Is there any practical difference between a conventional capacitor microphone and a back-electret type? I always get the impression that the back-electret is the poor cousin of the 'true' capacitor mic.
Martin Metcalfe via email
SOS Editor In Chief Paul White replies: The short answer is no, back-electrets are not the poor relations anymore. Certainly electret mics had a bad reputation in the 1970s and '80s, but the technology was pretty crude back then. It has advanced massively since, and today some of the very best mics are electret designs — including all of the small-diaphragm models from DPA, some large-diaphragm AKG mics and many Audio-Technica models.
The difference between a traditional capacitor mic and
a back-electret model is in the way the capsule is polarised. Any
capacitor microphone needs to have an electrical charge applied to the
capacitor, formed by a conductive
diaphragm that's placed in close proximity to a fixed backplate in
order to produce any signal. As the diaphragm moves relative to the
backplate, whenever it's forced into vibration by sound waves, the
electrical value of the capacitor changes. As the diaphragm gets closer
to the backplate, the capacitance value increases, and as it moves
further away it decreases. A simple formula, Q = CV (where Q is charge, C
is capacitance and V is voltage between the two plates), states the
relationship between the three key electrical parameters. From this it
can be seen that if Q is kept essentially constant (through the use of
a charging circuit with a very long time constant), then as C is
modulated by air movement, so must V be modulated. This variation in the
voltage between the diaphragm and backplate is amplified to produce the
audio signal, but to avoid draining away the vital electrical charge
from the capsule, a very high-impedance preamp is required, usually
involving either a FET (Field Effect Transistor) or a valve.
diaphragm that's placed in close proximity to a fixed backplate in
order to produce any signal. As the diaphragm moves relative to the
backplate, whenever it's forced into vibration by sound waves, the
electrical value of the capacitor changes. As the diaphragm gets closer
to the backplate, the capacitance value increases, and as it moves
further away it decreases. A simple formula, Q = CV (where Q is charge, C
is capacitance and V is voltage between the two plates), states the
relationship between the three key electrical parameters. From this it
can be seen that if Q is kept essentially constant (through the use of
a charging circuit with a very long time constant), then as C is
modulated by air movement, so must V be modulated. This variation in the
voltage between the diaphragm and backplate is amplified to produce the
audio signal, but to avoid draining away the vital electrical charge
from the capsule, a very high-impedance preamp is required, usually
involving either a FET (Field Effect Transistor) or a valve.
In a true capacitor mic, the electrical charge is
invariably derived either from phantom power or from a separate power
supply, as is the case with the vast majority of valve microphones.
However, the electret microphone uses a different method of keeping the
capsule charged. An electret material is one that carries a permanent
electrical charge sealed within an insulating film, and its manufacture
involves high temperatures and very high voltages. Early electret mics
used this material to form the diaphragm but, as it is both thicker and
heavier than a
typical
gold-coated Mylar diaphragm, high-frequency performance and sensitivity
were compromised. These simple electret mics are used extensively in
all manner of consumer devices, including mobile telephones.
typical
gold-coated Mylar diaphragm, high-frequency performance and sensitivity
were compromised. These simple electret mics are used extensively in
all manner of consumer devices, including mobile telephones.
The quality breakthrough came when the electret
material was fixed to the backplate (hence the term back-electret),
allowing it to be used in conjunction with a conventional gold-on-Mylar
diaphragm, providing designers with the means to achieve the same
performance as from a conventional capacitor mic but without the need
for an external polarising voltage to maintain the charge. This allows
some live back-electret mics to be powered from batteries when phantom
power isn't available, as only the preamplifier needs power and this can
often be run at a much lower voltage than a typical capsule's
polarising voltage.
A good back-electret mic can perform every bit as well as a traditional capacitor design, and in some cases a little better, though the electrical charge sealed in the electret material can leak away very slowly over a period of several decades, resulting in a slight decrease in sensitivity. In reality, though, this leakage is generally so small as not to be a practical issue.
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