What they do and why. Chris Dale ponders the sound, shape and purpose of modern microphones

The phenomenon of the microphone has always been a source of wonderment to me. My first conscious encounter with this instrument gave me to believe that I'd discovered a magic of fundamental importance. Herein lay a clue to the mysteries of the ancients; you speak in one end and it comes out the other, considerably louder. When I subsequently found out that the hollow tube along which the sound travelled was actually solid, my destiny was sealed.

Finding out about these basic principles of electro-acoustic transduction turned out to take somewhat less than a lifetime, but even so a little of that initial feeling of awe stays with me. Interesting chaps microphones.

But why are there so many different types on the market? What of cost, shape and size, why are capacitor mics better than dynamics and why do some people insist on spending hundreds of pounds on old, fifth hand valve mics when they can get a new FET version for half the price?

While different mics undoubtedly do display individual characteristics, just as with any other large commercial market, we are to an extent dealing with fashion. It's a fashion created by the industry expressly to sell product, and what more fashion conscious industry is there than Rock-and-Roll?

In the next three issues I hope to answer some of these questions and cover the main points of the most commonly used microphone types, plus a squint at the enduring appeal of the valve and the stigma attached to using the "right" mics. First things first, though, so I shall start right here and now with a look at what is the basic job of a microphone.

What we perceive as sound is actually rapid fluctuations in air pressure commonly referred to as sound waves. It is the job of the microphone to convert these acoustic changes into some kind of electrical equivalent. This is achieved by placing within the field of the pressure fluctuations an elastically suspended diaphragm which is moved to and fro according to the nature of the changes.

The next step is to cause the movements of the diaphragm to create the equivalent electrical voltage of which I spoke earlier. There are several ways in which this can be done, but as far as rock PA and recording are concerned, two methods stand way above the others in terms of popularity:

1) The moving coil dynamic
2) The condenser

The diaphragm of the dynamic has attached to it a lightweight coil of wire which is suspended between the poles of a permanent magnet. A long time before you were born, nature made up some laws which have been baffling us human types ever since. One such law states that when you move a piece of wire within a magnetic field an electrical voltage is induced in the wire. As we still don't really know what electricity is, we must accept, for the purposes of this explanation, that the law works.

As the diaphragm plus its coil is moved to and fro by the sound waves, a voltage is induced in the coil. The nature of the voltage relates directly to the sound waves causing it in terms of amplitude, phase and frequency.

The simplest form of dynamic exhibits an "omni-directional" polar response pattern, that is to say it is equally sensitive to sound coming from all directions. For general studio and PA work this type of mic tends to get left in the cupboard. The reason is that for most applications a high degree of "separation" is required between instruments, meaning that you don't want the piano mic picking up the drums etc. A microphone which has no directional discrimination will pick up anything, irrespective of where it's come from — I've got a mate who's a bit like that... no names, no pack drill, no hospital bills.

By providing a carefully designed acoustic path to the rear of the diaphragm, various kinds of "unidirectional" or "cardioid" type responses can be achieved. The latter name is derived from the fact that a graph of such a polar pattern resembles the classic shape of a heart.

Such mics have an area of maximum sensitivity directly in front of them and minimal pick up directly behind. Although they're not all that narrow in their selectivity their rugged construction, ease of use, relatively low cost and, in the best cases, very acceptably high performance make them by far the most commonly used stage microphone.

Next month I'll make mince meat out of nature's electrostatic principles and compare the performance of the condenser with that of the trusty dynamic. Terms such as transients, frequency response, sensitivity, balancing, impedance and proximity effect will be bandied about. I really wouldn't miss it if I were you.