A Dangerous Science Fair Project
Mar 1, 2004 12:00 PM, By Eddie Ciletti
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 (see Figs. 1a/1b), 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. In Fig. 2, 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 weight.
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. Figure 3 depicts a normal CK-1 capsule side-by-side with the mutant space blanket version.
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.
Visit Eddie Ciletti at www.tangible-technology.com for more fun!
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