ABSTRACT
Consider a sound source in a room with an open door or window, as illustrated in Fig. V.9. As the sound passes through each point of the door, it acts like a local source and generates a spherical wave front. Several are shown spanning the doorway. In addition, there are an infinity of others from each point in the doorway. The net result is for destructive interference to be worse the farther one is off the perpendicular from the middle of the door, as sketched in the sound distribution. The exact mathematical analysis of the infinity of interferences involved is complex, but the answer is simple. If dAc denotes the distance interval of significant sound along a line parallel to the side of the room at the distance D from the doorway of width W, then it can be shown that:
where c denotes the speed of sound, f is the frequency of a pure tone sound source, and where all distances, including that involved inc, are in the same units.