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Guide to Electronic Music Techniques

Microphone techniques for 'Musique Concrete'


I've already talked about how to manipulate sounds on tape and these ideas apply to any sounds that you might want to use. Now I want to talk about ways of getting the sound you want from acoustic sources.

In order to do this it is necessary to convert acoustic energy into electrical energy by means of some form of microphone. There are many different types of microphone and they can be used in many different ways.

Microphones can be divided into two broad classes - air mics and contact mics. Air mics detect variations in voltage that can then be recorded and/or manipulated electronically. There are various categories of air microphone and it is worth knowing a bit about them. First, they can be divided according to the method of energy conversion. The two you are most likely to encounter are dynamic (or moving coil) and capacitor (used to be called condenser). A dynamic mic is effectively the inverse of a speaker - a diaphragm which is moved by pressure waves in the air is attached to a coil which moves with the diaphragm in a magnetic field thereby generating current in the coil. A capacitor microphone, on the other hand, uses the diaphragm as one plate of a capacitor with a charge on it. As the diaphragm is moved by the air, the distance between the two plates and therefore the capacitance, changes causing a change in the voltage across the capacitor. As the capacitor must have a charge on it, and because, for technical reasons, most capacitor mics contain a preamp it is necessary for them to have some form of power supply, which may make their use more complicated. Apart from that, you mostly won't need to worry all that much about the different types. If you don't understand why these two approaches work you'd better bone up on basic electronic theory; there are a number of good books available and Robert Penfold's articles in E&MM will help.

Figure 1. Microphone Polar diagrams.

A more important consideration is the polar diagram, or directional characteristic, of the microphone. The two characteristics you are most likely to encounter are omni-directional and cardioid. An omni-directional mic is equally sensitive to sound from all directions, while a cardioid mic has a roughly heart-shaped characteristic with a null at the rear of the mic and the most sensitive point at the front (Figure 1). This is the most commonly used directional characteristic and this type of mic will usually be the most useful to you. Generally we are concerned with recording a particular sound from a particular source and we want to direct the most sensitive point of the microphone towards it.

Contact mics also come in different types. There are two common types. The first is not really a contact mic at all, but a magnetic pickup which senses the movement of a metal object within its magnetic field. This is the type of pickup used on electric guitars and similar instruments and can be useful for some applications provided the vibrating body is metal. Magnetic pickups are mounted close to the vibrating object whilst contact mikes detect vibrations directly. The most common method for achieving this is to use a piezoelectric crystal which produces a small output voltage when stressed. The vibrations of the sound-producing object to which the mic is attached produced stresses in the crystal, thus generating a waveform analagous to the vibrations.

The other point to note about microphones is impedance. Most professional microphones are low impedance (200 ohms or less), but cheaper microphones and most contact microphones are usually of much higher impedance and some domestic tape recorders only accept high impedance mics. The most important point is to get the impedance matched or else you will get hum, noise and/or overload problems. Also, if you are using high impedance mics (particularly contact mics, which sometimes have a very high impedance) keep the leads as short as possible in order to reduce hum and other interference.

Finally, before talking about what you can do with your microphones, a few comments about what microphones to use.

There are a number of mics available in what is usually called the semi-professional range. I'll first mention a couple which I have found useful for this sort of work. For a dynamic mic the AKG D190 is hard to beat for price vs. performance, giving a lot of the quality of a really professional mic for about £50. A "cheap" capacitor mic is the Calrec 652 (which I use quite a lot) which is a similar price to the D190 but, of course, you'll need a power supply, which is extra. It's worth trying some of the electret mics (a cheaper form of capacitor, usually powered by a small battery in the mic itself). There are a number of types available at various prices.

For contact mics there are various types ranging from a few pounds up to hundreds. Many of the very cheap ones are apt to fall apart unless you are extremely gentle with them, so may prove a false economy, but there are several in the £10-£20 range that are worth looking at. I use a number of Schaller mics which are really sold as acoustic guitar pickups, but can be attached to all sorts of things and they are fairly robust. Also Coloursound do a range of pickups that are worth looking into. In the more expensive range there is a new capacitor contact mic by DI Tapes Ltd. (at (Contact Details)) which was designed as a piano pickup and sells for about £50. This mic is very sensitive and has a good frequency response (often a problem with the cheaper models). There is also an extensive range of contact mics by Barcus Berry at various prices. Lastly, there are all sorts of odd pickups that can be bought very cheaply on the surplus "junk" market and it is worth getting some of these to experiment with; and, don't forget, amazing things can be done with dismembered telephone receivers! (To cover myself I must advise you not to use Telecom property!!)

Using microphones


The one really important problem with mics is where do you put them - how close to the sound source? - which part of the source should they point at? There are no fixed rules for this - just experiment until you get the sound you want; but it will help to have some idea of the principles involved. The most important thing to realise is that a vibrating body does not vibrate in the same way all over and therefore does not produce the same sound from all parts.

Figure 2. Modes of vibration of a string.


Let us take a simple example; a vibrating string will have a number of modes of vibration (Figure 2) corresponding to the different harmonics; placing a mic over different parts of the string will emphasise different harmonics and, therefore, alter the timbre. Electric guitars with more than one set of pickups exploit this principle to get different tone colours. But if we have a moveable pickup or an air mic we can take the idea further by running it along the vibrating string. If we are over the middle of the string (Point A in Figure 2) all the odd harmonics, and especially the fundamental, will be emphasised and all the even harmonics will be at a nodal point and, therefore, de-emphasised. At point B, the second harmonic and its odd multiples (6, 10, 14, etc.) will be emphasised; at point C the third harmonic and its odd multiples (9,15, 21, etc.) will be emphasised; and so on, with the fundamental getting progressively weaker as we move toward the end of the string. While other vibrating bodies tend to be more complex than a string, the same principles apply, and the best bet is to experiment by moving the mic over the surface - indeed you may want to use the filtering effect of moving the mic while you are recording.

Another matter to consider is the distance of the mic from the point of excitation (hammer striking object, bow scraping string, etc.). If the mic is very close to the point of excitation the noise made at this point will be emphasised, while if the mic is further away the characteristic resonance of the vibrating body will be emphasised.

All these points apply to both air and contact mics, but remember that contact mics, because they are actually attached to the vibrating object, may affect the way it vibrates and, therefore, the sound it makes. This should be taken into account, particularly on small or light objects, or if you are trying to get a long resonance (the extra weight of the contact mic will tend to damp out resonances more quickly).

Remember that we can use the mic as part of the whole process of making the sound we want to hear; it is not just a mechanical device, but can be used as a creative instrument — all it needs is imagination and a willingness to experiment.



Previous Article in this issue

PPG Wave 2 Synthesiser

Next article in this issue

Sound on Stage


Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.

 

Electronics & Music Maker - Jul 1981

Topic:

Microphones


Previous article in this issue:

> PPG Wave 2 Synthesiser

Next article in this issue:

> Sound on Stage


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