The sound absorption characteristics of wooden and porous materials including fibrous media and porous foams vary with the nature of their surface porosity. The low frequency absorption, which is of interest in noise control engineering, of these material types is generally enhanced by introducing an air gap of a certain width or a flexible support at the rear face of the material posterior to the impinging sound waves. This study investigates the effect of air gap with and without hind support on the low frequency absorption of a kenaf/epoxy composite, a Meranti wood, a low-density rigid foam and a high-density rigid foam. The sound absorption coefficient of these materials are determined using the two-microphone impedance tube transfer-function method according to ISO 10534-2 standard in the frequency range between 20 Hz and 1600 Hz. In this study, 5-mm, 10-mm, 50-mm and 100-mm wide air gaps are introduced at the posterior of the specimens to measure the influence of the quarter-wavelength absorption on sound absorption coefficient. Backing panels made from polyethylene are used to study the effect of hind support on sound absorption coefficient at different frequencies. The dBFA Suite 4.8.1 program is used to acquire and process the acoustic signals and the SCS8100 software package is employed for the computation of sound absorption coefficient. The sound absorption coefficient and characteristics of each material within the range of frequency tested are discussed based on the width of air gap (i.e., the quarter-wavelength effect) in tandem with the presence or absence of the hind support. It is found that the low frequency absorption of the high-density foam with the hind support improved the absorption by 50% as the air gap is increased from 0 to 100 mm. The opposite effect is observed for the other materials tested whereby the sound absorption coefficient is reduced by 30 to 40% with the hind support.

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