Effect of Air Gap, Thickness of Polyurethane (PU) Foam, and Perforated Panel on Sound Absorption Coefficient for Acoustic Structures

Abstract There are many applications and sectors in today’s era where noise pollution is reaching very high levels. This prolonged noise exposure leads to a detrimental health effect. This may cause loss of concentration, working efficiency, headache, increased blood pressure level, annoyance, and even workplace accidents. Hence, the need to reduce the high noise level is becoming a significant issue. Varieties of absorbing materials are being used in the recent age for noise reduction and sound attenuation. Most of the acoustic enclosures consist of polyurethane foam as a sound absorber. The noise control is achieved by modifying the noise source characteristics, path modification, or muffling at the receiver. The best way to control noise is noise absorption, where acoustic materials are designed to absorb sound. Polyurethane foam is widely accepted because of its good properties like wide absorption frequency range, structural stabilization, low cost, easy handling, and moisture resistance. It has been observed that the sound absorption coefficient (SAC) depends on the thickness of polyurethane foam and the air gap. This paper discusses the different acoustic properties of various acoustic structures consisting of the perforated panel, air gap, and polyurethane foam for the frequency range of 100–4000 Hz. Different combinations of acoustic structures are proposed to observe the effect of absorptivity in terms of sound attenuation. The effect of layer sequence on the acoustic absorption of the structure has been investigated. The relation between the thickness of the material, air gap, and SAC are also analyzed. FEA can accurately analyze the absorption characteristics at normal incidence of sound-absorbing structure. The acoustic structure consisting of polyurethane foam gives the maximum value of SAC for a frequency range of 2300–4000 Hz. The acoustic structure consisting of the perforated panel, air gap, and polyurethane foam give the maximum SAC value for the lower frequency range of 100 to 2300 Hz. In the current study, it has been seen that the acoustic structure consisting of polyurethane foam with an air gap gives the minimum value of SAC. The SAC varies with the air gap and thickness of the foam. The study affirmed that using the perforated panels in acoustic structure gives the maximum value of SAC in the lower frequency range. The SAC of a material is calculated with an analytical method and verified with the experimental method and FEA software. For verification of SAC for different acoustic structures, COMSOL Multiphysics 5.5 results are seen to be in good agreement with a maximum difference of 8.1% with that of the experimental results.