Perforated Facing and Sound Absorption

In the analysis of the absorption characteristics of a porous layer with perforated facing, it is usually assumed that the facing contributes only a mass reactance to the total impedance. In the present paper it is shown that the facing also causes an additional acoustic resistance which is often larger than the acoustic resistance of the porous layer itself. This resistance obtained only when the porous material is in close contact with the facing and becomes practically zero when the air space between facing and material exceeds approximately one perforation diameter. The effect is due to the near field (higher mode) losses in the porous material around the perforations. The resulting effect on the absorption characteristics considerable, as demonstrated in some examples, and may at least partly explain reported disagreement between measurements and previous calculations.