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.
Acoustic signals received in the northeast coastal regions of the United States from rockets launched at Cape Kennedy show strong seasonal effects. For the Saturn V rockets, strong signals are received in winter, very weak signals in summer, and weak signals in the transitional months of early fall and spring. These seasonal effects are attributed to the winds in the stratosphere (around an altitude of 50 km). In winter, when strong signals are received, the stratospheric winds have strong components in the direction of propagation of the signals. These components are weak during the transitional months, and during summer the stratospheric winds have components in the direction opposite to that of the signal propagation. It is shown that calculations of horizontal trace velocities provide an indirect method of estimating upper atmospheric winds.
Constant-frequency infrasound (about 5 to 10 Hz for different sources) has been detected at a number of locations from different directions. In an intensive study of 8.5-Hz infrasound detected regularly at Lamont-Doherty Geological Observatory of Columbia University in Palisades, N. Y., positive sound fixes have been obtained at the Tappan Zee Bridge. Geophones placed at several locations on the bridge recorded vibrations of the same frequency. We conclude that the vibrational motions of bridges radiate infrasound into the atmosphere and that the infrasound is most readily detected during times when atmospheric wind and temperature structure favor acoustic channeling near the ground. The acoustic signal occurs in pulses having a duration which, although fairly constant on a given occurrence, may vary from a fraction of a minute to several minutes.
Summary Microphones and seismographs were co‐located in arrays on Skidaway Island, Georgia, for the launchings of Apollo 13 and 14, 374 km to the south. Simultaneous acoustic and seismic waves were recorded for both events at times appropriate to the arrival of the acoustic waves from the source. Significant comparisons of the true signals are (1) the acoustic signal is relatively broadband compared to the nearly monochromatic seismic signal; (2) the seismic signal is much more continuous than the more pulse‐like acoustic signal; (3) ground loading from the pressure variations of the acoustic waves is shown to be too small to account for the seismic waves; (4) the measured phase velocities of both acoustic and seismic waves across the local instrument arrays differ by less than 6 per cent and possibly 3 per cent if experimental error is included. It is concluded that the seismic waves are generated by resonant coupling to the acoustic waves along some 10 km of path on Skidaway Island. The thickness of unconsolidated sediment on the island is appropriate to a resonant ground wave frequency of 3.5 to 4 Hz, as observed. Under appropriate conditions, ground wave observations may prove more effective means of detecting certain aspects of acoustic signals in view of the filtering of wind noise and amplification through resonance.
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