energy density under measurement conditions can cause discrepancies between the measured value and the theoretical random incidence absorption coefficient. Therefore the angular distribution of the incident acoustic energy onto an absorber sample should be taken into account. The angular distribution of the incident energy density was simulated using the beam tracing method for various room shapes and source positions. The averaged angular distribution is found to be similar to a Gaussian distribution. As a result, an angle-weighted absorption coefficient was proposed by considering the angular energy distribution to improve the agreement between the theoretical absorption coefficient and the reverberation room measurement. The angle-weighted absorption coefficient, together with the size correction, agrees satisfactorily with the measured absorption data by the reverberation chamber method. At high frequencies and for large samples, the averaged weighting corresponds well with the measurement, whereas at low frequencies and for small panels, the relatively flat distribution agrees better.

Non-uniform sound intensity distributions when measuring absorption coefficients in reverberation chambers using a phased beam tracing

“…4͑a͒ and 4͑b͒. The spatially averaged intensity distributions at higher frequencies agree well with a frequency-independent incident energy density calculated by the beam tracing method, 17 which is displayed as the thick solid line.…”

“…Using the geometrical acoustics models, the actual incident energy distribution in a room can be estimated and used as a correction factor. 17 Phased geometrical acoustics techniques, 18 which retain phase information during ray/beam tracing, can also be used in such analyses. A phased beam tracing technique has been used for calculating sound intensities in this study, and a brief introduction to the phased beam tracing method is given in Sec.…”

“…20 Makita and Hidaka 21 discussed possible variations of absorption coefficients in non-uniform energy field. Recently, Jeong 17 investigated incident energy density distributions for a rectangular and an irregular room using a beam tracing technique. The simulated incident energy density distributions as a function of the incidence angle for different source locations were used as weighing functions for correcting theoretical absorption coefficients.…”

The equivalent incidence angle for porous absorbers backed by a hard surface

“…14,16,17 Therefore, the increased absorption coefficients may lead to larger incidence angles, particularly for high absorption coefficients above 0.8 as can be seen in Figs. 5 and 6.…”

Predicting the Sabine absorption coefficients of fibrous absorbers for various air backing conditions with a frequency-dependent diffuseness correction

“…5 a Sab is also known to vary with the specimen size due to diffraction by the specimen edge. [6][7][8] Many round robin tests reported a poor inter-chamber reproducibility, indicating that a Sab depends largely on the reverberation chamber. [9][10][11] Some chambers systematically overestimate, while others underestimate the absorption coefficient.…”