A Uni-Directional Ribbon Microphone

Abstract

Directivity has been found to be desirable in sound collecting systems to improve the ratio of direct to generally reflected sounds and to otherwise discriminate against undesirable sounds. The bi-directional ribbon microphone is a pressure gradient instrument in which the response corresponds to the velocity component in a sound wave. By using a mass controlled element the velocity of the ribbon is in phase with the velocity in a sound wave. By a suitable combination of this microphone with a pressure-operated nondirectional ribbon microphone, a uni-directional ribbon microphone is obtained. The pressure ribbon microphone is resistive controlled and the response is a measure of the pressure component in a sound wave. The velocity of the ribbon in this type of pressure microphone is in phase with the pressure. Combination of the outputs of the pressure and velocity ribbon microphones produces a uni-directional characteristic. The response as a function of the angle θ between the normal to the ribbon and the direction of propagation of the incident sound is expressed by R = R0(1 + cos θ), where R0 is the sensitivity of either the pressure or velocity microphone for θ = 0. Results with an experimental microphone utilizing these principles verify the theory with respect to the phase relations and the directional characteristics. This microphone has been found to be useful in sound motion picture recording, radio broadcasting and sound reinforcing systems where the direct sounds originate in front and undesired sounds to the rear of the microphone.