[ARC5] NDRC & Aircraft Audio above 20,000 Ft

[Michael Hanz]

On 9/22/2017 1:20 PM, Brooke Clarke wrote:
> I've been studying the various projects by the National Defense 
> Research Committee and came across a discussion of aircraft audio.
> https://en.wikipedia.org/wiki/National_Defense_Research_Committee
>
> In the book: /Scientists Against Time/, 1952, James Phinney, 3rd ed 
> (Wiki <https://en.wikipedia.org/wiki/Scientists_Against_Time>) they 
> mention the problems with the microphones and earphones at high 
> altitude and that throat mikes were poor above 20,000 feet.
> Also speakers and common headsets did not work well and were replaced 
> by units that fit into the ear, maybe the HS-30.
> http://prc68.com/I/HS30A.shtml
>
> Is there information on what audio headsets and microphones were used 
> at the start of W.W.II vs. what was being used at the end of the war 
> in high altitude bombers?

A fair amount exists, Brooke - at least from the U.S. perspective. I've 
posted some material in my web page documents section from two sources:

http://aafradio.org/docs/NDRC_Interphone_Equipment-carbon_microphone_characteristics.pdf
and
http://aafradio.org/docs/NDRC_Division_17_excerpts.pdf

The NDRC Div. 17 book is entitled _Combat Instrumentation II_ in gold 
letters on the cover, but this particular volume (Volume III) is 
entitled _Transmission and Reception of Sounds Under Combat 
Conditions._  It is essentially a compendium of reports on the wartime 
work in the area of acoustic communication of the human voice, and has 
chapters on earphone and earphone "socket" (ear pad) development by 
Harvard and Bell Labs, as well as altitude effects on intelligibility 
and interphone system design.

The microphones available for the U'S. military at the beginning of the 
war were few - the venerable 1930s' T-17, the Navy T-38 (RS-38 and NAF 
213264-6 in their system), and the almost universally disliked T-30 
throat mike, the latter which was evaluated in this report as a device 
which "would probably been a very effective instrument but for the fact 
that the speech signal available at the larynx is intrinsically 
unintelligible".  This was true at any altitude, but certainly amplified 
above 20,000 feet.  The wartime work led to a plethora of different 
earphone sockets and microphone types to adapt to the role of oxygen 
masks and hands-free requirements in a combat situation, so the last 
part of your question is a bit more complex.  Perhaps the best overview 
is in Tech Order T.O. 16-1-29, Handbook of Maintenance Instructions for 
Headsets and Microphones dated 25 April 1945 (revised 1 September 
1952).  It's an 87 page manual.  I think Robert Downs has a reprint, and 
like all his work it will be flawless.

73,
Michael Hanz - KC4TOS

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