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What It All Means

Microphones

It's the microphone's turn to have all its technical terms translated into understandable language.


'It' being the manufacturer's spec sheet. What good are all those figures if you don't know what the words mean. This month MAKING MUSIC'S regular dictionary of terms looks at...

BALANCED: A balanced mike cable has two conductors arranged so that any outside interference — such as hum — acts on them identically. With the right connections at the far end of the cable, via a transformer, the two lots of hum will meet each other out of phase and cancel each other out (silence), while the true signal will come out unscathed. Unbalanced leads have single conductors and no self-cancelling facilities.

BINAURAL: Also known as artificial or 'dummy' head stereo. Two mikes are placed in precisely the position your ears would be - hence the dummy head - and a stereo recording made. On playback, it should produce exactly the same sort of sounds as your lug'oles would cop.

CARDIOID: See directivity. A heart-shaped pickup pattern, with the greatest sensitivity straight out in front (0 degrees), and the greatest power to 'ignore' sound directly behind it (180 degrees).

CONDENSER: Based on the principal of the capacitor. Two facing plates are charged - one positive, one negative - so that a voltage is created across the plates. The closer they are, the higher the voltage. One plate is fixed while the other can be moved by air waves falling upon it (sound, as we all know from Physics Is Fun, is no more than fluctuations in air pressure). As the plate moves the voltage will change, and there's your electrical representation of the sound heard.

DIRECTIVITY: Some mikes, like some blokes, will pickup anything within a 360 degree radius. This is not always desirable. There are disadvantages with mikes, too. Microphones can be shaped to ignore, say, sounds behind them and to the side, and concentrate on the stuff in front. These shapes, represented on a circular graph, are the pickup patterns - cardioid, omni, etc.

DYNAMIC: A dynamic microphone has a light diaphragm attached to a wire coil, suspended between the poles of a magnet. As this moves with the incoming sound waves, the wire is shifted between the poles so inducing a tiny electrical current - again, a voltey representation of the sound.

ELECTRET: Original condenser mikes were delicate and needed an outside power source to keep up the charge. Electrets are tougher these days and feature permanently charged plates. Still needs a battery for the built-in amp, but it's very small.

HYPERCARDIOID: This pattern produces an even tighter focus on the sound in front of the mike (better than cardioid) but only a 50 per cent rejection at 180 degrees (worse than cardioid).

IMPEDANCE: see open circuit sensitivity.

OMNI-DIRECTIONAL: A pick-up pattern that will record sounds from all sides uniformly.

OPEN CIRCUIT SENSITIVITY: Mikes, like many other electrical devices, should be measured when loaded. That is, you gauge the output they can produce not in theoretical mid-air but connected to a real mixing desk. The desk will put up some opposition to the incoming signal and this will reduce its actual strength. This reaction is called impedance, and in this instance, the higher the impedance, the lower the amount of opposition. Rather than drag an entire desk around you can use a few simple components to 'load' the mike with an equivalent impedance for test purposes. Unfortunately, Europe and America can't agree. We (and japan) measure at 1KOhm, or thereabouts. America may use 300, 150 or 600 Ohms. They produce quite different results, and microphone manufacturers, despairing of quoting half a dozen standards to satisfy everyone, plumped for their own - open circuit sensitivity, equivalent to a theoretically huge impedance and therefore negligible opposition. While, surprisingly, 1 or 2K Ohms of impedance may offer little extra grief to the incoming signal, 600 Ohms will. Open circuit sensitivity is great for the theoreticians, but in practice, it's more useful to remember that mikes churning out the power on a British 1KOhm desk, might lose 4-9dB of oomph (two or three times their strength) on an old American job.

PAD: For mikes faced with lots of loud stuff, a pad will knock back the incoming signal to a manageable level, and prevent distortion.

PHANTOM POWERING: Those original electrets were kept powered by a DC supply (usually 48V) fed down their own balanced cables, normally from the mixing desk.

PRESSURE GRADIENT: The various pickup patterns are developed by forming paths within the microphone that lead some of the sound pressure round to the back of the diaphragm. The general term for this family is Pressure Gradient Microphones.

RIBBON: A microphone which has an aluminium ribbon as its moving diaphragm.

SENSITIVITY: How well a mike will react to the softest sounds it can hear. A condenser mike will be more sensitive than a dynamic because it will produce a greater electrical output for any given sound. Condensers also generally have a better frequency response as the moving plate is many times lighter than the dynamic's diaphragm, and can react more quickly and accurately. On the other hand, dynamics are tougher and better able to withstand the high pressure levels put out by, say, a bass drum. But, technology being what it is, the barriers are growing ever blurrier.

SIDE ATTENUATION: A measure of how much the sound recorded from a particular direction is reduced compared with the main direction the mike is dealing with. Expressed in dB. If a mike had a side attenuation at 100 degrees and 1200Hz of 3dB, this means that the sound level at that angle and frequency would generate a voltage 3dB less (half) of what it would at 0 degrees.

SOUND POWER CONCENTRATION: See directivity. A measure of how good a mike is at concentrating its attention in the right direction. An average cardioid figure might be 3, hypercardioid at 4 would be better still. However, upping the SPC can distort the pickup pattern elsewhere. You might reduce its ability to reject sounds behind it, for example.

SOUND PRESSURE LEVELS: Put (very) crudely, a way of measuring the loudness of an incoming signal. Sound pressure is measured in Pascals (Pa). Talking directly into a microphone from 1m may produce 0.1 Pa. Reduce that to 10cm and you're up to 1 Pa. For convenience, SPLs are expressed in dB, where 0dB is taken to be the audibility threshold (quietest signal you can hear: 2x10-5 Pa if you were curious). Each increase of 3dB represents a doubling of the sound pressure. An SPL of 9dB is 16 times louder than the threshold of hearing (still around the mouse fart area). A loud concert could be 110dB SPL. Threshold of pain is around 125 dB SPL.

SUPERCARDIOID: A compromise between Cardioid and Hyper. High SPC figures, but better rejection round the back.



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Production Values

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The Six Lives of Rick Wakeman


Making Music - Copyright: Track Record Publishing Ltd, Nexus Media Ltd.

 

Making Music - Jun 1986

Topic:

Microphones


Feature

Previous article in this issue:

> Production Values

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> The Six Lives of Rick Wakema...


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