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Problems of Theory and Designing for Directional Interference Microphones

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The mathematical model and theoretical analysis of the work of the directional microphone which con-sists of the interference tube and pressure - gradient capsule are presented. The analytical expressions for angular dependence of the geometrical path length and directional characteristic are received. The physical reasons of the difference between polar pattern of such microphones and separate capsule in the low frequencies range are ascertain. The dependence of the required rear aperture acoustic resistance on the acoustic antenna length is shown. The reasons of the polar pattern axis asymmetry are analyzed. The practical recommendations for such microphones designing are given.

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Scott Dorsey
Scott Dorsey

Comment posted February 20, 2021 @ 17:40:49 UTC (Comment permalink)

Much of the information in this paper can be found in Harry Olson's work at RCA where he describes the basic physics of interference tube microphones and the concept of how correlated signals from the front appear at the capsule unchanged while correlated signals from the side are deliberately smeared with respect to time. 

However this paper goes one stage farther than Olson's work and goes to describe the axial asymmetry that results from the entrance holes being on one side of a tube that has a diameter that is a large portion of a wavelength.

Note that Olson's spiral design as used in the RCA10006A avoids this but brings along with it a whole other set of problems resulting from the limited number of entrance holes. 

This is a useful paper for anyone interested in interference tube microphones.

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