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Electret condenser microphone capsules
A typical electret microphone
preamp circuit uses an
FET in a
common source configuration. The two-terminal
electret capsule contains an FET which must be
externally powered by supply voltage V
+.
The resistor sets the gain and
output impedance. The audio signal appears at
the output, after a DC-blocking
capacitor.
An electret microphone is a relatively new type of
condenser microphone, which eliminates the need for a
high-voltage bias supply by using a permanently-charged
material.
An
electret is a stable
dielectric material with a permanently-embedded static
electric charge (which, due to the high
resistance of the material, will not decay for hundreds of
years). The name comes from electrostatic and magnet;
drawing analogy to the formation of a magnet by alignment of
magnetic domains in a piece of iron.
Electret materials have existed since the 1920s, and were
proposed as condenser microphone elements several times, but
were considered impractical until the foil electret type was
invented at
Bell laboratories in 1962 by
Gerhard Sessler and
Jim West, using a thin metallized Teflon foil.[1]
This became the most common type, used in many applications from
high-quality recording and
lavalier use to built-in microphones in small
sound recording devices and telephones.
Though electret mics were once considered low-cost and low
quality, the best ones can now rival capacitor mics in every
respect apart from low noise and can even have the long-term
stability and ultra-flat response needed for a measuring
microphone. There are three major types of microphone, depending
on the way the electret material is used:
- Foil-type or diaphragm-type
- A film of electret material is used as the diaphragm
itself. This is the most common type, but also the lowest
quality, since the electret material doesn't make a very
good diaphragm.
- Back electret
- An electret film is applied to the back plate of the
microphone capsule and the diaphragm is made of a superior,
uncharged material.
- Front electret
- In this newer type, the back plate is eliminated from
the design, and the condenser is formed by the diaphragm and
the inside surface of the capsule. The electret film is
adhered to the inside front cover and the metalized
diaphragm is connected to the input of the
FET.
Unlike other condenser microphones they require no polarising
voltage, but normally contain an integrated
preamplifier which does require power (often incorrectly
called polarizing power or bias). This preamp is frequently
phantom powered in sound reinforcement and studio
applications.
While few electret microphones rival the best DC-polarized
units in terms of noise level, this is not due to any inherent
limitation of the electret. Rather, mass production techniques
needed to produce electrets cheaply don't lend themselves to the
precision needed to produce the highest quality microphones.
External links
-
Electret condenser Microphone structure and theory
introduction
-
Guide for Electret Condenser Microphones
-
Rane schematic
-
Integrated Circuits for High Performance Electret
Microphones
Audio Engineering Society Convention Paper
-
Foil Electret Microphone: Sessler & West (1960)
-
Modern electret microphones and their applications
