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Sensitivity pattern for an omnidirectional microphone. It is equal in all directions |
An omnidirectional (or nondirectional) 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 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 gives the best omnidirectional characteristics at high frequencies.
The wavelength of sound at 10 kHz is little over an inch (3.4 cm), so the smallest measuring microphones are often 1/4" (6 mm) in diameter, which practically eliminates directionality even up to the highest frequencies. Omnidirectional microphones (unlike cardioids below), do not employ resonant cavities and therefore add very little to the original sound. Being pressure-sensitive they can also have a very flat low-frequency response down to 20 Hz or below. Pressure-sensitive microphones also respond much less to wind noise and plosives (such as popping the letter P) than directional (velocity sensitive) microphones.
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Sensitivity pattern for a bidirectional microphone. It has strong sensitivity to the front and rear but little sensitivity to the sides. |
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Sensitivity pattern for a cardioid microphone. It has little sensitivity to the rear. |
A cardioid microphone is so named because the sensitivity pattern is heart-shaped. The microphone has resonant cavities that delay some of the sound that comes from the rear in such a way that the delayed sound cancels the non-delayed sound.
Cardioid microphones are velocity sensitive rather than pressure sensitive. This means that the sensitivity drops-off at low frequencies (the pressure is changing more slowly at low frequencies). To compensate, many cardioid microphones incorporate low pass filters to boost bass response. However, when the microphone is very close to the sound source, such as with a person speaking directly into the microphone, a cardioid microphone begins to act like a pressure sensitive microphone. This results in an increased the sensitivity to low frequencies. This is known as bass tip-up or the proximity effect.
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Sensitivity pattern of a shotgun microphone. It is strongest directly in front of the microphone |
Shotgun microphones are highly directional. They have small lobes of sensitivity to the left, right, and rear but are significantly less sensitive to the side and rear than other directional microphones. This results from placing the element at the end of a tube with slots cut along the side; wave cancellation eliminates much of the off-axis sound. Due to the narrowness of their sensitivity area, shotgun microphones are commonly used on television and film sets, in stadiums, and for field recording of wildlife.
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A shotgun microphone. |
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Parabolic microphone. |
A parabolic microphone is a microphone that uses a parabolic reflector to collect and focus sound waves onto a receiver, in much the same way that a parabolic antenna (e.g., satellite dish) does with radio waves or a telescope mirror focuses light.
A parabolic microphone is useful for picking up sounds from a considerable distance. However, they tend to have a poor low-frequency response. This is because the wavelength of the sound waves must be smaller than the reflector to be affected. A parabolic microphone that works at frequencies as low as 60Hz would be about 20 feet in diameter. At low frequencies a parabolic microphone is essentially a cardioid microphone pointed in the wrong direction.
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