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Home Recording (Part 4)

Taking The Mike

Taking the Mike — All you'll ever need to know


This month we look at mikes and explain what all those funny sounding terms like 'Phantom Powering' really mean. When you understand the terminology it makes your choice so much easier...

I remember as a young lad viewing microphones with some awe. They were the first things in sound reproduction that came to my notice as doing the impossible, that is to send a sound down hundreds of feet of solid cable, and even have it coming out the other end louder than when it went in. Even though I now know the theory of their operation, the feeling of magic still hangs over the whole transduction phenomenon, whether the transducer is a microphone or some other piece of sound processing equipment.

Transduction is the conversion of changes in one quantity into those of another, and that's what a microphone does — it converts changes in acoustic energy into parallel or analogous changes in electrical energy, so that in its new form it can be processed by the rest of the recording or sound reproduction chain which can only deal with electrical signals. How they make the conversion depends on the type of microphone, and there are basically three kinds in common use in the studio today. Of these three the most common are the dynamic and the capacitor, with the third type, the ribbon, having relatively little exposure, especially in the rock world. Before taking a look at each type individually let's go over the parameters by which any microphone's performance is normally judged.

Frequency response — is one which most people are aware of as being important. The human hearing system is sensitive to a wide range of frequencies measured in Hertz (Hz), with the lowest audible note being 20 Hz and the highest around 20,000Hz (20 KHz). When you adjust the treble control on an amplifier you are altering the amount by which a band of frequencies, predetermined by the design of the tone control, in the upper regions of the spectrum is amplified, and similarly with the bass control, a band of frequencies in the lower region is affected, if the balance of high and low components is not right, you will end up with either a brittle harsh sound or an over-bassy, muddy sound. In the same way, it is important that when a microphone converts the soundwave into its electrical equivalent, it does so equally at all useful frequencies, thereby giving what is referred to as a 'flat response', or if there are irregularities, that they produce a desirably 'contoured' sound.

The word 'flat' refers to the shape of the graph showing the microphone's output level in relation to the frequency of the sound wave incident upon it; generally speaking it is good to have a mike with a wide response, meaning that it is reasonably flat across a wide band of frequencies.

Directional response — this refers to a microphone's response to sound coming at it from different angles. Although there are many variations it is simplest to think of them all as falling into three basic categories: Non-directional or omni-directional, where the device responds equally to sound from all angles. Bi-directional or figure-of-eight, where it responds equally to sound coming from the front and the rear, but is relatively insensitive to sound coming at it from the sides. Unidirectional microphones are most sensitive to sound arriving from one direction along the axis of response.

The directional characteristic of a microphone is frequency dependent, generally becoming less directional as the sound gets lower. With the separation of different instruments usually being of great importance in modern multitrack techniques, it is the unidirectional mike which is used in the vast majority of cases, and although studios spend a fortune buying units which can switch through a range of responses, for one reason or another, they usually get left in the 'cardioid' position, which gives a unidirectional response with a fairly wide pickup angle looking graphically heart shaped — hence the name.

Most high quality dynamics will exhibit a cardioid response, capacitor mikes which aren't switchable will either have a number of interchangeable capsules, providing a range of responses or have a fixed response which will normally be cardioid. Ribbons are likely to be either unidirectional or figure-of-eight. Assuming your budget is fairly limited I would advise you to stick to unidirectional dynamic and capacitor models for maximum flexibility. One type of unidirectional microphone is the 'rifle' or 'shot gun' mike, characterised by its long thin rifle-like appearance, it exhibits a very narrow angle of pickup, and in the recording studio it is often used as an overhead ambient mike for the snare drum, whilst keeping pickup of the rest of the kit to a minimum. This however would be a later addition to your collection.

Sensitivity — refers to the magnitude of the microphone's electrical output for a given sound pressure level and is measured in millivolts per pascal (mV/Pa). For most purposes, this figure isn't critical.

Maximum SPL — gives the maximum sound pressure level which a mike can handle with a given percentage distortion, usually 0.5% THD. Most good quality dynamics and capacitor mikes will take in excess of 120dB, and many capacitor models will have an attenuator, allowing them to handle well over 130dB, which should cope with most problems. On the other hand it's better not to become too relaxed as various parts of a drum kit like the snare, kick drum or large crash cymbal can peak over 125dB as far as a mike placed a few inches away is concerned, likewise a heavy brass section can peak in the same region. When using a capacitor mike, if in doubt start with the attenuator in.

Impedance — can be seen as a force acting against the flow of the output of the microphone or in fact any other alternating electrical voltage. The-input to the mixer has an impedance which it offers up against the incoming signal and the microphone has an output impedance which can be seen as an indication of the input impedance it is designed to be matched with. Both are frequency dependent, increasing with frequency, and so ratings are usually given for a 1 KHz (1,000Hz) signal. Most good quality mikes designed for studio or PA use come within the range called low impedance, which means they are usually around the 200ohms (ohm is pronounced like home, but without the h) although anything from 50ohms to 400ohms can still be thought of as low. Roughly speaking 600ohms to 2,500ohms is termed middle impedance, with anything over 10,000ohms being high.

One of the main advantages of a low impedance mike compared to the others, is that you can use much longer cable runs without picking up significant noise or interference, or losing the high frequency part of the signal. High impedance models are generally limited to cable runs of under twenty feet.

It is important to realise that the input impedance of the mixer or amplifier should not be the same as the microphone's output impedance, but should be at least three times it to allow optimum frequency response, noise figures and maximum useful output from the mike, and in fact five times is probably a better figure although not always possible with modern equipment formats.

The low impedance microphone inputs on most modern mixers will be in excess of 1,000ohms, and will therefore be capable of taking mikes up to around 300ohms. If however you have either a high impedance mike and a low impedance input or vice versa, it is possible to purchase a small transformer to convert either way. Although by no means foolproof, a good indication of a microphone's impedance is its cable: A single conductor and screen normally means high impedance whilst a twin conductor and screen suggests low impedance.

Balanced or Unbalanced — A balanced line operation has an advantage over the unbalanced one, in that by means of a balancing transformer at either end of the line, (one in the microphone and one in the input of the mixer) any noise, hum or interference picked up along the length of the cable run is cancelled out, leaving a relatively noise free signal.

A balanced line requires a cable with two conductors and a screen, whilst an unbalanced line requires only one conductor and a screen. Most low impedance mikes are balanced, and that's why a two core cable is a good indication as I have suggested. An unbalanced line only requires a two-pole connector, usually a mono jack, whilst a balanced line requires a three-pole connector such as a stereo jack, or more common in professional usage, a three-pole XLR connector.

The Dynamic Moving Coil - If you read the first part of this series you will recall that one of the fundamental laws of physics is that when a piece of wire is moved within a magnetic field, an electrical voltage is induced in the wire, which is analogous to the movement, that is it corresponds directly with the movement. A moving coil mike makes good use of this by mounting a coil of wire on a lightweight diaphragm, and elastically suspending the combined structure between the poles of a permanent magnet.

Thus, as soundwaves hit the diaphragm, causing it and the coil to move to and fro, an analogous electrical signal is induced in the coil, which basically constitutes the output of the mike. Modern moving coil mics are capable of very good performance, they are robust, don't need any batteries or power supplies, and are relatively inexpensive. On the other hand they are often fairly large and don't exhibit such a flat and extended frequency response as a good quality capacitor. Omni-directional models are referred to as 'pressure operated' where an acoustic path is provided only to the front of the diaphragm. As the frequency of the sound gets higher, such that the wavelength approaches the dimension of the microphone head, the evenness of the omni-directional response is lost.

The other commonly found directional characteristic for a moving coil is the cardioid response, which relies for its response pattern on 'pressure gradient operation'. In this case acoustic paths are provided both to the front and to the rear of the diaphragm, and it is the difference between these two wave fronts, dependent on the angle from which the sound is coming, which provides the movement of the diaphragm and hence the signal. At certain angles there is equal pressure on either side and so no movement takes place, whereas in the case of sound arriving directly on axis the difference is great and so a large output results. The polar response of the moving coil mike, whether omni or cardioid, is fixed and cannot be changed.

The Capacitor or Condenser - A capacitor, often called a condenser, consists of two plates. These plates can be charged-up by an electrical supply so that a voltage exists across them. If the distance between the plates is altered, the voltage is changed correspondingly. In a condenser mike, an extremely thin lightweight foil diaphragm is used as one of the plates of the condenser, and as the diaphragm is moved by incident soundwaves, so the voltage across the condenser changes, thus producing the basic signal for the output of the mike. The condensers need a 'polarising voltage' power supply, to set up the original voltage, and by means of multielement systems and through varying their relative electrical polarities, a variety of responses can be provided in one mike. Alternatively a system of interchangeable heads can provide a similar facility, without the simplicity and convenience of a switch, but usually in a much more compact form and at less expense. Most good quality recording consoles have a standard 'phantom power supply' which supplies 48 volts DC for the polarisation usually via pins two and three on the XLR mike input sockets, using the earth connection for a 0 volt line. It's called a 'phantom' supply because the way it's connected up allows it to polarise the condenser whilst appearing to the audio circuit not to be there at all, and thus not interfering with its fidelity.

An electret mike is the same as a capacitor except that the foil of the diaphragm undergoes a special process which leaves it with a permanent charge, thus eliminating the need for a polarizing power supply. However, all capacitor mikes need a built in amplifier to change the impedance and level of the original signal and so a small battery is housed within the body of an electret to power it. The quality of this kind of microphone ranges from not very good, at around £12.50 each to excellent costing several hundred pounds. Capacitor mikes in general are more fussy than moving coil models, and are more susceptible to damp and mechanical damage.

The Ribbon — Here a thin, corrugated foil ribbon acts as both the diaphragm and a single turn coil. It moves within a permanent magnetic field as soundwaves are incident upon it, and the voltage is actually induced in it. Ribbon mikes are either figure-of-eight or unidirectional, they are capable of very high quality but are generally very delicate and don't like being hit by drum sticks or dropped even on carpeted floors. I wouldn't bother until you've got a selection of the other types.



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Brass & Woodwind

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Westone Concord Bass


Music UK - Copyright: Folly Publications

 

Music UK - Sep 1982

Topic:

Home Studio

Microphones

Recording


Series:

Home Recording

Part 3 | Part 4 (Viewing) | Part 7 | Part 8


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Previous article in this issue:

> Brass & Woodwind

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> Westone Concord Bass


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