[Milsurplus] throat mike technique

[Mike Hanz]

WA5CAB@cs.com wrote:
> Is there any technique that is effective in unpacking the granules?

It all depends how bad the moisture and packing is, I suspect.  Rather 
than rewrite it, I'll just quote a note I sent to the ARC-5 reflector a 
couple of years ago:

 > Apropos to further exploring this question, in pursuing my interphone
 > interest I recently
 > received an interesting book from an epay source - "Response
 > Characteristics of Interphone Equipment", an NDRC pub dated 1 March
 > 1943.  In honor of its 58th birthday last Thursday, I started going
 > through it, and there are some interesting excerpts which deserve
 > consideration.  With respect to the question about T-17 usage, here's
 > a relevant clip: "Test data indicate that the RS-38A microphone is not
 > as stable when test currents as high as 65mA are used as when lower
 > currents are used."  [(snip)  The 65mA level was a Bell Labs figure
 > specified for carbon mike response tests]  "It is not necessary that
 > lower test currents be used on the ANB-M-C1 [oxygen mask mike] and the
 > T-17 types of microphones, even though they are to be used with Navy
 > equipment."  My editorial comments are marked by [ ].  I'm afraid it
 > doesn't go into any more detail than that, but it justifies my
 > previous caution in never saying never.
 >
 > The question about the carbon mike response characteristics is a bit
 > more complex to answer  There are some fascinating aspects which were
 > probably widely known at the time, but no one seems to remember today.
 > It appears that the response could vary widely from one test to
 > another,
 > and a procedure called "conditioning" was necessary to get consistent
 > results.  Bell Labs was consulted before the start of the tests, and
 > they had a strong influence on the approach.  The original
 > conditioning procedure is described as follows:  "Condition the
 > microphone by shaking
 > it thoroughly in all directions.  This action is similar to, but more
 > thorough than, the action a pilot goes through in taking the
 > microphone
 > from its holder and getting it in position before his lips."  The
 > image
 > of a pilot shaking the mike before he uses it seems mildly amusing,
 > but it appears that it was a necessary evil with the carbon mikes of
 > the day
 > (up to 1943, at least.)  The test procedure then twice rotated the
 > mike through a 270 degree arc over a period of two seconds, presumable
 > to distribute the carbon granules more evenly.  Later on, this
 > interesting
 > observation about the RS-38A is noted - "Data showing the value of
 > shaking are given in the next section.  The differences in the
 > response
 > curves are most pronounced for the RS-38A."  I won't go any further
 > into
 > the details unless someone is really interested, but it does say,
 > "...the repeatability is very poor" [for the RS-38A] and,  "The data
 > were even less repeatable when the microphone was not thoroughly
 > shaken..." and "None of the T-17 or ANB-M-C1 microphones of any
 > manufacturer showed this effect.  It must be inferred, therefore, that
 > this instability is inherent in the design of the microphone."
 > Presumably later RS-38s were modified to improve their performance, in
 > the face of that rather scathing indictment.
 >
 > Some other little gems:  "It is seen that the microphones become more
 > sensitive with increasing carbon current (about 5dB as the current is
 > increased from 30mA to 80mA." and  "For any given microphone there is 
 > an
 > optimum carbon current which yields the lowest "burning" (button
 > current) noise."   One last interesting test result was the two
 > frequency measurement of nonlinear distortion.  The T-17 was the worst
 > in this test, running from 12 to 40% distortion over the 300-3000Hz
 > band
 > they tested, highest at the upper end.  The RS-38A averaged around
 > 5-15%
 > over that same range.  Moral of the story is to talk like James Earl
 > Jones (Darth Vader's voice) if you want to be understood, I guess.
 > Ladies need not apply...
 >
 > That's about it after wading through 107 pages of graphs and charts.
 >
 > Okay, I'll have to admit this post has probably sunk the list to new
 > lows of arcane and abstruse minutia, but I needed to get it down in
 > bits and bytes while I had a chance.
 >
 > So...remember to condition all your carbon mikes once a day.  One
 > conditioning tool described is an electromagnetically actuated rocker
 > arm which raps the mike with a hard rubber pad for a number of times.
 > I have a photograph of this conditioning fixture for anyone who wants
 > it...  :-)