A Non-Directional Microphone

TN many situations--such as when a microphone is used as a pick-up -- for large orchestras or choruses, or in sound picture studios--the 1 sound reaching the microphone directly may be only a small part of the total. Most of the sound arrives at the microphone from directions other than normal to the plane of the diaphragm. If the microphone response differs in these various directions, the output will not truly represent the sound at the point of pick-up--and this is, of course, a form of distortion. This distortion was minimized in the Western Electric 618-A type moving coil microphone 1 by selecting the constants of the instrument so that the field response would be as uniform as possible for sound of random incidence. Still there remained a considerable change of response with the angle of sound incidence and with frequency as is shown in Fig. 2. In the non-directional microphone this variation (Fig. 3) has been greatly reduced so that it is imperceptible to the ear. Moreover, the new microphone is designed to be mounted so that its diaphragm is horizontal.* In this position the instrument is symmetrical with respect to a vertical axis through the center of the diaphragm. If a sound source is placed at some arbitrary location we may rotate the microphone around this vertical axis without changing its response. Hence the instrument is entirely non-directional with respect to the vertical axis. If the microphone is rotated around an axis in the plane of and through the center of the diaphragm a very slight residual directional effect remains and it is this one which has been plotted.