The camera’s lens and the recordist’s microphone are not equivalent devices. A lens merely processes light rays so that they can then be captured on film or converted into electrical signals. Light rays enter from the front and leave from the rear. The microphone, on the other hand, is the device which actually performs the conversion. The task of the microphone is to receive vibrations from the surrounding air and change them into electrical impulses which can be recorded onto some medium, usually magnetic tape. Numerous different means have been tried over the years to achieve this, but for a long time, two devices have been in principal use: the dynamic microphone and the capacitative.
A dynamic microphone depends on the fact that when a wire moves through a magnetic field an electrical current is induced in it. If a magnetic field is moved past a static wire the same phenomenon occurs. The induced current is normally very small, but by increasing the length of the wire it can be made of measurable size. In a dynamic microphone, the wire is fixed to a diaphragm which moves in sympathy with the vibrations in the air. The wire is extended to a considerable length and wound into a coil, the whole coil vibrating within the field of a powerful magnet. The wire, though rather long and thin, has relatively little impedance—resistance to the flow of alternating current—and the result is a current flow of some magnitude, which does not need very high preamplification to bring it up to a level at which it can be successfully recorded. The simplicity and low cost of this arrangement makes it very attractive.
However, there are drawbacks to dynamic microphones. The coil assembly has a significant mass and a significant size and shape. These two characteristics mean that it has resonances of its own and vibrates more easily at some frequencies than at others, rather than moving totally and exactly in sympathy with the vibrations in the air. In particular the highest and lowest frequencies are less exactly reproduced. Moving coil microphones thus give a noticeable ‘colour’ to the sound they record, emphasising the mid-range, and giving less response at the extremes of the frequency scale.
Capacitative microphones depend on a different physical property. When two conductive plates are brought close to each other and an electrical potential applied across the two, an electrical field is brought into being in between. Movement of the plates in relation to each other, as when responding to vibrations in the air, induces a change of voltage between them. The voltage change is extremely small but can be amplified to a usable level. The plates of the capacitor have very little mass and can be made to respond almost equally to all the sound frequencies which impinge upon them.
Fig. 7.1 A plot of microphone sensitivity as seen from above.
Fig. 7.2 Plot of microphone sensitivity as seen from the front.
The result is a microphone which has an extended and rather flat frequency response with little unwanted colouration. Of course capacitative microphones have their disadvantages too. One is that they need a relatively high voltage source to make them work; this is usually derived from the mixer. Cheap ‘electret’ microphones, which are based on the capacitative principle, are often battery powered, but their sound quality is generally below professional standard. Another difficulty is that their internal impedance—resistance to alternating current flow—is so high, and the signal produced is so small, that capacitor microphones are very susceptible to electrical noise.
Whichever kind of microphone is used, the sound recordist takes care to capture only the sounds which are wanted, and to exclude those which are irrelevant, or disturbing, to the shot. To make this possible, microphones have directionality, that is, they are designed to treat sound differently depending on the direction from which it comes. Most microphones have either an ‘omni-directional’ or a ‘cardioid’ response—though since sound is a three-dimensional phenomenon, the same microphone may have both, depending on the plane in which it is operated. Thus a cardioid (heart-shaped) microphone mounted horizontally has a heart-shaped sensitivity to sound arriving in its own plane (Fig. 7.1). The same microphone looked at from the front is omnidirectional (Fig. 7.2).
For full directionality, the microphone capsule itself is set into a carefully audio-engineered housing which uses phase differences in the sound waves to cancel out, as far as possible, all sound from unwanted directions. Such ‘gun’ microphones, usually themselves housed in fluffy fur-covered, sausage-shaped windshields, have become a trademark of the sound recordist on location. However, many more kinds of microphones are regularly used. In particular, personal microphones, attached to the clothing of the speaker, and either connected to the mixer by a long cable or by a radio link, are among the most frequently seen. Such microphones require considerable skill in their use if rustles from the clothing and other unwanted sounds are to be avoided.
In the last few years, sound recording has become an even more demanding operation because of the introduction of stereophonic sound. Stereo sound is captured by using two microphones. At its simplest, the ‘A and B’ system, the stereo effect is obtained by recording separate left and right hand channels from separate left and right microphones, corresponding to the left and right ears of an auditor present at the scene. This demands great skill and sensitivity from the sound recordist as the final sound-world of the shot is decided at this time. The width of the stereo ‘spread’, once recorded on tape, cannot thereafter be altered; only the strength of the effect can be changed.
To offer greater flexibility a different technique, the ‘M and S’ system, has been developed. This also uses two microphones but in a different way. One microphone records a standard monaural signal. The other in effect detects and records the differences between the stereophonic sound of the scene and the monaural signal. Using specialized equipment, a stereo signal can be reconstituted from these two channels, while allowing for the stereo effect to be substantially changed. Thus final and irrevocable decisions about the sound quality can be deferred until later.