A Uni-Directional Ribbon Microphone

Abstract

Directivity has been found to be desirable in sound pickup systems to improve the ratio of direct to generally reflected sounds and to otherwise discriminate against undesirable sounds. The ribbon microphone is a simple directional system in which the directional characteristics are independent of the frequency. The response is a function of the angle between the normal to the ribbon and the direction of propagation of the incident sound. The ribbon microphone is a pressure gradient microphone and the response is a measure of the velocity component in a sound wave. Using a mass controlled element the velocity of the ribbon is in phase with the velocity in the sound wave. By a suitable combination of this microphone with a pressure operated ribbon microphone a uni-directional microphone may be obtained. The pressure ribbon microphone should be resistive controlled and the response is then a measure of the pressure component in the sound wave. The velocity of the ribbon in this microphone is then in phase with the pressure. By combining the outputs of two such microphones 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 the pressure and velocity ribbon microphone for θ=0. Preliminary experimental results verify the theory with respect to the phase relations and the directional characteristics.