A Tubular Directional Microphone
W. P. Mason, Bob Marshall · The Journal of the Acoustical Society of America · 1939
A tubular directional microphone is described which consists of a pressure type microphone coupled to an acoustic impedance element composed of a large number of tubes whose lengths vary by equal increments. The function of this variation in length is twofold. First, the multiple resonances of the individual tubes occur at intervals so close together that the net effect of the bundle is that of an acoustic resistance over a fairly wide frequency range and so does not impair the high quality of the attached microphone. Second, high directivity is secured, because for sound incidence other than normal each tube introduces a different path length with phase cancellation resulting in a composition chamber between the microphone and the ends of the tubes. The theory of operation is summarized and data are presented to show the performance of the instrument which is in fair agreement with the theory.