Some Data on a Room Designed for Free Field Measurements

Abstract

This paper gives some data on the absorbing material used in a room recently completed and intended for acoustical measurements under essentially free field conditions. The absorbing material, which consists of a number of layers of cloth separated by air spaces, is shown to have an absorption coefficient of the order of 98 to 99 percent over a wide frequency range. Some data are also given showing the degree of departures of the sound field in the room from the field to be expected in free space. It is shown that it may not be possible to duplicate a free field in an enclosed room, regardless of the freedom from stationary waves obtained, in the sense that the pressure may decrease more rapidly than the inverse distance law applying to a free field.