This paper defines the factors that lead to a change in the frequency characteristic (increase of thesensitivity in the lower frequencies, when the distance from the sound source is decreasing) of directionalmicrophones, when the sound wave changes its form from a plane to a spherical wave front. An equationis presented in this paper that shows the factors which define the change in the sensitivity. These are -the frequency, the distance between the microphone and the sound source, the distance between the twoacoustic entrances and the directional characteristics (polar pattern) of the microphone.
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