The use of the four-microphone standing wave tube to assess the acoustical properties of sound absorbing materials is becoming increasingly popular. The objective of the present work was to demonstrate that it is possible, under certain conditions, to predict the random incidence transmission loss of sound absorbing materials based on normal incidence measurements in a four-microphone standing wave tube. If a porous material may be modeled as being isotropic and either perfectly rigid or limp, it may be characterized as an effective fluid in terms of its complex density and wave number. The latter quantities may then be used in conjunction with plane wave theories to predict random incidence properties. That procedure is demonstrated here by using two different porous media. Good agreement was found between the estimated and directly measured random incidence transmission losses. The accuracy of the procedure may however, be limited at both low and high frequencies. At low frequencies, the estimated material properties may be affected by the constraint of the sample around its edge. At high frequencies, higher order mode propagation within the sample may affect the results owing to the typically subsonic wave speeds in porous media.
Fibrous Media, Transmission Loss, Random Incidence
Acoustics and Noise Control
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