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NASA NTRSDate not supplied

Coherent Vorticity Meteorological Infrasound Sensing Flight Research

Under collaborative flight research project with NASA and, separately, Air Canada, the NRC has investigated meteorological infrasound sensing, using NASA-developed Infrasound sensing microphones. The project involved the installation of 1.5-inch NASA microphone in the nosebay of the NRC DHC-6 research aeroplane. The microphone was plumbed to a pitot-static tube line, connected to a research pitot-static tube on the starboard side of the aircraft nose. Microphone signals on the DHC-6 installation were validated during orbiting flight through its own wake vortices, and flight in the vicinity of wake vortices from Medium Category jets landing at Ottawa Airport. Thereafter, flights were undertaken, to measure the infrasound sensing of meteorological features. Possible acoustic ducting of infrasound energy was measured in stratified lower atmospheric conditions. Infrasound sensing was also accomplished, in the vicinity of meteorological flow-fields of relatively concentrated vorticity, notably line features of vorticity. An increase in sensed infrasound energy was recorded as distance to the line features reduced. Convective uplift infrasound signature sensing was also conducted, and indicated a similar result.

A P Brown↗
NASA NTRS1969

Probing the atmosphere with infrasound

Recent studies are reported which have contributed to the knowledge of atmospheric structure and have established the practicality of infrasonic techniques for probing the atmosphere to heights of 120 km or more. Observations of a few types of infrasound are reviewed, and the theories used to account for the infrasound propagation and the deduced atmospheric structures are discussed.

Posmentier, E. S.↗
NASA NTRS1981

Detection of infrasound generated by Clear Air Turbulence (CAT)

A unified data acquisiton system which is an AM carrier system consisting of a converter signal, conditioning electronics, and peripheral equipment was checked and calibrated and installed in a mobile van. A microphone array in the shape of a 244-m equilateral triangle was connected to the data acquisition system using 457-m long cables. Development of techniques for signal processing for interpreting the infrasonic signature (differentiating between CAT and other sources of infrasound) is summarized. Once patches of CAT are located in the atmosphere, corroboration can be achieved through test flights of aircraft into the suspected region.

Meredith, R.↗
NASA NTRS1984

Design of infrasound-detection system via adaptive LMSTDE algorithm

A proposed solution to an aviation safety problem is based on passive detection of turbulent weather phenomena through their infrasonic emission. This thesis describes a system design that is adequate for detection and bearing evaluation of infrasounds. An array of four sensors, with the appropriate hardware, is used for the detection part. Bearing evaluation is based on estimates of time delays between sensor outputs. The generalized cross correlation (GCC), as the conventional time-delay estimation (TDE) method, is first reviewed. An adaptive TDE approach, using the least mean square (LMS) algorithm, is then discussed. A comparison between the two techniques is made and the advantages of the adaptive approach are listed. The behavior of the GCC, as a Roth processor, is examined for the anticipated signals. It is shown that the Roth processor has the desired effect of sharpening the peak of the correlation function. It is also shown that the LMSTDE technique is an equivalent implementation of the Roth processor in the time domain. A LMSTDE lead-lag model, with a variable stability coefficient and a convergence criterion, is designed.

Khalaf, C. S.↗
NASA NTRS1990

The correction of infrasound signals for upper atmospheric winds

Infrasound waves propagate in the atmosphere by a well known mechanism produced by refraction of the waves, return to earth, and reflection at the surface into the atmosphere for subsequent bounces. A figure illustrates this phenomenon with results from a ray trace model. In this instance three rays are returned to earth from a region centered at about 50 kilometers in altitude and two from a region near 110 kilometers in altitude. The control of the wave refraction is largely dominated by the temperature-height profile and inversions; however, a major influence is also produced by the atmospheric wind profile. Another figure illustrates the considerable ray differences for rays moving in the wind direction (to the right) and in the counter direction (to the left). It obviously can be expected that infrasonic signal amplitudes will be greatly influenced by the winds in the atmosphere. The seasonal variation of the high altitude atmospheric winds is well documented. A third figure illustrates this with average statistics on the observed zonal wind in the region of 50 plus or minus 5 kilometers in altitude. The results are based upon a survey by Webb; Webb terms this parameterization the Stratospheric Circulation Index (SCI). The very strong seasonal variation has the ability to exert a major seasonal influence on infrasonic signals. The purpose here is to obtain a method for the correction of this effect.

Mutschlecner, J. Paul↗
NASA NTRS2008

Sub-Surface Windscreen for the Measurement of Outdoor Infrasound

A windscreen has been developed that features two advantages favorable for the measurement of outdoor infrasound. First, the sub-surface location, with the top of the windscreen flush with the ground surface, minimizes the mean velocity of the impinging wind. Secondly, the windscreen material (closed cell polyurethane foam) has a sufficiently low acoustic impedance (222 times that of air) and wall thickness (0.0127 m) to provide a transmission coefficient of nearly unity over the infrasonic frequency range (0-20 Hz). The windscreen, a tightly-sealed box having internal dimensions of 0.3048 x 0.3048 x 0.3556 m, contains a microphone, preamplifier, and a cable feed thru to an external power supply. Provisions are made for rain drainage and seismic isolation. A three-element array, configured as an equilateral triangle with 30.48 m spacing and operating continuously in the field, periodically receives highly coherent signals attributed to emissions from atmospheric turbulence. The time delays between infrasonic signals received at the microphones permit determination of the bearing and elevation of the sources, which correlate well with locations of pilot reports (PIREPS) within a 320 km radius about the array. The test results are interpreted to yield spectral information on infrasonic emissions from clear air turbulence.

Shams, Qamar A.↗
NASA NTRS1976

On meteor-generated infrasound

The characteristics of generation and propagation of infrasonic pressure waves excited during meteor entry into the earth's atmosphere are studied. Existing line source blast wave theory is applied to infrasonic airwave data from four bright fire-balls. It is shown that the strong shock behavior of the blast wave is confined to a cylinderical region with a radius proportional to the product of the meteor Mach number and its diameter. A description of the wave form far from the source is provided. Infrasonic data reported elsewhere are analyzed. All the results should be considered as preliminary, and additional work is under way to refine the estimates obtained.

Revelle, D. O.↗
NASA NTRS2014

Sub-Surface Windscreen for Outdoor Measurement of Infrasound

A windscreen is configured for measuring outdoor infrasonic sound. The windscreen includes a container and a microphone. The container defines a chamber. The microphone is disposed in the chamber and can be operatively supported by the floor. The microphone is configured for detecting infrasonic sound. The container is advantageously formed from material that exhibits an acoustic impedance of between 0 and approximately 3150 times the acoustic impedance of air. A reflector plate may be disposed in the container. The reflector plate operatively can support the microphone and provides a doubling effect of infrasonic pressure at the microphone.

Zuckerwar, Allan J.↗