PREDICTING THE ABSORPTION OF OPEN WEAVE TEXTILES AND MICRO-PERFORATED MEMBRANES BACKED BY AN AIR SPACE

Abstract

Although theories for both membrane absorbers and micro-perforated plate absorbers have been well developed, it appears that no theory has been given for their combination, such as a glass-fibre textile or a micro-perforated membrane mounted over an airtight cavity. This paper presents a theoretical method for predicting the absorption of such a structure. The basic idea of the theory is to regard an open weave textile or a micro-perforated membrane as a parallel connection of the membrane and apertures. The predictions for both normal and random incidence have shown very good agreement with measurements. It has also been demonstrated that the absorption performance of such structures can be very high. Typically, with appropriate parameters the absorption coefficient of a glass-fibre textile or a micro-perforated membrane mounted at 100 mm from a rigid wall can exceed 0·4 over 3–4 octaves. With two layers of the material over the same total air space of 100 mm this range can extend to 4–5 octaves.