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Thomas B Gabrielson
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ABSTRACT: In-situ measurement of the frequency-response of infrasound array elements has proven to be a useful tool in the assessment of element performance. In order to transition to a true calibration process, the uncertainties inherent in the method must be determined. It is critically important to distinguish between bias errors and random errors and to recognize that the ambient pressure fluctuations are typically not stationary in a statistical sense. The time evolution of the cross-spectrum is particularly useful for identifying non-stationary behavior and for isolating high-quality data intervals. Three important cases are tractable: high coherence between the reference sensor and the infrasound element; low-to-moderate coherence resulting from uncorrelated noise in one channel; and moderate coherence resulting from uncorrelated noise in both channels. For a fixed number of averages, the confidence limits for the frequency-response estimate are often considerably tighter than the corresponding limits for the estimated spectral densities.
The Journal of the Acoustical Society of America 09/2012; 132(3):2048. · 1.55 Impact Factor
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ABSTRACT: Wind-noise-reduction systems for infrasound monitoring stations often take the form of networks of pipes and cavities. The acoustical response of these wind-noise-reduction systems can be determined using ambient noise and comparison to a reference sensor. Faults in these systems can sometimes be detected by such response measurements; however, identification and localization of a fault is more challenging. Another approach for performance assessment is to measure the acoustical impedance at accessible points in the pipe network. This approach has the potential for high signal-to-noise ratio, less dependence on atmospheric conditions, and the ability to isolate sub-sections of the network. A portable apparatus has been designed for field measurement of acoustical impedance. The impedance apparatus generates a controlled volume velocity and measures acoustic pressure at the driving point.
The Journal of the Acoustical Society of America 09/2012; 132(3):2048. · 1.55 Impact Factor
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Thomas B Gabrielson
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ABSTRACT: A worldwide network of more than 40 infrasound monitoring stations has been established as part of the effort to ensure compliance with the Comprehensive Nuclear Test Ban Treaty. Each station has four to eight individual infrasound elements in a kilometer-scale array for detection and bearing determination of acoustic events. The frequency range of interest covers a three-decade range-roughly from 0.01 to 10 Hz. A typical infrasound array element consists of a receiving transducer connected to a multiple-inlet pipe network to average spatially over the short-wavelength turbulence-associated "wind noise." Although the frequency response of the transducer itself may be known, the wind-noise reduction system modifies that response. In order to understand the system's impact on detection and identification of acoustical events, the overall frequency response must be determined. This paper describes a technique for measuring the absolute magnitude and phase of the frequency response of an infrasound element including the wind-noise-reduction piping by comparison calibration using ambient noise and a reference-microphone system. Measured coherence between the reference and the infrasound element and the consistency between the magnitude and the phase provide quality checks on the process.
The Journal of the Acoustical Society of America 09/2011; 130(3):1154-63. · 1.55 Impact Factor
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ABSTRACT: A network of rigid pipes with multiple inlets is one commonly employed strategy to filter out pressure fluctuations generated by wind when making infrasound measurements. This talk presents a model of the transfer function of such a rosette filter using a computer model (DeltaEC) that includes thermo-viscous effects at the walls of the pipes. Measurements of this transfer function have been made utilizing the coherence between a calibrated reference microphone and a calibrated microphone shielded by the wind-reduction rosette and these data will be compared to the model. Rosettes with pipes of varied lengths were tested and modeled. The response of the rosette as a function of the vertical arrival angle of the incident plane wave will also be presented. [Work supported by US Army Space and Missile Defense Command.].
The Journal of the Acoustical Society of America 03/2010; 127(3):2038. · 1.55 Impact Factor
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ABSTRACT: The high cost of many infrasonic transducers can be a setback for universities interested in performing research in the field of infrasound. Reasonably effective infrasonic transducers can be constructed in the laboratory, however, and for a fraction of the cost. A simple, inexpensive infrasonic microphone that operates on the principle of carrier demodulation has been constructed. Noise analysis of the microphone demonstrates its potential usefulness for cost-effective field research and deployment in temporary infrasonic arrays. [Research funded by the Penn State Applied Research Laboratory Educational and Foundational Fund.].
The Journal of the Acoustical Society of America 05/2009; 125(4):2595. · 1.55 Impact Factor
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ABSTRACT: Characteristics of the "ideal" shock, a one-dimensional step change in pressure, are reasonably well established; however, the freely propagating shock with finite duration is challenging to model and measure. These challenges are reflected in an extensive body of literature over several decades. In this paper we present a method for producing free-space propagating shocks, measurements of rise time with minimal contamination by turbulence, and model results that account for spreading, absorption (including molecular relaxation), and nonlinearity. The shocks are produced using an acoustic pipe filter to condition the irregular pressure release of either balloon pops or starter-pistol shots. The shocks range in peak pressure from 10 to 1000 Pa and in rise time from 10 to 0.5 mus. Rise-time measurements are made using a custom piezoelectric polymer wideband microphone with a measurement range that overlaps with that of a diffraction-corrected 18-inch measurement microphone. The exponential-decay time constant beyond the pressure peak is typically less than 100 mus, which may be too rapid to permit development of characteristics associated with relaxation mechanisms: rise times over the entire range measured here are well predicted by accounting for classical and rotational absorption only.
The Journal of the Acoustical Society of America 05/2009; 125(4):2631. · 1.55 Impact Factor
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ABSTRACT: To address the question of the role of nonlinear effects in the propagation of noise radiated by high-power jet aircraft, extensive measurements were made of the F-22A Raptor during static engine run-ups. Data were acquired at low-, intermediate-, and high-thrust engine settings with microphones located 23-305 m from the aircraft along several angles. Comparisons between the results of a generalized-Burgers-equation-based nonlinear propagation model and the measurements yield favorable agreement, whereas application of a linear propagation model results in spectral predictions that are much too low at high frequencies. The results and analysis show that significant nonlinear propagation effects occur for even intermediate-thrust engine conditions and at angles well away from the peak radiation angle. This suggests that these effects are likely to be common in the propagation of noise radiated by high-power aircraft.
The Journal of the Acoustical Society of America 07/2008; 123(6):4082-93. · 1.55 Impact Factor
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ABSTRACT: Acoustic intensity processing of signals from directional sonobuoy acoustic subsystems is used to enhance the detection of submerged bodies in bi-static sonar applications. In some directions, the scattered signals may be completely dominated by the incident blast from the source, depending upon the geometry, making the object undetectable by traditional pressure measurements. Previous theoretical derivations suggest that acoustic vector intensity sensors, and the associated intensity processing, are a potential solution to this problem. Deep water experiments conducted at Lake Pend Oreille in northern Idaho are described. A large, hollow cylindrical body is located between a source and a number of SSQ-53D sonobuoys positioned from 5 to 30 body lengths away from the scattering body. Measurements show changes in the acoustic pressure of less than 0.5 dB when the scattering body is inserted in the field. However, the phase of the acoustic intensity component formed between the acoustic pressure and particle velocity component orthogonal to the direction of incident wave propagation varies by as much as 55 degrees. This metric is shown to be a repeatable and strong indicator of the presence of the scattering body.
The Journal of the Acoustical Society of America 05/2007; 121(4):1909-15. · 1.55 Impact Factor
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ABSTRACT: From 1998 to 2001, 115 h of acoustic recordings were made in the presence of the well-studied St. Lawrence population of blue whales, using a calibrated omnidirectional hydrophone [flat (+/- 3 dB) response from 5 to 800 Hz] suspended at 50 m depth from a surface isolation buoy. The primary field site for this study was the estuary region of the St. Lawrence River (Québec, Canada), with most recordings made between mid-August and late October. During the recordings, detailed field notes were taken on all cetaceans within sight. Characterization of the more than 1000 blue whale calls detected during this study revealed that the St. Lawrence repertoire is much more extensive than previously reported. Three infrasonic (<20 Hz) and three audible range (30-200 Hz) call types were detected, with much time/frequency variation seen within each type. Further variation is seen in the form of call segmentation, which appears (through examination of Lloyd's Mirror interference effects) to be controlled at least partially by the whales. Although St. Lawrence blue whale call characteristics are similar to those of the North Atlantic, comparisons of phrase composition and spacing among studies suggest the possibility of population dialects within the North Atlantic.
The Journal of the Acoustical Society of America 11/2006; 120(4):2340-54. · 1.55 Impact Factor
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ABSTRACT: The use of Directional Frequency Analysis of Recording (DIFAR) acoustic subsystems as underwater acoustic intensity sensors in various Navy applications is the subject of this report. Specifically, acoustic intensity processing of DIFAR signals is used to enhance the detection of submerged bodies in the forward-scattered occlusion zone. In this zone, the incident and scattered pressure components arrive at the receiver at essentially the same time. The received signal is thus completely dominated by the incident blast from the source which renders the object undetectable. The problem of scattered wave detection in the forward-direction has been a fundamental unsolved problem of acoustics, electromagnetics and optics. The experimental results presented in this report for submerged-body acoustic scattering provide some very encouraging indications that acoustic vector intensity sensors, and the associated intensity processing, is a potential solution to this problem.
09/2005;
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ABSTRACT: An underwater acoustic intensity sensor is described. This sensor derives acoustic intensity from simultaneous, co-located measurement of the acoustic pressure and one component of the acoustic particle acceleration vector. The sensor consists of a pressure transducer in the form of a hollow piezoceramic cylinder and a pair of miniature accelerometers mounted inside the cylinder. Since this sensor derives acoustic intensity from measurement of acoustic pressure and acoustic particle acceleration, it is called a p-a intensity probe. The sensor is ballasted to be nearly neutrally buoyant. It is desirable for the accelerometers to measure only the rigid body motion of the assembled probe and for the effective centers of the pressure sensor and accelerometer to be coincident. This is achieved by symmetric disposition of a pair of accelerometers inside the ceramic cylinder. The response of the intensity probe is determined by comparison with a reference hydrophone in a predominantly reactive acoustic field.
The Journal of the Acoustical Society of America 01/2005; 116(6):3384-92. · 1.55 Impact Factor
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The Journal of the Acoustical Society of America 02/2003; 113(1):45-8. · 1.55 Impact Factor
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ABSTRACT: The fundamental goal of this project was to measure acoustic intensity in the strong interaction region of a parametric source. The expected benefits of such a measurement are (1) a clear definition of the source-generation region, and (2) an ability to separate local generation (the reactive field) from propagation (the real or active field). The fundamental goal of this project was to measure acoustic intensity in the strong interaction region of a parametric source. The expected benefits of such a measurement are (1) a clear definition of the source-generation region, and (2) an ability to separate local generation (the reactive field) from propagation (the real or active field).
11/2002;
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ABSTRACT: Acoustic intensity is measured by a submerged probe consisting of two passive geophones mounted in spaced relationship with their sensing axes aligned. The geophones are connected through a cable to a remote spectrum analyzer in which acoustic intensity is computed from the velocity gradient between the two geophones. The exclusive use of geophones, which inherently have low impedance outputs, eliminates the need for preamplification in the probe. The geophones are mounted inside an acoustically transparent, thin rubber shell, which reduces the effects of noise due to flow.
01/2001;
