a,
tion” (WT-930), AR
M. H. Oleson, Project
Officer.
—
or to rate-of-change of pressure for signal periods
ranging from approximately 5 to 300 seconds.
A total of 16 stations, one close-in (320 km) and
the balance at distances, were operated ior the
AR
Standard detection equipinent (Data Recording
System M-2 or NBS Infrasonic Microphone Svstem)
was operated at all SCEL stations. Both types of
equipment utilized condenser microphones as the
electromagnetic experiments.
Sotrttoad-band measurements (up to 40 Me at
close-in distances and approximately 100 ke a: greater
pressure-sensitive transducers, wire lines for trans1:ussi0on to the recording central, and Esterline-
distances) and narrow~band measurements (approxi-
Angus graphic recorders.
mately 200 cycles) were madeof the_vertical field
component. Close-in wave forms and field strengths
were recordedfor all shots except Shot i. Signals
were received, and wave forms, field strengths, and
azimuths were recordedat distances exceeding 12,000
km for both a north-south and an east-west path.
The National Bureau of Standards (NBS) operated
the close-in station: a 2-meter vertical antenna with
a cathode follower feeding a coaxial line to recording
oscilloscopes set at various sweep speeds and gains.
At this close distance (320 km), signal strengths were
several volts per meter, and interference from natural sources or transmitting stations in proximity
was no problem. Band widths were about 13 and 40
Mc, Hmited by the type of scopes used; the low-
The M-zZ equipment responded mainly to pressure
‘hanges in the range of periods from 1 to 50 seconds
and the NBS from 1 to 35 seconds.
The maximum
sensitivity for the M-2 was of the order cf 15-mm
deflection for a pressure change of 1 dyne/cem’, that
for the improved M-2 was about 45 mm/(dyne/cm’),
and that for the NBS was approximately 30 mm, (dyne/
cm’).
Recording speed was 3 in/'min.
Very-low-
frequency equipment was also operated by SCEL at
some stations. This equipment consisted of a specia)
condenser microphone designed for low-frequency
response (5- to 4uvQ-second geriods) through use of a
very-large reference voluire, a high-resistance
acoustic leak, and elaborate thermal insulation. The
electronic and control circuits were similar to that
employed in the improved M-2 cquipment, and the
frequency limit was about 160 cps.
Distant stations were operated by the NBS and the
Defense Research Laboratory (DRL) using 30-foot
vertical antennas with standard cathode followers.
inaamumsensitivity was approximately the same.
Recording speed was 1.5 in/min.
Each standard microphone was equipped with a
Both narrow-band (about 200-cps) and broad-band
(about 1- to 70-kce) recordings were made.
Agencies participating in this project under the
sponsorship of AF
were the National Bureau of
linear, multiple-inlet pipe array 1,000 feet in lenzth,
designed to reduce the noise background from atmosBheric turbulence. No effective array was available
for use at very-low frequercies.
Standards (NBS), the Navy Electronics Laboratory
(NEL), and the Signal Corps Engineering Laboratories
(SCEL). The Geophysics Research Directorate of the
Air Force Cambridge Research Center (AFCRC) conducted additional measurements under a different
program.
Each station operated by the Signal Corps consisted
of four microphone outposts, one at each corner of a
quadrilateral, approximately square, 4 to 10 miles
on a side. Each outpost was connected to a recording
central.
The NEL operated arrays of two to five microphone
outposts spaced from 3 to 15 miles apart at three lo-
cations. In most cases, microphone outposts were
connected to a recording central.
The NBS station consisted of six microphone outposts located at the corners of two roughly equilateral
triangles, one having 2'4-mile sides and the other
14-mile sides. The small triangle was roughly centered inside the larger triangle. Each outpost was
connected by wire lines to a recording central.
The AFCRC stations were similar to those of SCEL,
except that individual recordings were made in the
immediate vicinity of each microphone outpost.
Two main types of equipment were used: (1) stand-
The NEL operated two types of very-low-frequency
equipment. One type operated at some stations consisted of a Ricber vibrotron microphone modifted for
response to periods from §& to 265 seconds. Output
was recorded on a Brush graphic recorder at speeds
of 0.2 and 0.5 in/min. The second type, operated at
ali NEL stations, consisted of a Signal Corps T-21-B
condenser microphone modified to respond to periods
from 6 to 300 seconds.
Output was recorded on
Esterline-Angus graphic recorders at 0.75 in/min.
At maximum sensitivity, the modified Rieber equipment gave a deflection of approximately 0.2 mm for
a pressure change of 1 dyne/cem? and the modified
T-21-B equipment gave approximately 0.7 mm/(dyne/
cm’). No effective noise-reducing arrays were avatlable for use at very~-low frequencies.
All NBS stations were equipped with standard NBS
equipment. The microphone was modified to increase
the sensitivity, but to retain the same frequency response. At maximum sensitivity, the equipment gave
ard detection equipment most responsive to atmos-
pheric-pressure changes having periods ranging
roughly from 1 to 60 seconds and (2) very-lowfrequency equipment responsive to change in pressure
116
a deflection of approximately 50 mm/(dyne/cm’). A
standard, linear, pressure-averaging pipe array of
Signal Corps design was used for noise reduction.
Recording speed was 3 in/min.
The three microphones making up the large triangle and one of the microphones from the small tri~angle were also connected to special multivibrator-
type discriminators and low-passfilter amplifiers