The Dependence of Sound Transmission Measurements on Microphone Position

Abstract

In the customary method of determining the transmission loss of a wall or floor partition it is necessary to measure the difference in sound levels existing in two rooms which have the partition as a separating wall. Also, the ratio S/A2, where S is the transmitting area of the partition and A2 the total sound absorption of the receiving room, must be known. It was found that despite adequate precautions the sound energy density in both test rooms was not entirely uniform. In particular, on the quiet side a sharp drop in level occurred within a region of 2 or 3 feet from the panel face. The sound energy density existing at the panel face on the quiet side may be considered to consist of a direct component arising from energy radiated directly off the panel and a reverberant component arising from reverberant energy existing throughout the entire room. On this basis a formula was computed which allows one to determine transmission losses by restricting measurements on the quiet side to positions at the panel face. The formula was found to be in very good agreement with experimental results over a wide range of variation of A2. All of the above data were obtained with a pressure microphone. A number of similar measurements, using a pressure gradient (ribbon) microphone of special design, were also taken. The results so obtained were independent of the amount of absorption in the room and the area of the panel, although to secure the actual transmission loss a constant correction is required.