[Milsurplus] throat mike technique

Thu, 27 Mar 2003 21:15:02 -0700

Mike et al.
I guess the question  I had ( sorry migraine  tonight)  was  did this unit
ever get a military  designation?
Thanks Ed Sharpe archivist for SMECC

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----- Original Message -----
From: "Mike Hanz" <[email protected]>
To: <[email protected]>
Cc: <[email protected]>
Sent: Tuesday, March 25, 2003 9:13 AM
Subject: Re: [Milsurplus] throat mike technique

> [email protected] 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...  :-)
>
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