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

[Richard Knoppow]

    That is what eventually happened. Throat mics have very poor 
articulation at best. They were replaced by small mics in oxygen masks 
and other types.  Headphones were also the subject of much research. The 
conventional bipolar magnetic phones first used in aircraft also have 
poor articulation under the best circumstances. Eventually they were 
replaced by the ANBH-1A type, a moving coil type receiver developed at 
the Harvard acoustical labs by a group led by Leo Beranek. I think this 
was also an NRDC project. These phones are still made by Telephonics and 
are the standard for use in ear testing equipment.
    At 20,000 feet the air is very thin so many acoustical devices have 
problems. However, the kind of microphones and headphones used in the 
early days of WW-2 were pretty bad especially under noisy combat 
conditions.
    There are some interesting patents in the list given by Brooke 
Clarke. The two issued to Halsey Fredericks are for the very familiar 
Western Electric bipolar magnetic headphone. The first is the master 
patent on the type and the second is for a modification of the magnetic 
properties to resist demagnetization from DC in the windings.  The 
Signal Corps P-2 is essentially the same as the WE 509-W but is earlier. 
I have a pair which is a century old, they still work fine but the cloth 
on the headband has mostly rotted away.  A great many other headphones 
of similar type were made with enough variation to avoid patent 
infringement. I also have a pair of the WE phones with the thin paper 
damping. There were better ways to do this but they sort of work. The 
idea was to reduce the Q of the acoustical and mechanical resonance at 
about 1Khz typical of this kind of headphone. There are a great many 
variations and permutations of the basic type but the best of them is 
far from being high fidelity. The resonance was chosen to be in the 
center of the sensitivity of the ear and the rather sharp peak is 
sometimes useful for CW.
    The Baldwin patent is for his famous balanced armature phones with 
mica diaphragms. These were very popular in the 1920s but are heavy and 
IMO no better than the WE type. Eventually, light weight phones, like 
the Trimm Featherweight, became popular because they worked well and 
were not head crushers like the WE and Baldwin phones. Enough.
On 9/22/2017 11:51 AM, arc5 at ix.netcom.com wrote:
> I have two throat mics that actually work. They sound bad at 20,000 
> angstroms above the floor. They sound bad on the floor. In fact, I think 
> they'd sound best if the whole idea of throat mics were buried under the 
> floor.

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL