Microphone

A microphone, sometimes called a mic (pronounced "mike"), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, tape recorders, hearing aids, motion picture production and in radio and television broadcasting.

Directionality

A microphone's directionality or polar pattern indicates how sensitive it is to sounds arriving at different angles about its central axis. The polar pattern represents the locus of points that produce the same signal level output in the microphone if a given sound pressure level is generated from that point.

Related Topics:
Locus - Sound pressure level

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An omnidirectional microphone's response is generally considered to be a perfect sphere in three dimensions. In the real world this is not the case. As with directional microphones, the polar pattern for an ?omnidirectional? microphone is a function of frequency. The body of the microphone is not infinitely small and as a consequence, it tends to get in its own way with respect to sounds arriving from the rear, causing a slight flattening of the polar response. This flattening increases as the diameter of the microphone (assuming it's cylindrical) reaches the wavelength of the frequency in question. Therefore, the smallest diameter microphone will give you the best omni characteristics at high frequencies.

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A unidirectional microphone is sensitive to sounds from only one direction. The diagram above illustrates a number of these patterns, with the microphone capsule being represented as a red dot. The top of the diagram is the front of the mic. The sound intensity for a particular frequency is plotted for angles radially from 0?360°. (Professional diagrams show these scales and would include multiple plots at different frequencies. These diagrams just provide an overview of the typical shapes and their names.)

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The most common unidirectional mic is a cardioid microphone, so named because the sensitivity pattern is heart-shaped (see cardioid). A hyper-cardioid is similar but with a tighter area of front sensitivity and a tiny lobe of rear sensitivity. These two patterns are commonly used as vocal or speech mics, since they are good at rejecting sounds from other directions.

Related Topics:
Cardioid - Cardioid - Hyper-cardioid

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Figure 8 or bi-directional mics receive sound from both the front and back of the element. Most ribbon microphones are of this pattern.

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Shotgun microphones are the most highly directional. They have small lobes of sensitivity to the left, right, and rear but are significantly more sensitive to the front. This results from placing the element inside a tube with slots cut along the side; wave-cancellation eliminates most of the off-axis noise. Shotgun microphones are commonly used on TV and film sets, and for location recording of wildlife.

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An omnidirectional microphone is a pressure transducer; the output voltage is proportional to the air pressure at a given time.

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On the other hand, a figure-8 pattern is a pressure gradient transducer; the output voltage is proportional to the difference in pressure on the front and on the back side. A sound wave arriving from the back will lead to a signal with a polarity opposite to that of an identical sound wave from the front. Moreover, shorter wavelengths (higher frequencies) are picked up more effectively than lower frequencies.

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A cardioid microphone is effectively a superposition of an omnidirectional and a figure-8 microphone; for sound waves coming from the back, the negative signal from the figure-8 cancels the positive signal from the omnidirectional element, whereas for sound waves coming from the front, the two add to each other. A hypercardioid microphone is similar, but with a slightly larger figure-8 contribution.

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Since directional microphones are (partially) pressure gradient transducers, their sensitivity is dependent on the distance to the sound source. This is known as the proximity effect, a bass boost at distances of a few centimeters.

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