Instrumentation Role in the Observation of Geoacoustical Phenomena from Artificial Sources

Fehr, Uri

doi:10.1121/1.2143662

Cited by 1 publication

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“…The instrumentation used to obtain the infrasound data consists of the sensors, analog recording equipment, and analog-to-digital conversion equipment. The sensors are Fehr and Fiske 'aerophones,' which employ thermistors to measure the flow in a tube connecting a reference volume to the environment [Fehr, 1967b]. Their low-frequency 3-dB point is 0.7 Hz, below which the falloff in sensitivity is 6 dB/octave, and their high-frequency 1.7-dB point is 20 Hz.…”

Section: Data Acquisition and Reductionmentioning

confidence: 99%

1- to 16-Hz infrasound associated with clear air turbulence predictors

Posmentier¹

1974

J. Geophys. Res.

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The 1-to 16-Hz infrasound detected by a four-element array of microphones was recorded for 75 s six times daily from April through June 1971. The spectra of these records are diurnally variable but stationary on a shorter time scale. The spectral slopes are inconsistent with theoretical spectral slopes of turbulent pressure fluctuations and of sound radiated by incompressible aerodynamic turbulence. It is hypothesized that the infrasound is radiated by a mechanism involving compressibility in aerodynamic turbulence. Four records were selected as being representative of extremely low amplitude conditions. Four other records were chosen as being representative of highly coherent strong amplitude records. These latter four records all exhibited very high horizontal phase speeds, implying near-vertical incidence. A search was made for meteorologic parameters that varied consistently with the acoustic data. It has been found that all four low amplitude records were from periods during which nonturbulent conditions probably existed between altitudes of 6 and 12 km. The four strong amplitude records coincide with times of high probability of clear air turbulence. The turbulence judgments are based on the wind speed, the directional shear, and Richardson's number, calculated from nearby rawinsonde data. It is concluded that 1-to 16-Hz infrasound may be radiated by clear air turbulence and may be a basis for a remote passive detection system. If this conclusion were confirmed, it would have significant implications for both practical and theoretical problems associated with clear air turbulence.Of the entire spectrum of infrasound the highest frequencies, those that are closest to audible frequencies, have probably been least subjected to scientific inquiry. The purpose of the study that is the subject of this report has been to explore the general nature of this higher-frequency infrasound, and the essential finding of the study has been the association of this infrasound with clear air turbulence (CAT) in the lower stratosphere.Wescott [1964] measured the spectra from 0.2 to 200 Hz of pressure fluctuations detected by balloon-borne acoustic probes at altitudes of 17-22 km. The median values of his spectra at 5 Hz were about 9 dB above those of our spectra, discussed below. Since his measurements were taken at high altitudes (and the pressure amplitude of a plane acoustic wave varies as the square root of the ambient pressure as it propagates vertically), they would normally be expected to be about 12 dB below ours. There is a net excess therefore of 21 dB. This is probably a real difference in observations, resulting from geographic, seasonal, or other effects, if it is assumed that we did not observe entirely different phenomena. 5 "' 0 4 ß -r I o z z I0 • i0 z 95% ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß X ß ß ß ß ß ß ß ß ß ß ß ß X o --ß --X X ß ß X ß _ X X ß ß ß ß _X X S x X X X X X ß ß ß

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Section: Data Acquisition and Reductionmentioning

confidence: 99%

1- to 16-Hz infrasound associated with clear air turbulence predictors

Posmentier¹

1974

J. Geophys. Res.

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Instrumentation Role in the Observation of Geoacoustical Phenomena from Artificial Sources

Cited by 1 publication

References 0 publications

1- to 16-Hz infrasound associated with clear air turbulence predictors

1- to 16-Hz infrasound associated with clear air turbulence predictors

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