Qamar A. Shams

Compact nonporous windscreen for infrasonic measurements

Infrasonic windscreens, designed for service at frequencies below 20Hz, were fabricated from a variety of materials having a low acoustic impedance, and tested against four specifications (the first three in a small wind tunnel): (1) wind-generated noise reduction (“insertion loss”) at a free-stream wind speed of 9.3m∕s, (2) transmission of low-frequency sound from a known source (subwoofer), (3) spectrum of sound generated from trailing vortices (aeolian tones), and (4) water absorption (to determine suitability for all-weather service). The operating principle is based on the high penetrating capability of infrasound through solid barriers. Windscreen materials included three woods (pine, cedar, and balsa), closed-cell polyurethane foam, and Space Shuttle tile material. The windscreen inside diameter ranged from 0.0254to0.1016m (1to4in.), and wall thickness from 0.003175to0.01905m (18to34in.). A windscreen made of closed-cell polyurethane foam revealed a wind noise reduction of 10–20dB from 0.7to25Hz, t...

Application of MEMS Microphone Array Technology to Airframe Noise Measurements

NASA Langley Research Center, Hampton, Virginia, 23681Current generation microphone directional array instrumentation is capable ofextracting accurate noise source location and directivity data on a variety of aircraftcomponents, resulting in significant gains in test productivity. However, with this gain inproductivity has come the desire to install larger and more complex arrays in a variety ofground test facilities, creating new challenges for the designers of array systems. Toovercome these challenges, a research study was initiated to identify and develop hardwareand fabrication technologies which could be used to construct an array system exhibitingacceptable measurement performance but at much lower cost and with much simplerinstallation requirements. This paper describes an effort to fabricate a 128-sensor arrayusing commercially available Micro-Electro-Mechanical System (MEMS) microphones. TheMEMS array was used to acquire noise data for an isolated 26%-scale high-fidelityBoeing 777 landing gear in the Virginia Polytechnic Institute and State University StabilityTunnel across a range of Mach numbers. The overall performance of the array wasexcellent, and major noise sources were successfully identified from the measurements.

Experimental investigation into infrasonic emissions from atmospheric turbulence

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.

Contribution of crosstalk to the uncertainty of electrostatic actuator calibrations

Crosstalk in electrostatic actuator calibrations is defined as the ratio of the microphone response to the actuator excitation voltage at a given frequency with the actuator polarization voltage turned off to the response, at the excitation frequency, with the polarization voltage turned on. It consequently contributes to the uncertainty of electrostatic actuator calibrations. Two sources of crosstalk are analyzed: the first attributed to the stray capacitance between the actuator electrode and the microphone backplate, and the second to the ground resistance appearing as a common element in the actuator excitation and microphone input loops. Measurements conducted on 1/4, 1/2, and 1 in. air condenser microphones reveal that the crosstalk has no frequency dependence up to the membrane resonance frequency and that the level of crosstalk lies at about -60 dB for all three microphones-conclusions that are consistent with theory. The measurements support the stray capacitance model. The contribution of crosstalk to the measurement standard uncertainty of an electrostatic actuator calibration is therewith 0.01 dB.

Low frequency electret condenser microphone

A condenser microphone has been fabricated for measuring low‐frequency sound pressure. The goal of this design is to keep the background noise as low as possible. The microphone features a high membrane compliance with a large backchamber volume, a prepolarized backplane, and a high impedance preamplifier located inside the backchamber. Methods for characterizing the performance of the microphone will be presented including background noise levels, which will be compared to commercially available microphones.

Soaker hose versus compact nonporous windscreen: A comparison of performance at infrasonic frequencies

A compact nonporous windscreen described previously [J. Acoust. Soc. Am. 114, No. 4, Pt. 2, 2323 (2003)] was tested in the field against a soaker hose array to compare performance at infrasonic frequencies. The cylindrically shaped compact windscreen, made of closed‐cell polyurethane foam, had dimensions 0.0762 m i.d. ×0.2286 m height ×0.0127 m wall (3×9×0.5 in.). The low acoustic impedance of the foam permits the propagation of infrasound through the walls of the windscreen with a transmission coefficient near unity. The soaker hoses were 15.24‐m (50 ft.) long and coupled to a Chaparral model 5 low‐frequency microphone. The hose plenum was removed and replaced with the compact windscreen for testing. A sonic boom simulator, located at a distance of 400 m (1/4 mile) from the microphone, generated tones at 3, 4, 5, and 6 Hz. Analysis of the signals received by the interior microphone revealed that the tones transmitted through the windscreen, as well as the reduction of background noise due to naturally oc...

Infrasonic emissions from aircraft wake vortices: Experimental results

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 broadban...

Infrasonic emissions from aircraft wake vortices: Field installation

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...

Spectrum of measured infrasonic emissions from clear air turbulence.

An array of three infrasonic microphones (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.

Response of infrasonic microphone field array to a controlled source.

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).