Acoustic impedance of perforations in contact with fibrous material

Acoustic impedance of perforations in contact with fibrous material, as well as air alone, is experimentally determined in the absence of mean flow. Different porosities (2.1, 8.4, 13.6, and 25.2 %) and hole diameters (0.249 and 0.498 cm) are applied for perforations, along with two fiber filling densities (100 and 200 kg m3) to illustrate the effect of variations in such parameters on the perforation impedance. A modified impedance tube setup is developed for the measurement of the acoustic impedance of perforations in contact with fibrous material, and the air alone. The complex wave-number and characteristic impedance of the fibrous material are also experimentally obtained in this study to utilize such properties for the calculation of acoustic impedance of perforations in contact with the material. The experimental results show that both resistance and reactance of the perforations in contact with air decrease as the porosity increases. It is also shown that the fibrous material significantly increases both the resistance and reactance. For a given filling density, both resistance and reactance decrease as the porosity increases, except for the specific samples with highest porosity (25.2%).