Experimental Study on the Effect of Air Gap, Thickness, and Density on Acoustic Impedance and Sound Absorption of Stone Wool and Rubber Samples with Transfer Function Method

Effects of air gap behind two sample thicknesses and two different densities are investigated in this study on the grounds of acoustic impedance and absorption aspects. An impedance tube was built for the measurements, and the study was carried out for different air gaps. In addition, the impedance and absorption of stone wool with densities of 80 and 120 kg/m3 and rubber with densities of 1058 and 1252 kg/m3 were measured. The 25-mm-thick stone wool had the maximum impedance value in frequencies below 250 Hz. The impedance of 25 and 50 mm thicknesses decreased in low frequencies, when the air layer was increased. Higher thicknesses of the stone wool in the frequency range of 160–2500 Hz had higher impendence rates. Maximum impedance in 50-mm-thick rubber was related to the 3-cm air gap and frequency of 160 Hz. As compared to lower densities, higher impedances were observed in all frequencies except for 630 Hz and lower than 1600 Hz, respectively, in 25-mm-thick and 50-mm-thick stone wool with the density of 120 kg/m3. For the sample of 25-mm-thick rubber, with density of 1252 kg/m3, yielded higher impedance in all frequencies except 200, 500, 800 and 1250–1500 Hz, and for 50-mm-thick rubber with density of 1252 kg/m3, impedance was higher in all frequencies except 250–315 and 630 Hz. The effects of air gap, thickness and density on impedance were unlike the effects of those on absorption. According to the results, the acoustic characteristics of materials can be determined in order to achieve the desired results for noise control methods.