oy tion” (WT-930), AL M. H. Olegon, 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 .or the AR electromagnetic experiments. Standard detection equipment (Data Recording System M-2 or NBS Infrasonic Microphone Svstem) was operated at all SCEL stations. Both types of WorkBtoad-band measurements (up to 40 Mc at equipment utilized condenser microphones as the close-in distances and approximately 100 ke ai greater distances) and narrow-band measurements (approximately 200 cycles) were made of the,vertical field component. Close~in wave forms andfield strengths were recorded for all shots except Shot 1. pressure-seusitive transducers, wire lines for trang- Luss.on to the recording central, and Esterline- Angus graphic recorders. The M-z equipment responded mainly to pressure ‘hanges in the range of periods from 1 to 50 seconds Signals were received, and wave forms, field strengths, and azimuths were recorded at distances exceeding 12,000 km for poth a north-south and an east-west path. and the NBS from 1 te 35 seconds. Yhe maximum sensitivity for the M-2 was of the order cf L3-im.m deflection for a pressure change of 1 dyne/cm’, that for the improved M-2 was about 45 mm/(dyne/2m?), and that for the NBS wus approximately 50 mm, (dyne/ 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 em?). Recording speed was 3 in/min. Very-low- oscilloscopes set at various sweep speeds and gains. frequency equipment was also operated by SCEL at several volts per meter, and interfcrence from natural sources or transmitting stations in proximity some stations. This -quipment consisted of a special condenser microphone designed for low-frequency response (f- to 3v0-second ceriads; through use of a very-large reference voluire, a nigh-resistance At this close distance (320 km), signal strengths were was no problem. Band widths were about 13 and 40 Mc, limited by the type of scopes used; the lowfrequency Limit was about 160 cps. Distant statiuns were operated hy the NBS and the acoustic leak, and elaborate thermal insulation. The electronic and control circuits were similar to that employed in the Improved M~-2 :quipment, and the maximumsensitivity was approximately the same. Recording speed was 1.5 in/inin. Defense Research Laboratory (DRL) using 30-foot vertical antennas with standard cathode followers. Both narrow-band (about 200-cps) and broad-band (about 1- to 70-ke) recordings were made. Agencies participating in this project under the sponsorship of AF Each standard microphone was equipped with a linear, multiple-inlet pipe array 1,000 feet in ler th, designed to reduce the noise background from atn.ospheric turbulence. No eff.ctive array wus available were the National Bureau of Standards (NBS), the Navy Electronics Laboratory for use at very-low frequercles. (SCEL). The NEL operated two types of very-low-frequency equipment. One type overated at some stutions con- (NEL), and the Signal Corps Engineering Laboratories 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. sisted of a Ricbe: 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. Ottput was recorded on Esterline-Angus graphic recorders at 0.75 in/min. The NEL operated arrays of two to five microphone outposts spaced from 3 to 15 miles apart at three lo- At maximum sensitivicy, the modified Rieber equip- cations. a pressure change of 1 dyne/em? and the modified ment gave a deflection of approximately 0.2 mm for In most cases, microphone outposts were connected to a recording central. The NBS station consisted of six microphone out~ posts located at the corners of two roughly equilateral T-21-B equipment gave approximately 0.7 mm/(dyne/ cm?). No effective notse-reducing arrays were avail- 14-mile sides. The small triangle was roughly centered inside the larger triangle. Each outpost was connected by wire linea to a recording central. The AFCRC stations were similar to those of SCEL, except that individual recordings were made in the equipment. The microphone was modified to increase the sensitivity, but to retain the same frequency response. At maximum senaitivity, the equipment gave triangles, one having 2'4-mile sides and the other immediate vicinity of each microphone outpost. Two main types of equipment were used: (1) standard detection equipment most responsive to atmospheric-pressure changes having periods ranging roughly from 1 to 60 seconds and (2) very-lowfrequency equipment responsive to change in pressure 116 able for use at very-low frequencies. All NES stations were equipped with standard NBS a deflection of approximately 50 mm/(dyne/em’). 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 smalli triangle were also connected to special multivibratortype discriminators and low-paas filter amplifiers