Effect of water-films on the absorption of membrane absorbers

The membrane absorber is a fibre-free absorber which consists of vibrating metal membranes on a lightweight metal honeycomb structure. An advantage of this absorber is that the smooth cover-membrane seals the component against humidity and solid particles. In some cases the cover-membrane still needs to be frequently cleaned, and a flowing water-film can be used for this purpose. In this research the effects of such a water-film on the absorption characteristics of the membrane absorber have been experimentally investigated in a reverberation room. The results show that with a water-film the absorption coefficient of a membrane absorber is generally decreased, and the frequency of maximum absorption becomes slightly lower. It is noted, however, that when the water-film is relatively thin, say 1 mm, with which a membrane absorber can be effectively cleaned, its effect on sound absorption is negligible. Generally speaking, a vertically flowing water-film has less effect on the absorption of membrane absorbers than a horizontal water-film. Measurements also show that the noise level caused by a flowing water-film is acceptable in practice. (C) 1998 Elsevier Science Ltd. All rights reserved.