Modelling and validation of defects on infrasound wind-noise-reduction pipe systems.

Infrasound signals are detectable from many different sources, such as earthquakes and man-made explosions. Wind-generated turbulent noise can mask incoming infrasound signals; however, pipe-array wind-noise-reduction systems (WNRSs) have been designed to reduce the level of noise in the observed pressure time series. Given that the arrival times of the signals need to be well-known to calculate the source back azimuth and trace velocity, the response of the WNRS must be known in magnitude and phase. Previous work has been performed to optimize these systems and effectively model them. The goal of this research is to determine the effects of different defects which may occur during normal operation in typical field-experiment conditions. The models were extended to include the effects of defective systems, such as blockages or leaks. It was found that these models could effectively recreate the responses observed in an experimental setting, and several different defects were tested and are summarized in this paper.

Farfield coherent infrasound generation using an air-propane burner.

An invaluable tool in the characterization of any receiver, propagation path, or detection system is a source with known and repeatable signal characteristics. This article presents the theoretical development and engineering design of a coherent (nonexplosive, periodic with controlled duration) infrasound source in the sub-hertz to several hertz band. Design of a sound source within this band is a difficult engineering challenge. The simple source equation, which will govern any portable human-fabricated infrasound source due to the long wavelengths, shows this fundamental difficulty. As frequency decreases, volume displacement must increase by the squared inverse factor of frequency in order to maintain an equal pressure amplitude at equal range. For this reason, the authors evaluate using the high energy density available in gas combustion to periodically displace large volumes of air within the open atmosphere. Prototype testing has verified the capability of generating continuous signals at a fundamental frequency of 0.25 Hz in the farfield-ranges in which pressure and particle velocity can be considered in-phase-where the product of the acoustic wavenumber and range is near 4.7. The generation of frequency content throughout the 0.25-4.0 Hz band with a reasonable signal-to-noise ratio was also demonstrated.

Measurements of two-dimensional spatial coherence of normal-incidence seafloor scattering.

Measurements have been made near normal incidence of the two-dimensional spatial coherence of the acoustic field scattered from the lakebed in Seneca Lake, New York. In the test region, the lakebed consists of a series of sediment layers created by a sequence of distinct depositional processes. The spatial coherence length of the scattered field is shown to be dependent on the structure of the underlying sediment sequences. Significant ping-to-ping variability in the spatial coherence surface was also observed for each sediment sequence. This variability is quantified by a two-dimensional spatial coherence metric that measures the coherence lengths and asymmetric coherence surface orientation. The ping-to-ping variation of the surface asymmetry appears to be linked to the spatial isotropy of the sediment scattering strength. The scattering strength of the deepest observed sequence in the sub-bottom is the most spatially isotropic and the ping-to-ping variability of the coherence lengths and surface orientations are random. The scattering strength of the shallower sequences is spatially anisotropic and the coherence lengths and surface orientations show intervals of non-random ping-to-ping behavior.

A metric for characterization of two-dimensional spatial coherence.

A metric is developed providing a quantitative measure of the two-dimensional spatial coherence of scattered fields. The metric is based on fitting a function similar to bivariate Gaussian to measured two-dimensional coherence surfaces. This function provides a robust fit to the measured data for a range of coherence lengths and surface asymmetries. Through an eigendecomposition of the bivariate Gaussian covariance matrix, it is possible to define surface orientation as well as coherence lengths along the major and minor axes. The metric is applied to normal-incidence scattering data collected in recent field trials at Seneca Lake, NY.

Analog model for thermoviscous propagation in a cylindrical tube.

Modeling acoustic propagation in tubes including the effects of thermoviscous losses at the tube walls is important in applications such as thermoacoustics, hearing aids, and wind musical instruments. Frequency dependent impedances for a tube transmission line model in terms of the so-called thermal and viscous functions are well established, and form the basis for frequency domain analysis of systems that include tubes. However, frequency domain models cannot be used for systems in which significant nonlinearities are important, as is the case with the pressure-flow relationship through the reed in a woodwind instrument. This paper describes a cylindrical tube model based on a continued fraction expansion of the thermal and viscous functions. The model can be represented as an analog circuit model which allows its use in time domain system modeling. This model avoids problems with fractional derivatives in the time domain.

Improving low-frequency response of sonic boom measurements through digital filtering.

High-fidelity measurement of sonic boom waveforms requires microphones and data acquisition hardware with flat frequency responses extending below 1 Hz. Hardware limitations can pose challenges meeting these requirements. This letter describes an engineering method involving digital pole-shift filtering that can be used in post-processing to extend effective hardware bandwidth. This approach is evaluated for sonic boom recordings from NASA's Quiet Supersonic Flights 2018 measurement campaign. Recordings of several booms at multiple measurement sites using different hardware/microphone combinations were used to design filters. Results demonstrate that the measurement-designed filters significantly reduce the mean square error between original and benchmark waveforms. Digital filter designs based on hardware manufacturer specifications also reduce error, but not as much. Residual errors after filtering, method limitations, and transferability to a launch vehicle reentry boom measurement are discussed.

In situ calibration of atmospheric-infrasound sensors including the effects of wind-noise-reduction pipe systems.

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 role of nonlinear effects in the propagation of noise from high-power jet aircraft.

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.

Bi-static sonar applications of intensity processing.

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.

St. Lawrence blue whale vocalizations revisited: characterization of calls detected from 1998 to 2001.

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.

Development of an accelerometer-based underwater acoustic intensity sensor.

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.