A Lapel Microphone of the Velocity Type

The requirements for satisfactory operation of a lapel microphone are shown to be (1) a wide band frequency characteristic compensated for diffraction of the voice around the head, (2) means for keeping the output constant while the head is turned, (3) satisfactory sensitivity, and (4) light weight. The first is accomplished by calculating the diffraction and compensating for it. The second is accomplished by utilizing the velocity principle and so orienting the microphone that the region of optimum sensitivity lies in the direction of the mouth when the head is turned to the side away from the microphone. Suitable compensation is used to make the output proportional to sound pressure rather than wave velocity. Statisfactory sensitivity is attained with only three ounces of total weight by carefully proportioning the microphone. The latter consists of a very thin aluminum ribbon suspended between the poles of a small permanent magnet. It occupies only one fortieth the volume of a standard velocity microphone. The over-all effective frequency characteristic is flat from 80 to 7000 cycles with a deviation of ±2 decibels.