From: JOSEPHSON@AppleLink.Apple.COM (Josephson Engineering,VCA)
Newsgroups: rec.audio.pro
Subject: Electret vs DC condenser mics
Date: 5 Jan 93 01:29:00 GMT

David Schachter <ds@netcom.com> asks:
 
>Why are high voltage condenser microphones better than electret condenser
>microphones?  (Or if they aren't "better," how do they differ?)
 
This isn't a stupid question, and it's the subject of a lot of discussion. Any
microphone type presents a set of compromises that the designer has to take
into account when making the mic do what's wanted. To my ear, electret and
DC-air condenser mics of otherwise identical construction often sound
different, with the electret types seeming to have higher distortion and often
a degree of compression in the output signal. There are a bunch of theoretical
possibilities for these effects, and some that aren't well explained at all.
 
Remember that the principle of a condenser mic (to avoid confusion, I still use
"condenser" here, and at the auto parts store, but "capacitor" for every other
such animal) is that a fixed charge of Q Coulombs is stored in the dielectric
between two electrodes, which are in this case the diaphragm and the backplate.
Q is equal to CV, where C is the capacitance and V is the voltage to which the
cap has been charged. At rest, it's a small capacitor, typically 10-100 pF.
When a positive sound pressure wave pushes the diaphragm closer to the
backplate, C goes up because the spacing goes down. Since Q is constant, V goes
down (i. e. the microphone inverts polarity of the signal). On the negative
side of the input sound waveform, C goes down and V goes up. This V can come
from one of two sources: either a DC power supply (through a very high value
resistor), or a static charge permanently implanted in the dielectric between
the two plates of the capacitor (this is called an electret). Electret charge
is typically implanted in PTFE (Teflon) film. Mics can be made either by using
this film as the diaphragm, metallizing one side which faces outward, and
placing the electret side toward the backplate (a "film electret" mic), or by
bonding a film of charged electret film to the backplate ("back electret").
 
The first problem is that this PTFE film isn't very good for making microphone
diaphragms. The thinnest you can get (and this is the type used in nearly all
film electret mics) is 12 micrometers thick. It's also very stretchy and will
not hold much tension at all, versus say polycarbonate or polyester
(polyethylene terephthalate) which are typically used in DC-air condenser mics.
These films are typically used in 1.5 - 6 micrometer thickness, or else nickel
or titanium metal films 0.6 - 1.5 micrometer thick. The thick film and low
values of tension you can achieve pretty much dictate that you can only make a
certain range of mic types with a film electret. If you put the electret on the
backplate, you now have extra space between the backplate and the diaphragm
caused by the thickness of the electret film, which is typically 25 micrometers
or more. Since the diaphragm-to-backplate air film thickness is ideally in this
range (typically 20-50 micrometers) anyway, the extra spacing reduces the
amount of capacitance change that can be generated by the movement of the
diaphragm, while the fixed capacitance is increased by the dielectric effect of
the electret film. The result of this is lower output levels than a DC
condenser mic of the same dimensions, and a lower delta-C to total-C ratio,
which causes higher distortion. Better electrets, however, are in effect
conductive, so that the electrically active surface of the backplate is the
outer layer of the electret rather than the metal behind it.
 
Distortion in condenser microphones? Nearly every text that mentions this comes
up with a different comment. Some even contradict themselves -- see the
estimable Leo Beranek, _Acoustical Measurements_, 1988 revision, p 208: "...the
output voltage, therefore, is proportional to the displacement of the
diaphragm." and p. 215, "...[the condenser microphone] is nonlinear in
principle, for the potential across the condenser is not directly but inversely
proportional to its capacitance." (Capacitance change _is_ directly
proportional to displacement, for small displacements.) He goes on to give a
formula (p. 216), saying "The percentage of second harmonic distortion produced
by this effect is given approximately by the relationship
                                   peak mic output voltage
   Percentage of distortion = 5 * -----------------------------
                                   polarizing voltage
 
and quite frankly I'm not sure what to make of that. The AC output voltage of a
condenser mic is roughly the change in capacitance (delta-C) divided by the
total (resting) capacitance, times the polarizing voltage. The second harmonic
is not related to the output and polarizing voltages, but rather to delta-C and
total-C. The higher the stray capacitance, or the lower the ratio of delta-C to
total-C, the lower the output will be and the higher the distortion will be. So
stray capacitance is bad, and many electret mics have higher stray capacitance
due to the dielectric effect of the electret.
 
The last source of sonic problems in electrets is a mechanism I haven't
explained fully at all. Many listeners have reported a "duller" sound from
electret mics than from DC-air condenser mics of otherwise identical
performance, and a loss of low level detail much as would be expected from some
sort of spectral contamination distortion. I mused once that this might be
caused by the electret charge being depleted in certain regions of the electret
during loud sounds, reducing sensitivity for some period afterward. But the
structural theory of electrets, proven by the long lifetime of the implanted
charge, doesn't permit this kind of behavior. Maybe it's some signal-dependent
variation in the dielectric constant of the electret akin to dielectric
absorption? Something is going on here, and I haven't heard a good explanation
yet. Any ideas?
 
David Josephson/Josephson Engineering/San Jose CA/ 408-238-6062
Josephson@AppleLink.apple.com
 
Newsgroups: rec.audio.pro