This paper defines the factors that led to a change in the frequency characteristic (an increase of sensitivity in the lower frequencies when the distance from the sound source is decreasing) of directional microphones when the sound wave changes its form from a plane to a spherical wave front. An equation is presented in this paper which shows the factors that define the change in the sensitivity. These are the frequency, the distance between the microphone and the sound source, the distance between the two acoustic entrances, and the directional characteristics (polar pattern) of the microphone.:
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