Varieties
Microphones are referred to by their
transducer
principle, such as condenser, dynamic, etc., and by their directional
characteristics. Sometimes other characteristics such as diaphragm size,
intended use or orientation of the principal sound input to the
principal axis (end- or side-address) of the microphone are used to
describe the microphone.
Condenser microphone
Inside the Oktava 319 condenser microphone
The
condenser microphone, invented at Bell Labs in 1916 by E. C. Wente
[17] is also called a
capacitor microphone or
electrostatic microphone—capacitors were historically called condensers. Here, the
diaphragm acts as one plate of a
capacitor,
and the vibrations produce changes in the distance between the plates.
There are two types, depending on the method of extracting the
audio signal from the transducer: DC-biased microphones, and radio frequency (RF) or high frequency (HF) condenser microphones. With a
DC-biased microphone, the plates are
biased with a fixed charge (
Q). The
voltage maintained across the capacitor plates changes with the vibrations in the air, according to the capacitance equation (C =
Q⁄V), where Q = charge in
coulombs, C = capacitance in
farads and V = potential difference in
volts. The capacitance of the plates is inversely proportional to the distance between them for a parallel-plate capacitor. (See
capacitance for details.) The assembly of fixed and movable plates is called an "element" or "capsule".
A nearly constant charge is maintained on the capacitor. As the
capacitance changes, the charge across the capacitor does change very
slightly, but at audible frequencies it is sensibly constant. The
capacitance of the capsule (around 5 to 100
pF) and the value of the bias resistor (100
MΩ
to tens of GΩ) form a filter that is high-pass for the audio signal,
and low-pass for the bias voltage. Note that the time constant of an
RC circuit equals the product of the resistance and capacitance.
Within the time-frame of the capacitance change (as much as 50 ms at
20 Hz audio signal), the charge is practically constant and the voltage
across the capacitor changes instantaneously to reflect the change in
capacitance. The voltage across the capacitor varies above and below the
bias voltage. The voltage difference between the bias and the capacitor
is seen across the series resistor. The voltage across the resistor is
amplified for performance or recording. In most cases, the electronics
in the microphone itself contribute no voltage gain as the voltage
differential is quite significant, up to several volts for high sound
levels. Since this is a very high impedance circuit, current gain only
is usually needed, with the voltage remaining constant.
AKG C451B small-diaphragm condenser microphone
RF condenser microphones use a comparatively low RF voltage,
generated by a low-noise oscillator. The signal from the oscillator may
either be amplitude modulated by the capacitance changes produced by the
sound waves moving the capsule diaphragm, or the capsule may be part of
a
resonant circuit
that modulates the frequency of the oscillator signal. Demodulation
yields a low-noise audio frequency signal with a very low source
impedance. The absence of a high bias voltage permits the use of a
diaphragm with looser tension, which may be used to achieve wider
frequency response due to higher compliance. The RF biasing process
results in a lower electrical impedance capsule, a useful by-product of
which is that RF condenser microphones can be operated in damp weather
conditions that could create problems in DC-biased microphones with
contaminated insulating surfaces. The
Sennheiser "MKH" series of microphones use the RF biasing technique.
Condenser microphones span the range from telephone transmitters
through inexpensive karaoke microphones to high-fidelity recording
microphones. They generally produce a high-quality audio signal and are
now the popular choice in laboratory and
recording studio
applications. The inherent suitability of this technology is due to the
very small mass that must be moved by the incident sound wave, unlike
other microphone types that require the sound wave to do more work. They
require a power source, provided either via microphone inputs on
equipment as
phantom power
or from a small battery. Power is necessary for establishing the
capacitor plate voltage, and is also needed to power the microphone
electronics (impedance conversion in the case of electret and
DC-polarized microphones, demodulation or detection in the case of RF/HF
microphones). Condenser microphones are also available with two
diaphragms that can be electrically connected to provide a range of
polar patterns (see below), such as cardioid, omnidirectional, and
figure-eight. It is also possible to vary the pattern continuously with
some microphones, for example the
Røde NT2000 or CAD M179.
A
valve microphone is a condenser microphone that uses a
vacuum tube (valve)
amplifier. [18] They remain popular with enthusiasts of
tube sound
