The influence of finite and infinite wall cavities on the sound insulation of double-leaf walls

Abstract

Theories used to predict the sound insulation of double-leaf cavity wall systems are usually based on the assumption that the wall is of an infinite extent. To account for the effect of the finite extent of the wall, limiting the angle of incidence, a finite radiation efficiency model or the spatial windowing method is used in order to obtain realistic predictions. However, the effects of the finite extent of the cavity are often not included. This paper presents an extension of a finite two-dimensional cavity theory to include limp panels on each side of the cavity. It is shown that the oblique incidence mass-air-mass resonance can only occur for certain frequencies and certain angles of incidence. This is the reason why the infinite extent theories under-predict the sound insulation. The results of the predicted sound insulation agree with measurements when the wall cavity is empty. To obtain agreement when the cavity is full of a porous sound absorbing material, a flow resistivity of about one-fifth of the measured value has to be used. Use of the actual flow resistivity gives sound insulation values that are 10 dB too high.

A review of the different approaches to predict the sound transmission loss of building partitions

“…If there is no sound-absorbing material in the air cavity of an infinite double-leaf cavity wall, London 46 showed that its sound transmission coefficient above ω n is actually greater than the sound transmission coefficient of a single-leaf wall with the same total mass per unit area. This is not the case for finite double-leaf cavity walls because, as Cambridge et al 49 showed in this case, in any finite frequency range, there are only a finite number of discreet angular frequencies and angles of incidence at which an oblique incidence mass-air-mass resonance occurs. This is because there must be resonances along the length and along the height of the cavity as well as across the cavity in order to obtain an actual resonance.…”