Omnidirectional MicrophonesOmnidirectional implies equal sensitivity in all directions, but because the microphone mechanisms inevitably have some directional character, a true omnidirectional microphone is not achieved in practice. It is practical to achieve "omnidirectional" behavior in one plane, and that is what is usually implied by the word in practice. A directional pattern such as the cardioid has its maximum sensitivity along the microphone axis, and that directional pattern is a figure of revolution with uniform sensitivity in every direction perpendicular to the microphone axis. Therefore, if a cardiod microphone is pointed at the ceiling, then it is omnidirectional in the horizontal plane. This is in fact a useful microphone technique if you wish to pick up sound over a wide angle without excessive focusing on one direction. Another physical fact that prohibits a truly omnidirectional microphone is the action of diffraction in limiting the high frequency response of a microphone at large angles to the opening port to the microphone transducer. A high frequency (short wavelength) sound arriving at the rear of a microphone cannot bend around the microphone to enter the port. So at extreme angles, "omnidirectional" microphones may lack clarity because of tipping the response toward the bass frequencies.
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Index Sound reproduction concepts Reference White & White p299. | ||
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Figure-Eight MicrophonesThe "figure-eight" or bidirectional microphone pattern just refers to a microphone element which has symmetric pickup lobes to the front and rear. For a dynamic microphone where the air pressure moves a diaphragm attached to a voice coil, just providing equal air access to the front and back of the diaphragm will produce a figure-eight pattern. One use of figure-eight is in a geometry called a "Blumlein pair". A pair of such microphones is place so that if you drew lines from the microphones to the sound source, they would intersect at right angles. This has been found to be a good microphone placement for live music in an acoustically desirable room, providing a pleasing ambience. It is also sometimes used for pianos and drums.
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Index Sound reproduction concepts Reference White & White p299. | ||
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