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====Technology====
In a condenser microphone, also known as a capacitor microphone, the diaphragm acts as one plate of a [[capacitor]], and the vibrations produce changes in the distance between the plates.
There are two methods of extracting an audio output from the transducer thus formed. They are known as DC biased and RF (or HF) condenser microphones.
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where Q = charge in [[coulomb]]s, C = capacitance in [[farad]]s and V = potential difference in [[volt]]s. The capacitance of the plates is inversely proportional to the distance between them for a parallel-plate capacitor. (See [[capacitance]] for details.)
The charge across the capacitor is not maintained perfectly constant. As the capacitance changes, the charge across the capacitor changes to make the voltage drop across the capacitor equal to the bias voltage. However, the rate of this change is kept slow by using a series resistor of a very high value (of the order of 10 MΩ). Note that the time constant of a [[RC circuit]] equals the product of the resistance and capacitance.
Within the time-frame of the capacitance change (on the order of 100 μs), the charge thus appears practically constant and the voltage across the capacitor adjusts itself instantaneously to reflect the change in capacitance. The voltage across the capacitor varies above and below the bias voltage. The voltage difference between the bias and the capacitor is seen across the series resistor. The voltage across the resistor is amplified and reproduced to audio or recording.
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=====Technology=====
A small movable [[induction coil]], positioned in the [[magnetic field]] of a [[permanent magnet]], is attached to the [[diaphragm (acoustics)|diaphragm]]. When sound enters through the windscreen of the microphone, the sound wave moves the diaphragm. When the diaphragm vibrates, the coil moves in the magnetic field, producing a varying [[current (electricity)|current]] in the coil through [[electromagnetic induction]]. The frequency content of the generated signal is proportional to the perceived frequency. So a 1 kHz sine wave would generate an identical frequency output from the microphone. A single dynamic membrane will not respond linearly to all audio frequencies. Some microphones for this reason utilize multiple membranes for the different parts of the audio spectrum and then combine the resulting signals. Combining the multiple signals correctly is difficult and designs that do this are rare and tend to be expensive. There are on the other hand several designs that are more specifically aimed towards isolated parts of the audio spectrum. AKG D112 is for example designed for bass content rather than treble. In audio engineering several kinds of microphones are often used at the same time to get the best result.
The dynamic principle is exactly the same as in a [[loudspeaker]], only reversed.
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The most common unidirectional mike is a '''[[cardioid microphone|cardioid]]''' microphone, so named because the sensitivity pattern is heart-shaped (see [[cardioid]]). A '''[[hyper-cardioid microphone|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 mikes, since they are good at rejecting sounds from other directions. Because they employ internal cavities to provide front-back delay, directional mikes tend to have more coloration than omnis, and they also suffer from low-frequency roll-off. These problems are overcome to a large extent by careful design, but only the best cardioids can begin to approach the performance of a tiny low-cost omni in terms of absolute accuracy. This is not always recognised, but is the price paid for directionality, often needed to exclude ambient reverberation wherever very close placement is impossible.
'''Figure 8''' or '''[[bi-directional microphone|bi-directional]]''' mikes receive sound from both the front and back of the element. Most ribbon microphones are of this pattern.
[[Image:shotgun_microphone.jpg|thumb|200px|right|An Audio-Technica shotgun microphone]]
'''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.
An omnidirectional microphone is a pressure transducer; the output voltage is proportional to the air pressure at a given time.
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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