Allan J. Zuckerwar’s research while affiliated with STAR Analytical Services and other places

Infrasonic emissions from aircraft wake vortices were investigated at the Newport News-Williamsburg International Airport early in the year 2013. Signals received by the microphones situated along an airport runway were processed in 10-s intervals. As an aircraft accelerates toward takeoff, it produces a large pressure burst as it passes each microphone. Following the burst, there appear low-frequency signals of high coherence among microphone pairs. These are interpreted as emissions from the aircraft wake vortices, as suggested by theory. In successive 10-s intervals, the coherence gradually diminishes to background levels, signifying the disappearance of the vortices. On landing the intervals of high coherence precede the bursts at aircraft touchdown, and then diminish. The pressure burst serves as a time stamp for the ensuing vortex emissions and thereby permits the tracking of successive takeoff or landing events on the same runway or on adjacent runways. The emission spectrum is essentially broadband, lacking spectral features (e.g., tones). Data were taken for takeoff of Airbus 319, DC-9, MD-88, CRJ, Lear Jet, Corporate Jet, and Dash-8 aircraft, and for landing of the Airbus 319. The pattern of pressure burst, high-coherence intervals, and diminishing-coherence intervals was observed for all takeoff and landing events without exception.

An infrasonic field installation was set up at Newport News-Williamsburg International Airport in early 2013. The system is made up of three PCB 377M06 microphones installed into non-porous subsurface windscreens [POMA 1pNS9, 18, 040005 (2013)], which limit the bandwidth to 100 Hz. The microphones are placed 250 ft (76.2 m) orthogonal to the runway and 200 ft (60.96 m) apart. The data acquisition system is the B&K Pulse, from which time histories, spectra, and coherence between microphone channels are derived. The system is placed inside an instrumentation vehicle just behind the center microphone. Perforated drainage hoses are installed from the subsurface windscreens to adjacent drainage ditches and weight is added to the windscreens for additional stability. The drainage system has proved successful even on occasions of heavy downpour, revealing a truly all-weather system. A pistonphone calibration at 14 Hz in the field reveals that the three channels are matched to within 2 dB. This capability permits long-term monitoring of the health of the system. A sample time history of signals received from an aircraft takeoff will be presented.

This article reviews Acoustics: Sound Fields and Transducers by Leo L. Beranek, Tim J. Mellow , an imprint of Elsevier, 2012. 695 pp. Price: $119.95. ISBN: 0123914213

The present invention is an extremely low frequency (ELF) microphone and acoustic measurement system capable of infrasound detection in a portable and easily deployable form factor. In one embodiment of the invention, an extremely low frequency electret microphone comprises a membrane, a backplate, and a backchamber. The backchamber is sealed to allow substantially no air exchange between the backchamber and outside the microphone. Compliance of the membrane may be less than ambient air compliance. The backplate may define a plurality of holes and a slot may be defined between an outer diameter of the backplate and an inner wall of the microphone. The locations and sizes of the holes, the size of the slot, and the volume of the backchamber may be selected such that membrane motion is substantially critically damped.

Clear air turbulence (CAT) is the leading cause of in-flight injuries and in severe cases can result in fatalities. The purpose of this work is to design and develop an infrasonic array network for early warning of clear air turbulence. The infrasonic system consists of an infrasonic three-microphone array, compact windscreens, and data management system. Past experimental efforts to detect acoustic emissions from CAT have been limited. An array of three infrasonic microphones, operating in the field at NASA Langley Research Center, on several occasions received signals interpreted as infrasonic emissions from CAT. Following comparison with current lidar and other past methods, the principle of operation, the experimental methods, and experimental data are presented for case studies and confirmed by pilot reports. The power spectral density of the received signals was found to fit a power law having an exponent of -6 to -7, which is found to be characteristics of infrasonic emissions from CAT, in contrast to findings of the past.

Measurements of wind noise reduction were conducted on a box-shaped, subsurface windscreen made of closed cell polyurethane foam. The windscreen was installed in the ground with the lid flush with the ground surface. The wind was generated by means of a fan, situated on the ground, and the wind speed was measured at the center of the windscreen lid with an ultrasonic anemometer. The wind speed was controlled by moving the fan to selected distances from the windscreen. The wind noise was measured on a PCB Piezotronics 3" electret microphone. Wind noise spectra were measured with the microphone exposed directly to the wind (atop the windscreen lid) and with the microphone installed inside the windscreen. The difference between the two spectra comprises the wind noise reduction. At wind speeds of 3, 5, and 7 m/s, the wind noise reduction is typically 15 dB over the frequency range 0.1-20 Hz.

A dual transmission model of the fetal heart sounds is presented in which the properties of the signals received on a sensor, installed on the maternal abdominal surface, depend upon the position of the fetus. For a fetus in the occiput anterior position, the predominant spectral content lies in the frequency band 16-50Hz ("impact" mode), but for a fetus in the occiput posterior position, it lies in the frequency band 80-110Hz ("acoustic" mode). Signal processing comprises digital bandpass filtering, matched filtering, Teager energy operator, autocorrelation, and figure of merit algorithms. The digital filter permits the user to select the frequency band that best conforms to the prevailing signal mode. Clinical tests on twelve patients, with some in the occiput anterior and some in the occiput posterior fetal positions, support the validity of the dual transmission model.

Governing equations including the effects of pressure relaxation have been utilized to study an incompressible, steady-state viscous axial vortex with specified far-field circulation. When sound generation is attributed to a velocity gradient tensor-pressure gradient product, the modified conservation of momentum equations that result yield an exact solution for a steady, incompressible axial vortex. The vortex velocity profile has been shown to closely approximate experimental vortex measurements in air and water over a wide range of circulation-based Reynolds numbers. The influence of temperature and humidity on the pressure relaxation coefficient in air has been examined using theoretical and empirical approaches, and published axial vortex experiments have been employed to estimate the pressure relaxation coefficient in water. Non-equilibrium pressure gradient forces have been shown to balance the viscous stresses in the vortex core region, and the predicted pressure deficits that result from this non-equilibrium balance can be substantially larger than the pressure deficits predicted using a Bernoulli equation approach. Previously reported pressure deficit distributions for dust devils and tornados have been employed to validate the non-equilibrium pressure deficit predictions.

An array of three infrasonicmicrophones (0.2–20 Hz), operating continuously in the field at NASA Langley Research Center, on several occasions received a class of signals interpreted as infrasonic emissions from clear air turbulence. The presence and location of the turbulence were confirmed by pilot reports (PIREPS), and the direction of emitted signals toward the array was determined by slowness mapping. The coherence of the signals among the three microphone pairs in the array was close to unity. The amplitude spectrum of the received signals was found to fit a power law having an exponent of −7/2, which disagrees with the exponent of −7/4 of Meecham and Ford [J. Acoust. Soc. Am. 30, 318–322 (1958)], based on turbulence self‐noise and with the exponent of −1 of Meecham [J. Acoust. Soc. Am. 33, 149–155 (1971)], based on mean shear fluctuations. Thus the above models do not account for the observed spectrum. Two case histories are described in detail.

A field test on a three‐microphone array at NASA Langley Research Center was conducted using a mobile controlled infrasonic source. A Helmholtz resonator, used to provide a simulated point source for infrasonic propagation studies, had an output SPL of 99 dB (at 1 m) at its resonance frequency of 9.45 Hz. The three‐microphone array was arranged as an equilateral triangle with microphone spacing of 30.48 m (100 ft) and at a distance of more than 85.3 m (280 ft) from the source. The signal level was 40 dB above the background noise in a 1‐Hz band. Measurements of the acoustical response for each of the array microphones were recorded, and the received signal was measured at the nearest microphone to be 60 dB (6 dB per doubling of distance).