The Microphone

AB INITIO

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A microphone, (often shortened to ‘mic’), is a transducer between sound waves in air and, (usually), electrical signals in wires. Sound waves themselves are periodic fluctuations in air pressure and the consequent displacement of air molecules. These can be caused by a wide variety of artefacts, but the main ones we are concerned with are caused by vibrating objects such as the vocal chords of the human larynx and the specialised parts of musical instruments. To sense these pressure or displacement changes, a microphone usually uses a thin metal or plastic “diaphragm”, whose movement is coupled to an electrical circuit to produce the electrical signal.

History
In the early days of radio, (the 1920s and 30s), when it was considered a great achievement to communicate over a distance of a few thousand miles, amateurs took pride in describing their home made equipment in detail, including their microphone. Speech quality and clarity were a prime consideration. However, things have changed, and there is little point in the modern amateur using a high quality microphone because extensive filtering out of both high and low audio frequencies is now usually used to achieve a minimum bandwidth. This narrow band filtering applies to both transmitters and receivers in the interest of “punching through” the QRM using as little of the spectrum as possible. In days of yore, this pride in the quality of transmission by both amateurs and professional public service broadcasters, led to the development of high quality microphones, (of so called “arm chair” quality). In this context, “quality” means the faithfulness of reproducing the original sound waves as electrical signals.

Although Early experiments by Alexander Graham Bell and Thomas Edison were capable of transmitting the sound of a vibrating reed by electromagnetic induction, their original apparatus was not capable of transmitting intelligible speech. This was first done, in 1877 by Bell using a “liquid microphone”. This consisted of a diaphragm the centre of which was connected to a needle, and the point of which dipped into a bowl of acidified water. As sound waves moved the diaphragm and needle, the depth of penetration into the water, and hence the electrical resistance of the contact varied. However, this was never going to be a commercial device. Nevertheless, also in 1877, Thomas Edison used the same principle of “variable resistance”, to invent and patent the “Carbon microphone”. This consisted of a thin metal diaphragm and a rigid back plate between which were loosely packed some carbon granules. The vibrations of the diaphragm varied the contact pressure between the granules and thus the electrical resistance through them. A small voltage between the diaphragm and the back plate produced a current through the device which was modulated by the varying pressure, thus reproducing the variations in the incident sound pressure wave. This rugged and sensitive device formed the backbone of the world telephone system for the next hundred years, although the quality of the sound reproduction was fairly poor by modern standards. The voice modulation of the current was sufficiently large that, when a common diaphragm was used, shared between an ear-phone and the carbon microphone, the composite device formed a speech amplifier of considerable gain and was used in early telephone repeaters. The voice varying resistance of the carbon granules could not only be used to modulate a DC current, but could also be used to modulate the RF output of a CW transmitter by placing the carbon microphone in the earth lead of the aerial-earth system. This formed the basis of the first radio transmitter of speech.

A variation on the carbon granule method was the “Transverse Current Microphone” where the diaphragm was made of mica or rubber and the back plate was an insulator. The carbon granules were packed between the diaphragm and the back plate and the current flowed parallel to the diaphragm, i.e. from side to side. This resulted in considerably improved speech and music quality due to the high mechanical damping of the diaphragm but lower sensitivity. This type was used by broadcasters and radio amateurs for several years during the 1930s and just post war. However, the quest for “more perfect reproduction of sound” led to the several types which are still in use today.

Modern Times
Although moving coil microphones, (sometimes known as “dynamic microphones”), constructed like miniature loud speakers gave acceptable broadcast quality, the first really high quality microphone, suitable for broadcasting concerts, was the “Ribbon Microphone”. This did not have a diaphragm in the conventional sense but a very thin, slightly corrugated aluminium strip mounted between the poles of a powerful magnet. Unlike a diaphragm, which inevitably has some stiffness resulting in a resonant frequency, the ribbon was relatively free to float, and was responsive to air movement rather than fluctuations in air pressure. Although capable of superb sound quality over an extended frequency range, it was rather heavy and the aluminium strip was susceptible to shock and wind damage. It also suffered from very low source impedance and low output voltage, requiring an adjacent “pre-amplifier”, sometimes known as a “head amplifier”. These latter, and the low output, were significant disadvantages in the days of valve amplifiers which often suffered from mains hum and noise problems. However, these problems could be alleviated by using a high ratio step-up transformer between the microphone and the amplifier. In modern times, the use of a small transistor amplifier incorporated into the microphone and supplied with DC via the coaxial connecting lead has meant that the ribbon microphone has survived in some applications to the present day.

A close contender in quality, and contemporary with the ribbon mic was the moving coil mic referred to above. Unlike the carbon mic, it required no polarizing voltage and its output was sufficient to drive a modern transceiver through several metres of screened cable without the need for an adjacent head amplifier. It works by inducing an emf into a small coil which sits in the annulus of a specially shaped powerful magnet, the coil being attached to a metal or plastic diaphragm. It is capable of good quality and can be made quite small, (less than an inch in diameter), and is probably the commonest type in use today.

Another high quality microphone, but one that has gone out of fashion, is the “condenser mic”, in today’s terminology more properly called the “capacitor mic”. This has a light weight metal or metalised plastic diaphragm, unburdened by any attachment such as a coil. The diaphragm is mounted close to a rigid back plate and a voltage is applied between the two via a high value resistor. As the diaphragm vibrates, the capacitance between the metalised diaphragm and the back plate varies in sympathy generating a voltage. The output is taken from across the high value resistor. Although capable of high quality, it cannot be used without a high input impedance head amplifier in close proximity. This is because the tiny changes in capacitance due to the sound waves would be dwarfed by the capacitance of the screened lead connecting it to a main amplifier or transceiver. It also requires an inconveniently high polarising voltage of up to 100 Volts. It has largely been replaced by the “Electret mic”.

The electret mic works on a similar principle to the condenser mic and there are several forms. The operating principle is roughly as follows. Instead of a metal diaphragm, it uses a thin plastic film as its diaphragm, thinly metalised on its outer surface. The plastic film, (the electret or ferro-electric material), carries a permanent electro-static charge, and so it does not require a separate polarising supply. The electrical output is taken between the metalised film and the closely spaced back-plate. It usually incorporates a single transistor head amplifier into the body of the unit. The electret mic is also capable of somewhat higher output voltage than the condenser mic. It can also be made quite small, and is often seen clipped to the lapel or shirt of a TV broadcast interviewer. Its sound reproduction quality is fairly high but this is influenced by the body shape of the microphone and where it is worn on the clothing. It is also cheap and can be mass produced.

Another form of relatively common microphone uses a piezo-electric crystal. This produces an emf across it when subjected to pressure or other forms of distortion. (This is the same principle as that employed in the quartz crystal used to control the frequency of RF oscillators). The most common sort of “Crystal mic” uses a diaphragm connected mechanically to the crystal, (with the attendant problems of diaphragm resonance), but a higher quality version but with a much lower output voltage uses direct impingement of the sound waves upon the crystal. Both types produce a rather lower output than the electret mic and usually require a head amplifier integrated into the body of the mic. All microphones have a “polar diagram of sensitivity”, much like aerials, which depends on the shape of their housing and whether they are mainly sensitive to variations in air pressure or air velocity.

There are also various kinds of optical microphone which use optical fibres or lasers to sense the vibrations of a diaphragm, but these have not yet found much application in amateur radio.

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