VISIT TO (SANITIZED)

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03172A000300050028-5
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 3
• Document Creation Date: December 22, 2016
• Document Release Date: April 10, 2012
• Sequence Number: 28
• Publication Date: October 3, 1956
• Content Type: MEMO

Declassified in Part - Sanitized Copy Approved for Release 2012/04/10: CIA-RDP78-03172A000300050028-5
3 October 1956
NNUORMfl 4 FOR : THE RECORD
1. TDIZ AND PLACE OF MZVMG: The meeting was held 27 September
1956 at
2. AT1R NfANCE:
3. PUFPOBE OF MBRTI*J2 To discuss the progress on the Contact
Microphone Project and the Wall Measurement Program (Ad Hoc #25)
4. DISCMION:
as Contact Microphone ? P-1098
=found that their previous calibration tests, using
wall mounts, were not accurate ate. In this test setup, a
Massa Accelerometer was used as a standard and the unknown unit
compared with it. Both units would be mounted in the same spot
on a wall and driven with the sear frequencies and intensities.
However under this test setup, the high frequencies did not show
up. has now gone to the use of avAbratig?n detector and a
shake table, sitilar to the one used b
has found that nearly every crystal microphone
tested by somewhere around 1200 cps, regardless of
who made it. calculated the pssisasters of what they wanted
in a crystal crephor+e mathematically, but have found so far,
that in actual practice, their calculations do not hold true.
More work is being done to ascertain why. The basic formulas
is using are:
fr wT
Sensativity 2 LW
fr ? frequence response
T a Thickness
A a Area
L ? Length
W ? Width
Tr 23JU-'{ br0 B Y?.
?- WEXT RE4 .?O/L A014. HR I
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Declassified in Part - Sanitized Copy Approved for Release 2012/04/10: CIA-RDP78-03172A000300050028-5
 Declassified in Part - Sanitized Copy Approved for Release 2012/04/10: CIA-RDP78-03172A000300050028-5
(
stated that the unloaded crystals cut to 25X1
specific t peaked at 1000 cps, t
hen dr
opped 25X1
sharply. The mounted crystals were
tested
and it vas 25X1
found that they peaked at less than 1000 cps, then gradually
dropped off in response. However, the sensitivity of these
microphones was exceptionally good.
U found that it was difficult to find a crystal
microphone which possessed a good frequency response above
3000 ape. In testing eoesosrcial units, they found two which
they considered acceptable. They are the model 198
and a Brush BL301. This is a reversal of previous stand
that they could find no comsnarcial units which they considered
acceptable. Apparently this revere decision is based on the
fact that their earlier test results were not too accurate.
(designed several crystal units themselves, but
found that none of their designs were as good as the two
com ercial units mentioned above. Both of these units have
a broad frequency response, the Brush from 200.4000 cps and the
Shure from 400-to about 4000 cps. 0 feels that these
aoceleromsters most the criterion they want, both frequency
and sensitivity vise. These two units will now be field
tested on various wall and articulation curves made of the
results. =feels that than microphoned are sensitive
enough so that no preaap will be necessary,
L] made a small barium titanite transducer, but
the results were poor. Further investigations
undertaken. In their articulation test setup,
high pass filter which chops off everything be
db levels of 30,40, 50, 60, and 80 are used.
not been
cps.
U has now moved into bhe new Physics Building and
the acoustic section has found, unfortunately, that hhe ambient
noise level in the walls of their offices and labs is very high.
This is due to the constant running of air co^pressore, located
on the same floor and used with the electronic calculators on
the upper floors. This has forced them to conduct field tests in
other areas, with the inherent disadvantage of setting up and
moving the required test equipment.
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The next "s of this project will be to field test
the Brush, Shure,microphones and one
microphone to
get a good monaural system. Mathematical
culations will
also continue. It appears to the undersigned that
has to do the basic research work which they had op
0 now
timistically
skipped before. It will be interesting to learn if the commercial
units
likes prove to be good and if so, the question can be 25X1
asked why
who tested many types, did not reach the same 25X1
conclusio
ns.
Declassified in Part - Sanitized Copy Approved for Release 2012/04/10: CIA-RDP78-03172A000300050028-5
 Declassified in Part - Sanitized Copy Approved for Release 2012/04/10: CIA-RDP78-03172A000300050028-5
0
be Wall Measurement Progrea - Ad Hoc
=has conducted a series of tests using a Sperry
Reflective type ultra sonic measuram.nt gauge. The results
obtained so far have been negative. 1~ a frequency range of
this instrument is from .5 to 50 MC. stated that the
higher frequencies are not satisfactory and that the lower
frequencies offer awe promise. =found that, although a
signal could be obtained on some samples at the lowest
frequency of this instrument, the three major obstacles to
overcome weret (1) absC~ the wall, (2) dispersion of
the` m- due to paebles, etc., in the wall, and (3)
good contact on rough Walls. The transducer must make a good
contact or no results will be obtained.
It appears that started this project without such
thought of the problems invo ved. They now feel that their
next step will be to make a literature check of the field to
see what has been dons; to determine the optimum frequency
range and transducer simae, and to try the instrument they
have with a lower frequency a and a higher intensity. If
poor results are still obtain will try the resonance
type ultrasonic gauge.
has calculated that the reflective type of
instrument is not capable of measuring wells less than 2"
thick. According to their figures, at 40 KC, in a wall 2"
or less the pulse travel time in and out is of such a
motion that it becomes buried in the next pulse.
L__] tried the device on wood with no results.
Measureamenta of a brick wall gave fairly good results, mainly
due to the fact that a brick is fairly homogenous and the
contact surface is fairly em oth. Measurements on cement
walls up to 6" could be obtained if a transducer was used on
each side. If only one transducer was used, ~as~ could
be picked up; but the correlation was poor. L-1 reels that it
should be possible by the use of ultra sonics to measure
cement walls up to 12" and possibly 20". However, they admit
that at the moment they cannot substantiate this statement.
Distribution:
1 - P-1098
1 - AH-25
1 - Chrono
1 - AWS
rMa/ls
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Declassified in Part - Sanitized Copy Approved for Release 2012/04/10: CIA-RDP78-03172A000300050028-5