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Microphones

A microphone is a transducer that converts acoustical sound energy into

electrical energy. It performs the opposite function to a loudspeaker, which converts electrical energy into acoustical energy. The three most common principles of operation are the moving coil or 'dynamic', the ribbon, and the capacitor or condenser.


The Moving Coil or Dynamic Microphone

The moving-coil microphone is widely used in the sound reinforcement industry, its robustness making it particularly suitable for hand-held vocal use. Wire-mesh bulbous wind shields are usually fitted to such models, and contain foam material which attenuates wind noise and 'p-blasting' from the vocalist's mouth. Built-in bass attenuation is also often provided to compensate for the effect known as bass tip-up or proximity effect, a phenomenon whereby sound sources at a distance Of less than 50cm or so are reproduced with accentuated bass if the microphone has a directional response. The frequency response of the moving-coil mic tends to show a resonant peak Of several decibels in the upper-mid frequency or 'presence' range, at around 5 kHz or so, accompanied by a fairly rapid fall-off in response above 8 or 10 kHz. This is due to the fact that the moving mass Of the coil diaphragm structure is sufficient to impede the diaphragm's rapid movement necessary at high frequencies. The shortcomings have actually made the moving coil a good choice for vocalists since the presence peak helps to lift the voice and improve intelligibility. Its robustness has also meant that it is almost exclusively used as a bass drum mic in the rock industry. Its sound quality is restricted by its slightly uneven and limited frequency response, but it is extremely useful in applications such as vocals, drums, and the micing-up of guitar amplifiers.



The Ribbon Microphone

The ribbon microphone at its best is capable of very high-quality results. The

comparatively 'floppy' suspension of the ribbon gives it a low-frequency resonance at around 40Hz, below which its frequency response fairly quickly falls away. At the high frequency end the frequency response remains smooth. However, the moving mass of ribbon itself means that it has difficulty in responding to very high frequencies, and there is generally a roll-off above 14kHz or so. Reducing the size (therefore the mass) Of ribbon reduces the area for the sound waves to work upon and its electrical output becomes unacceptably low. One manufacturer has adopted a 'double ribbon' principle which goes some way towards removing this dilemma. Two ribbons, each half the length of a conventional ribbon is mounted one above the other and are connected in series. They are thus analogous to a conventional ribbon that has been 'clamped' in the center. Each ribbon now has half the moving mass and thus a better top-end response. Both of them working together still maintain the necessary output.


The Capacitor or Condenser Microphone

The great advantage of the capacitor mic's diaphragm over moving coil and

ribbon types are that it is not attached to a coil and former, and it does not need to be of a shape and size which makes it suitable for positioning along the length of a magnetic field. It therefore consists of an extremely light disc, typically 12—25 mm in diameter, frequently made from polyester coated with an extremely thin vapor-deposited metal layer so as to render it conductive. Sometimes the diaphragm itself is made of a metal such as titanium. The resonant frequency of the diaphragm is typically in the 12—20kHz range, but the increased output here is rather less prominent than with moving coils due to the diaphragm's very light weight.

Occasionally capacitor microphones are capable of being switched to give a line level output, this being simple to arrange since an amplifier is built into the mic anyway. The high-level output gives the signal rather more immunity to interference when very long cables are employed, and it also removes the need for microphone amplifiers at the mixer or tape recorder. Phantom power does, however, still need to be provided.



Electret Designs

A much later development was the so-called 'electret' or 'electret condenser' principle. The need to polarize the diaphragm with 48 volts is dispensed with by introducing a permanent electrostatic charge into it during manufacture. In order to achieve this the diaphragm has to be of a more substantial mass, and its audio performance is therefore closer to a moving-coil than to a true capacitor type. The power for the head amplifier is supplied either by a small dry-cell battery in the stem of the mic or by phantom power. The electret principle is particularly suited to applications where compact size and light weight are important, such as in small portable cassette machines (all built-in mics are now electrets) and tie-clip microphones which are ubiquitous in television work. They are also made in vast quantities very cheaply.




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