On the Single Number Rating of Sound Absorption

The International Standards Organisation (ISO) is currently developing a standard by which the frequency dependent values of the sound absorption coefficient of building materials can be expressed as a single number rating. This paper describes the new rating procedure and relates it to measurement techniques currently employed. The practical implications are discussed and some examples of results presented. The new rating procedure was tested on various brickwork to be employed for building facades. Some of them were variants of common products, only differing in surface roughness; others were of new material not yet commercially available. Modifications to the production process allowed bricks with a perforated surface to be measured where the depth and the width of the holes could be varied to give a strong reactive absorption at various frequencies. The measurements were performed in an impedance tube, using both the conventional pure-tone technique and a variant of the transfer-function method, which is also described.