A Non-Directional Microphone 



By R. N. MARSHALL and F. F. ROMANOW 



A moving coil microphone is desc ril>etl which responds uniformly over a 

 wide frequency range to sound arriving from any direction. A study of 

 diffraction, the main factor causing directivity of microphones of the pressure 

 type, leads to the conclusion that a small spherical shape is the most desirable 

 for a non-directional microphone. But even fulfilling this requirement in 

 the design of the housing leaves a large directional effect. Hence an 

 acoustic screen has been developed which diminishes diffraction to an extent 

 necessary to make the change in response due to angle of sound incidence 

 imperceptible to the ear. The non-directional microphone is of simple and 

 rugged construction. Adecjuate precautions have been taken to prevent 

 atmospheric changes from affecting the stability. The small size and 

 unusual shape of the microphone contribute much to its attractive ap- 

 pearance. 



IN many situations — such as when a microphone is used as a pick-up 

 for large orchestras or choruses, or in sound picture studios — the 

 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 

 f.orm of distortion. This distortion was minimized in the Western 

 Electric 618-A type moving coil microphone ^ by selecting the con- 

 stants 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 micro- 

 phone 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 arbi- 

 trary 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. 



* Since the non-directional microphone is generally mounted with its diaphragm 

 in a horizontal plane the angles of incidence have been labeled 0°, ± 30°, ± 60°, 

 ± 90° retaining 0° as the angle of incidence for sound waves moving in the horizontal 

 plane. 



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