Two distinct groups of infrasonic waves from Saturn V, 1967, were recorded at Palisades, New York, 1485 kilometers from the launch site. The first group, of 10-minute duration, began about 70 minutes after launch time; the second, having more than twice the amplitude and a duration of 9 minutes, commenced 81 minutes after launch time. From information on the Saturn V trajectory and analysis of recorded data, it is established that the first group represents sound emitted either by the first stage reentry or by the second stage when its elevation was above 120 kilometers. The second, more intense wave group represents the sound from the powered first stage. A reversal of signal occurs because the rocket outran its own sound. Fourier analyses indicate that the energy extends to relatively long periods—10 seconds for the first stage and 7 seconds for the second. Trapping of sound in the upper atmospheric sound channel can be the cause of the separation of the signal into two distinct groups.
Long-range infrasound from large rockets launched at Cape Kennedy have been recorded and studied since 1957 by a variety of low-frequency acoustic sensors. Dominant energy ranges from 0.1 to 2 Hz. Signals can be grouped into early (apparently supersonic) arrivals, normal acoustic arrivals, and late (apparently subsonic) arrivals. The normal acoustic signals detected in the eastern United States from rockets launched in the northeast quadrant from Cape Kennedy show two prominent wave groups generated by the launch and first-stage reentry. The origin of this long-range infrasound is shown to be acoustic energy radiated from the cone of shock waves. The data of early arrivals (supersonic waves) suggests an origin from coupling between normal acoustic waves and those in a higher-speed region. Late arrivals having apparently subsonic velocities are also detected less frequently than the supersonic arrivals.
A quadrangle array of infrasonic detectors to monitor rocket engine noise during flight through the atmosphere and lower ionosphere monitored the atmospheric background noise, as well, for a period of two hours around the launching time. Data acquired gave a description of the rocket engine noise in the frequency range of 20–0.05 cps. It is proven that signals from rockets igniting in the upper atmosphere and ionosphere travel long distances and can be detected by the ground sensors. The atmospheric background noise consists of a great variety of sources, some of which are pressure fluctuations traveling with low velocity of approximately 5–50 m/sec. Other sources are infrasonic in nature. The data were analyzed by means of analog and mathematical bandwidth filters and power and cross spectra methods. Some studies were made by utilizing calculation of travel time and ray‐tracing techniques.
Many physical principles have been applied to transform atmospheric-pressure fluctuations 1 sec and longer into electrical signals for the purpose of monitoring infrasonic signals. A survey of some infrasonic sensors is taken, with calibration and testing of several at the UCLA calibration facility. An attempt was made to define the capabilities of each available sensor. Several experiments are described, which include the monitoring of static and dynamic firing of rockets as well as that of explosions. The difficulties in interpretation of data are explained in relation to various instruments, taking into account wind effects, ground vibrations, natural pressure fluctuations, and other phenomena producing noise at the frequency range of interest.
Many physical principles have been applied to transform atmospheric pressure fluctuations of 1 sec and longer into electrical signals for the purpose of monitoring infrasonic signals. A survey of some infrasonic sensors is taken, with calibration and testing of several sensors at the UCLA calibration facility. An attempt was made to define the capabilities of each available sensor. Several experiments are described, which include the monitoring of static and dynamic firing of rockets, as well as the monitoring of explosions. The difficulties in interpretation of data are explained in relation to various instruments, taking into account wind effects, ground vibrations (observed by magnetometers), natural pressure fluctuations, and other phenomena producing noise at the frequency range of interest.
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