The year was 1983 or 1984, and I was interested in doing some measurements of frequency response
of speakers. I was using a Radio Shack stereo mic at the time, was all I had. In looking through
a Digi-Key catalog, was very thin back then, I discovered the Panasonic line of inexpensive electret
capsules. The response seemed very flat, and so it was the data sheet supplied from Panasonic. It
was hard to see any fluctuation, but it was obvious this would be a load better than the Radio Shack
mike. The Radio Shack mic used one of the larger capsules which commonly has rather large peaks
at about 6-7 kHz. The Panasonic element was essentially flat in this region, and up to about 10kHz
would be my primary target. Also seemed flat down to a very low freq. I assembled the capsules and
battery powered interface, did some brief testing and was satisfied with the results. I then wrote up
a brief page about the system, and sent it to Speaker Builder magazine. It was published in the 3/85
monthly edition. Later I discovered a description of the Panasonic element in Audio Amateur magazine
which seemed to be published some months before. I don't recall any assembly instructions in that publication.
It seems people started using the capsule for various applications. On the cover of 6/88 Speaker Builder,
the capsule is shown hovering above the cone of a Keith Johnson designed woofer, yes, its the same model
woofer on my front audio page speaker. On the 6/90 edition of Speaker Builder, Joseph D'Appolito and
Richard H. Campbell introduced the Mitey Mike system. Later in 4/97, the Mitey Mike II would be introduced.
The original capsule is no longer produced, and the WM60AT seems to be the recommended replacement,
as is included with the Mitey Mike II system.
Some time after I constructed my mike system, I made a sound level meter, which was shown in the 1/86
Speaker Builder. I didn't bother mounting the mike out from the case, which was a bad idea.
Enough of the history, on with some stuff! I'm showing my basic interface circuitry, and pictures of my system.
As a general rule, the mic should be mounted in free space to keep undesired reflections, and refraction
when used in close spaced measurement situations. Ideally, the mic should also be shock mounted, which
can present problems if trying to mount in a narrow diameter tube. I used Plexiglas tubes, larger than the
capsule diameter. I suppose I could have filed off the end to form more of a point. Current Applications
use a brass tube of some length, with the capsule just fitting inside a thin tube. In the case of the Mitey Mikes,
the tube is rather long. Good for keeping obstructions away from the element. Back to the mounting of the element,
I was trying to use a thin bead of silicone rtv to shock mount the capsule. Difficult to achieve the proper elasticity
for decoupling. Probably would have to add weight to the capsule, which might also produce an undesired
low frequency resonance of its own.
After some experimenting, I decided to mount some capsules on Plexiglas plates to produce a pressure zone
mike system. The PZM is a Crown invention as far as I know. The idea behind the system, is to avoid
large secondary reflections from interfering with the prime pick-up of the element. This keeps a null from forming
at some frequency, determined from the spacing of the secondary reflection. The requirement is to also place
the platform on a larger wall or table, forming a much larger single plate. There are also some other traits
with using a smaller plate by itself. The plate acts to reinforce lower frequencies in relation to higher
frequencies. It also makes the mike directional. When doing some testing of my voice, I found that speaking
close to a small element produces excessive treble response. By speaking into the plate mounted element
my voice sounded much more balanced, or normal. When a small element is placed on a table or applied to
a lapel, or even used along side of the head, it is trying to mimic the effect of the PZM principal. To be fully
effective, the mike needs to be very close to that surface to couple the highest frequency without a secondary
reflection. I made a stereo mike set out of some small plates, bent at an angle. I made some successful
recordings with this setup. This is shown in the background.
I used a 6 volt lithium battery in my system. Very stable voltage source and long life. The element changes
sensitivity with applied voltage. For relative measurements, it's not necessary to calibrate the absolute
sensitivity of this system. It's also not necessary to use any additional buffering or amplifier stages for
short cable runs. Not shown in the schematic was a necessary attenuator switch. I found my portable,
rather large cassette recorder, input stage overloaded, no matter what the level setting on high decibel
sounds. I was recording cars at a race track. I put in about 25 dB of switchable attenuation.
The coupling caps in the interface are rather large, and can be reduced in value to roll off infrasonic, noises,
vibration, and wind pressure noise. These things respond to slow pressure changes. It's almost mandatory
to use windscreens outside. Adding foam or other material will mess up the highs, but is much better than
wind noise. You can probably see foam wedges on my plate mike.
As it turns out, the elements frequency response is fairly predictable. I suggested in the original article, to try
and select or test various samples, by using two with close coupling to weed out those that might be on
the far side of the normalized specs. This can all be done without using calibrated capsules, although
As modified in the Mity Mike II system, the capsule connection to case is broken, and a 150 ohm resistor
is added as a series connection. This along with increasing the bias resistor to 10 K will reduce distortion
at higher decibels markedly. I might not suggest doing this unless a buffer stage is added close to
the capsule, for longer cable runs. Hey, I suppose you can get much longer battery life with he mod
and get the other benefits as well!!!
Typical response of the mics are within .5 db from very low freqs. to up about 7kHz. There is a plus 2 dB
slow rising hill centered about 13 kHz, back to near zero level at 20 kHz. Plots can be seen on the link referenced
For those who need a quick and simple mike amp, here is my version.
The TI TLC070, 71,72 can also be used in this circuit. The OPA 134,234, series
has high bias requirments. About 10 ma. for the dual. I would prefer the Burr Brown
part for overall performance. The TLC series for a second choice. Well, Burr Brown
wil be part of TI now, sigh!! The LM6134 has low drain but is a wimp for driving current,
but can be used for longer battery life and more noise.
More on op-amps comming soon!
More on mods......HERE
Here is a web page devoted to this electret capsule, along with measurements, an amplifier system, and more.
Jason M. Neal
Another source is the pages from Wallin, with SLM mods, etc......
There are more references and web pages devoted to recording and whatnot, for those investing
the use of these mics.