MICROPHONE TECHNOLOGY

PHANTOM POWER

First of all we need to consider & understand the operation of a capacitor microphone. A thin plastic diaphragm coated with gold or aluminium is stretched over a shallow hollow cavity which has a flat metal back-plate. These two plates form a capacitor of some 5 to 75pF {Nominally 20 to 30 pF}. A polarising charging voltage is provided by a DC source. Electrostatic attraction keeps the diaphragm taut, but sound waves that impinge upon it will cause a small variation in the capacitive value which varies in sympathy with the air pressure waves. The output impedance is very high at around 100Mohms. To avoid the losses of HF, a long cable cannot be used and a very adjacent pre-amplifier is required. The low mass and inertia of the diaphragm gives a very flat and wide response, while the output is high because of the pre-amp.
The need of power is a drawback and ways and means have been sought to provide the small but necessary power from within the amplifier with which the microphone is used. This is known as "Phantom Powering" The generally adopted standard is for a 48V DC supply. This is sometimes obtained within the actual microphone from a small battery. Otherwise it must come from the amplifier itself which might mean having extra wires. This is often overcome by the following circuitry.
The positive is taken to a centre tap in the input transformer or to the junction of a pair of resistors across the winding. Another similar method avoids the disadvantage of having to have the screen connected at both ends, a practice known to induce hum & noise pick-up. In this circuit both the signal wires are used to carry the dc power and the screen does not form a part of any circuit.

This is achieved by the circuit configuration known as A-B powering thus: See how cleverly the routes for the ac signal and the DC power is preserved over the same wires whilst the dc is blocked from the ac signal with scarcely any loading.

There is much more to know about microphones which all have their pros & cons. We have to weigh up their performance in several important ways: - Cost, performance, ruggedness, weight, size and pick-up qualities are the main considerations.
Since the latter may be the most obscure we will start with that. We may simply need a microphone that will pick up in an even way all around its site. Or we may need a performance that is more DIRECTIONAL. Here is a diagram that shows the most popular choices.

"Directivity" also Rejection ratio" - "Discrimination" - "Cancellation" and "Front to back ratio" are common terms used. Typical values are 15 to 20 dB which is about a twelvefold difference.

A is Omni-directional
B has a cardioid pick up
C is a super cardioid
D is a figure of eight velocity
E is a gun/interference tube type.
The latter is a very directional specialised type mainly used in noisy crowded rooms. You point it at what you want to hear. An open ended tube with a series of slots along one side is attached to a cardioid microphone. It operates by phase displacement cancelling down to a frequency that is a half wavelength of the tube length.

NOISE CANCELLATION
It often happens that unwanted background noise is picked up. There have been, and still are, novel ways to deal with it. Two microphones can be connected in antiphase to produce almost absolute cancelling. Announcements are made through one or the other from close proximity and this balance is overcome. Some capacitor units have two diaphragms facing front & rear. They each have a cardioid pick-up area. Polarising voltages can be variously switched off or reversed to form pick-up lobes.