Condenser microphones are the topic this month and I have a great
story about them with a funny ending. But first, mes amis, I must tease
you with the usual geek diversions, so hang in, okay? I attempted to
repair a condenser mic capsule using a generic material that is most
likely available at your local science museum and, certainly, on the
Net. I don't have much self-control, but let's see if you can do me one
better and read the whole story—no peeking!
THE AGING PROCESS
A pair of AKG CK-1 cardioid capsules came in for evaluation, the type
used on the 451/452 Series preamp bodies. One capsule was dead, and the
other noisy. Outwardly, condenser microphone mechanics are relatively
simple (like any capacitor), with a pair of conductors separated by a
nonconductor. That is, of course, until it becomes desirable to expect
identical performance from each capsule. Parameters such as frequency
response, noise and output level rely on consistent diaphragm
tensioning, low-noise components (active and passive), high-quality
insulators and long-life materials, including adhesives. That's the
nature of it. Noise in condenser microphones can be elusive. More than
the typical spurious emissions from tubes and transistors, the capsule
itself can be a noise source.
For a better appreciation of how delicate, yet resilient, these little
critters are, let's mentally roll our own capacitor, then shrink it
down to “microphonic” size. Take two sheets of aluminum
foil sandwiched around two sheets of paper insulators, wrap around the
center connecting wire, add a wire to the outer foil and then
encapsulate with shrink tubing (or dip in wax if you're feeling
old-fashioned). I actually did this just for fun, creating a
0.0028-micro-Farad cap in the process.
For a condenser mic, the principle components are the diaphragm, air
and backplate. With the CK-1, there is an insulating ring around the
perimeter of the backplate. The diaphragm's base material is a thin
plastic sheet—a nonconductor—with a molecularly thin
coating of a conductor, such as gold, aluminum or nickel, on one side.
Total thickness is typically between 3 and 6 microns, thin enough to
see through to the backplate, which is a metal part with
“holes” that act as either air cavities or vents, as
needed. In the rear of the CK-1 is a vent capped with a cintered bronze
plug used to restrict airflow. AKG used a similar material as a
windscreen/pop filter on some of its earlier handheld mics.
The capsule has a measurable capacitance; for the CK-1, it's about
30 pico-Farads (pF). A DC polarizing voltage is applied to the capsule
so that changes in sound pressure vary the capacitance and, in turn,
the DC voltage. This tiny signal is fed to an ultrahigh-impedance
voltage amplifier before exiting the XLR and making the journey through
some length of cable. The impedance issues are not just electronic, but
also mechanical and acoustic, which contribute to sonic character and
directional characteristics. After achieving precision in the
manufacturing process, it's relatively simple to maintain repeatable
control over each facet.
Of the vulnerabilities, human breath and other forms of high
humidity will eventually deposit a layer of organic funk on the
diaphragm, plus there is a possibility of puncture wounds by vocalists
who have a propensity for projectiles. That makes a pop filter, if not
two, de rigueur. For some capsule designs, the funk can grow over areas
where there is no plating and the resulting conductivity may produce
intermittent noise. Even worse: A particularly “wet”
plosive can short out the capsule.
Of the two CK-1 capsules, the dead one was disassembled, revealing a
diaphragm that had separated at the edge either from age or, more
likely, during a previous disassembly/cleaning attempt. I normally send
capsules out for repair and had no intention of making a career change,
but a recent trip to the science museum with my kids yielded a space
blanket that seemed like it might serve as a crude diaphragm material.
I had to see just how far my science fair exercise would go.
The first step was to determine which side of the film was
conductive. Note that the space blanket has a gold and a silver side.
The gold side was conductive, so it would go on the outside.
Five-minute epoxy was chosen as the glue. Now, all I had to do was
suspend the film in such a way to eliminate wrinkles and minimize
tension; the backplate assembly would create tension by its own
A solder-wick spool was used to suspend the material, Scotch tape
secured the edges and glue was applied to the outer rim of the capsule
(a band of white plastic insulating material that, by my guess, is most
likely Teflon). After allowing sufficient time for the adhesive to
cure, the space blanket material was trimmed and the capsule reinserted
into its housing and screwed into the head amp. When it worked right
off the bat, I was completely blown away. That it compared quite
favorably to the reference capsule in terms of output and spectral
response added to my amazement.
This fascinating detour increased my appreciation for microphone
design and manufacture. I dove back into a short book, simply entitled
Microphones by Dr. Gerhart Boré, which was published by Georg
Neumann GmbH in 1973 and reprinted in 1989. The book details all types
of microphones, including the types of tensioning and dampening
employed in various designs to achieve what we ultimately take for
granted: a usable microphone. To say that I got more out of this book
the second time around after some hands-on experience is an
understatement that reinforces what this column is about: encouraging
the scientist within.
As challenging as it was to work with space blanket material that,
by my estimate, was somewhere between 10 and 20 microns thick (0.001
inches = 1 mil = 25 microns), I can only imagine the challenge of
working with 3-micron material. Of course, having the proper tools and
instruments would make the process easier. One can never have enough
tools—a micrometer is only $150.
In closing, I would like to thank David Josephson at www.josephson.com
for recommending Boré's book to me several years ago. Also, while
this article provided a fun excuse for some home-brew experimentation,
neither myself nor the publishers of Mix magazine accept any
responsibility for the action of readers who destroy irreplaceable
vintage or modern condenser microphones in their attempts to duplicate
the space blanket diaphragm-replacement procedure described herein.