Absorption characteristics of fibrous material covered with perforated facing and film

We propose a method for predicting the absorption characteristics of a fibrous material, that is glass wool, covered with a perforated facing and an impermeable film, typically used for noise barriers. The method is based on Ingard and Bolt’s model. It accounts for interactions among perforated facing, film and fibrous materials. The interaction occurs in areas where they are close to each other. That area was determined empirically as the coverage. The coverage is approximately 10 mm for a perforated facing with a 0.2 open area ratio. In the coverage, the perforated facing increases the acoustic impedance of film and fibrous material according to distance. The fibrous material causes acoustic resistance to the film when the film contacts the fibrous material. The formulae for their acoustic impedances were derived from many results of acoustic impedance measured using an impedance tube. The end correction of holes of the perforated facing was modified using the relationship between the measured values of resonance frequency for Helmholtz resonators with the perforated plate and their open area ratio. Results predicted by this method agree well with measured results obtained in most instances.