Transmission of Reverberant Sound through Double Walls

Abstract

The transmission of reverberant sound through a double wall, which consists of two identical single walls coupled by an airspace, is investigated both theoretically and experimentally. A theory is developed which gives good agreement with experiment. In order to compute the transmission loss of a double wall, it is necessary to know the impedance Zw of the single wall. Zw was determined from experiments conducted on the single wall and includes the effects of mass, dissipation, and flexural motion. The treatment shows that it is impossible to get a large improvement in transmission loss for a double wall relative to a single wall under reverberant sound field conditions if the single wall is considered to have only mass reactance. In addition, the customary normal incidence theory is totally inadequate in explaining the behavior of a double wall in a reverberant sound field. For double walls having air-coupling only, very shallow airspaces can produce appreciable increases in transmission loss over a single wall. An absorbent material when inserted in the airspace produces large improvements only when the mass of the walls is relatively light and has but little effect for heavy walls. Honeycomb or other non-absorbent cellular structures having no cell walls in a direction normal to the wall faces do not result in an increase in transmission loss. Air-coupled walls having no solid sound conducting paths between individual septa are extremely effective sound insulators as compared to conventional double wall constructions. The theory indicates that a large improvement in the transmission loss of a double wall can be obtained by using as components single walls with high internal dissipation.