Acoustic Noise Field Research Articles

Recovering the Acoustic Green’s Function from Ambient Noise Cross Correlation in an Inhomogeneous Moving Medium

We study long-range correlation of diffuse acoustic noise fields in an arbitrary inhomogeneous, moving fluid. The flow reversal theorem is used to show that the cross-correlation function of ambient noise provides an estimate of a combination of the Green's functions corresponding to sound propagation in opposite directions between the two receivers. Measurements of the noise cross correlation allow one to quantify flow-induced acoustic nonreciprocity and evaluate both spatially averaged flow velocity and sound speed between the two points.

Development of a triaxial pressure-velocity probe for deep submergence applications

A triaxial pressure-velocity probe has been developed for the purpose of measuring the ambient acoustic noise field in the ocean at frequencies below 500 Hz. The probe consists of an omnidirectional hydrophone, a compliantly suspended sphere containing a triaxial geophone, and an electronics housing that contains a four-channel preamplifier and a digital compass. All of these components are packaged in a frame which in turn is attached to a deployment cable (i.e., a strength member) and submerged to a prescribed depth. The performance specifications for the transducers include a submergence depth rating of 6 km and a noise floor of 40 dB re: 1 mPa2 per hertz. These two parameters essentially drive the design of the probe and the paper will address these topics from the standpoint of the design evolution that took place in order to meet the specifications. Other aspects of the probe’s development include efforts to mitigate flow-induced noise and shielding the compass from the magnetic field emitted by the geophones. Test data obtained on a prototype unit will be presented. [Work supported by C. M. Traweek at the Office of Naval Research Code 321MS.]

Finite Difference Time Domain Analysis of Underwater Acoustic Lens System for Ambient Noise Imaging

Much attention has been paid to the new idea of detecting objects using ocean ambient noise. This concept is called ambient noise imaging (ANI). In this study, sound fields focused by an acoustic lens system constructed with a single biconcave lens were analyzed using the finite difference time domain (FDTD) method for realizing an ANI system. The size of the lens aperture that would have sufficient resolution–for example, the beam width is 1° at 60 kHz–was roughly determined by comparing the image points and -3 dB areas of sound pressure fields generated by lenses with various apertures. Then, in another FDTD analysis, we successfully used a lens with a determined aperture to detect rigid target objects in an acoustic noise field generated by a large number of point sources.

Submarine volcanic activity, ocean‐acoustic waves and internal ocean tides

Submarine volcanic event often generates acoustic waves (T‐waves) traveling over long distances through the low velocity channel (SOFAR) of the ocean. By a method of coherent stacking of T‐waves from a submarine volcanic activity in northern Mariana, we found a significant semidiurnal variation of T‐wave travel times. The amplitude of variation is an order of larger than those reported in the previous ocean sound transmission experiments. Ray‐theoretical consideration for the numerically simulated ocean tides indicates that such large T‐phase travel time variation is a consequence of large up‐and‐down movement of seawater around the axis of the SOFAR channel due to the M2 internal tide effectively converted from external tidal forcing. T‐phases, a ubiquitous feature of the ocean acoustic noise field, can be used to infer internal tidal motion and the associated ocean mixing.

周囲雑音を用いた無音物体探知における高分解能アレイ信号処理手法の比較

Much attention has been paid to the new idea of detecting objects by using ocean ambient noise affirmatively. This concept is called acoustic daylight imaging. The authors have tried to detect a silent target object using ambient noise by employing a wide-band beam former consisting of an array of receivers. In experimental results obtained in air, we have applied the delay-sum array (DSA) method and the multiple signal classification (MUSIC) method to successfully detect a silent target object in an acoustic noise field generated by a large number of transducers by using a wide-band beam former. In this paper, we applied the minimum variance (MV) method to detect silent targets as one of the promising high resolution technique of the array signal processing method. The relative power levels near the target obtained with MV and MUSIC were compared with those obtained by DSA. Then, the effectiveness of the MV and MUSIC method was evaluated according to the rate of increase and the direction in the maximum relative power levels.

Experimental modeling of the processes of active-passive thermoacoustic tomography

Experiments confirming the major results of the theoretical study of an active-passive mode of acoustic thermotomography are described. Experimental results are obtained with a setup intended for physical modeling of the processes of correlation reconstruction of the temperature, absorption, and phase velocity of sound in the object under investigation in the presence of an additionally introduced “illuminating” acoustic noise field. A possibility of reconstructing the local values of absorption and inhomogeneity of sound velocity from analyzing the correlation dependences based on difference and summary delays and also with the help of a controlled anisotropic acoustic irradiation is demonstrated.

Applying the Multiple Signal Classification Method to Silent Object Detection Using Ambient Noise

The revolutionary concept of using ocean ambient noise positively to detect objects, called acoustic daylight imaging, has attracted much attention. The authors attempted the detection of a silent target object using ambient noise and a wide-band beam former consisting of an array of receivers. In experimental results obtained in air, using the wide-band beam former, we successfully applied the delay-sum array (DSA) method to detect a silent target object in an acoustic noise field generated by a large number of transducers. This paper reports some experimental results obtained by applying the multiple signal classification (MUSIC) method to a wide-band beam former to detect silent targets. The ocean ambient noise was simulated by transducers decentralized to many points in air. Both MUSIC and DSA detected a spherical target object in the noise field. The relative power levels near the target obtained with MUSIC were compared with those obtained by DSA. Then the effectiveness of the MUSIC method was evaluated according to the rate of increase in the maximum and minimum relative power levels.

Effect of Received Beam Width on Passive Detection of Silent Target Using Ambient Noise

Much attention has been paid to the new idea of detecting objects by using ocean ambient noise affirmatively. This concept is called acoustic daylight imaging. The authors have attempted to detect a silent target object using ambient noise by employing a wide-band beamformer consisting with an array of receivers. In the experimental results obtained in air, we applied the delay-sum array method to successfully detect the silent target object in the acoustic noise field generated by many transducers with a line array of receivers. In this paper, some experimental results of the effect of the beam width formed by the wide-band beamformer on target detection are reported. The ocean ambient noise is simulated by transducers decentralized to many points in air. The spherical target object is detected by changing the number of receivers. The beam width is compared with the angle subtended by the spherical target at the center of the receiver array.

Gradient-induced acoustic and magnetic field fluctuations in a 4T whole-body MR imager.

Both the acoustic and magnetic fluctuation frequency response functions for a Siemens AS25 body gradient coil inside a 4 Tesla whole-body MR system were measured and analyzed in this study. In an attempt to correlate the acoustic noise inside the gradient coil with magnetic field oscillations, triangular and trapezoidal gradient impulses of varying amplitudes and widths were used to excite the gradient coil. The acoustic and magnetic responses to these inputs were measured. The results show the existence of discrete resonances in both acoustic and uniform magnetic field fluctuation spectra, while gradient magnetic field fluctuation spectra show no such resonances. In addition, the dominant amplitude peaks in spectra fluctuate similarly with respect to trapezoidal gradient impulse flat-top widths. This implies that these phenomena are correlated, and that the trapezoidal impulse flat-top width may be used as a way to suppress both acoustic noise and uniform magnetic field oscillations.

Vertical noise coherence measurements in shallow water using Lagrangian drifters

The vertical coherence of an acoustic noise field in shallow water (at frequencies dominated by sea surface noise) can be used to infer certain ocean bottom properties such as the compressional sound speed of the upper sediment layer. As coherence is a normalized measurement, it is independent of background noise level, or wind speed/sea state, and remains a stable measurement over a wide range of environment conditions. A set of three Lagrangian ambient noise drifters (LANDs) were developed for vertical coherence measurements. The drifters (drogued at 15-m depth) follow the near-surface currents, and take hourly measurements of coherence from their two hydrophones located at 50-m depth. The hydrophone separation is 1 m, and the measurements are made over a usable frequency range of 100 Hz to 1 kHz. The drifters were deployed at Western Bank (Scotian Shelf, Canada) and drifted for up to 2.5 days. The sea conditions varied from calm to 3–4-m waves. The stability of the coherence data through the various sea states, and the impact of shipping noise on the measurements will be discussed.

Auger Recombination in an Intense Acoustic Noise Field

A study is given of the effect of an intense acoustic noise flux affecting charge carriers via deformation-potential interaction on Auger processes. The calculation of Auger coefficients is carried out within a semiclassical approach to the acoustic noise field and a non-degenerate carrier statistics. Simple analytic expressions are then obtained, which exhibit an exponential dependence of the Auger coefficients on flux intensity. The Auger recombination is found, in analogy with the case of a piezoelectric noise field, to be strongly enhanced as compared to that in no-noise conditions by up to several orders of magnitude at high flux intensity, short acoustic wavelength, small carrier concentration and low temperature.

Geoacoustic inversion using fish sounds

During a recent near-surf-zone experiment, the underwater acoustic noise field was dominated by biological sounds over the 50-Hz to 10-kHz band. At night, members of the croaker (Sciaenidae) family migrated from the surf zone out to the 20-m water, where two bottom hydrophone arrays were located. Often during the night, individual vocalizing fish came within several meters of an array element so that the fish’s sequence of 7–13 knocks, each with peak-to-peak source levels of about 160 dB re: 1 μPa @ 1 m, was clearly discernible above the background din. The arrivals from some of these knocks display low-frequency (around 60 Hz) oscillatory tails due to excitation of interface waves at the ocean bottom. These Scholte waves have phase velocities around 110 m/s and a group velocity near 30 m/s. Using adjacent array elements to locate the fish at each knock, its movement over the sequence creates a synthetic aperture that is used in inversion for the shear wave velocity in the uppermost part of the ocean bottom. [Work supported by ONR, Code 32.]

Active acoustic noise cancellation with audio signal enhancement based on an almost-symmetrical time-varying autoregressive-moving average model

The basic idea of active noise cancellation (ANC) is to generate a controllable secondary source which compensates a noise in a certain spatial region. In headset applications, it is desirable to generate a secondary source which cancels the acoustic noise field, while preserving the useful audio field. This requires that the cancellation algorithm possess an ability to differentiate between the useful audio signal and the cancellation residual at the secondary-sensor output. This is a formidable task for the conventional linear time-invariant filtering approach, since the Fourier spectra of acoustic noise and the audio signal occupy the same frequency bandwidth. In this paper, a two-sensor ANC scheme is presented with application to a noise-reduction headset. The active noise cancellation carried out is based on the concept of the separation of superimposed signals with complete overlapping spectra by using an almost-symmetrical time-varying autoregressive-moving average model (ASTV-ARMA). The significant advantages of our proposed selective noise cancellation approach are the cancellation of the undesirable noise field and the preservation of the useful audio wave field. Simulation results indicate that a noise reduction of over 30 dB can be achieved, thus resulting in a high-quality audio in the presence of a strong environmental acoustical noise.

Surf noise and biological sounds in the near‐surf‐zone environment.

The Marine Physical Lab’s Adaptive Beach Monitoring Experiment was conducted from 24 April to 14 June 1995, in and near the surf zone off the S. California coast. Other participants included NRaD/NCCOSC, NRL, ARL/PSU, and ARL/UT. The focus here is on the underwater acoustic noise field properties. Contributions of breaking surf are studied in the 1‐ to 400‐Hz band using a 35‐element subsection of a bottom hydrophone array oriented perpendicular to the coast and located 3.4‐km offshore in 20‐m water. A 3‐week time series of endfire‐beamforming results are compared to the ocean surface wave height data collected over the same period. Source tow data quantify the propagation effects of surf noise to the array site and permit inversions for bottom geoacoustic properties. The array data are supplemented by the distance dependence from shore of data from bottom‐mounted sonobuoys in the 100‐Hz–20‐kHz band, and air acoustic data recorded just landward of the shoreline. Biologics are a substantial component of the noise field and must be accounted for in evaluating the contribution from surf. Fish sounds themselves can be used as novel sources in inversion for bottom geoacoustic properties. [Work supported by ONR, Code 32.]

Acoustic coupler for high-speed modem

A data coupler is disclosed for use in connecting a high speed data modem to a typical telephone handset in a high ambient acoustic noise field. The data coupler features a method for conserving, through a magnetic detection process, the high signal to noise ratio (HSNR) available at the telephone line during the acoustic coupling process. The use of magnetic detection prevents the deterioration of acoustic SNR normally expected in a high ambient acoustic noise field.

Modeling of optical microphone performance

A number of modulation principles and an even larger variety of specific implementations may be considered for all-optical microphones. The procedures for designing optical microphones described in this paper include three principal steps. First the mechanical response of a membrane to an acoustic field is modeled by an analytical technique [A. Hu et al., 3049–3056 (1992)], and is used to obtain the magnitude of the membrane deformation and its frequency response. Next the behavior of the optical path under this flexure is studied using the beam propagation method. Predictions of the optical sensitivity (depth of modulation versus membrane displacement), and optical insertion loss are thus obtained. Third the signal-to-noise performance of the device is evaluated by representing all noise sources as equivalent acoustic noise fields at the microphone input. Applications of this methodology to optical microphone designs under consideration are described.

Unambiguous noise and reverberation measurements from a dual line horizontal array

Single horizontal line arrays are often utilized to resolve the azimuthal character of the low‐frequency noise dominated by long‐range shipping sources. However, such arrays are subject to an azimuthal ambiguity on conjugate bearings. Coherent beamforming of an array composed of two parallel horizontal lines was recently employed to eliminate this left/right ambiguity using a minimum variance adaptive algorithm with single‐point main response constraints under an assumed horizontally isotropic noise field. The ability of this array and beamformer to resolve the directional characteristics of the acoustic noise field was demonstrated in the Atlantic and the Mediterranean. Median left/right rejection of 20 dB or more was observed over a wide frequency and azimuthal band. The measured beam noise exhibits temporal and spatial variability that could not have been quantified with a single‐line system. Median noise gain improvements of up to 7 dB above that of a single line were measured. Distant low‐frequency reverberation ambiguity resolution was also measured using the reverberation from signals underwater sound (SUS) explosive charges deployed in the Atlantic. Reverberation left/right rejection of 20 dB was achieved despite imperfect, nonparallel line tow conditions.

The generation of infrasonic ambient noise in the ocean by nonlinear interactions of ocean surface waves

Expressions for the spectra of infrasonic ocean noise and microseisms induced by nonlinear wave interaction are derived theoretically for an ocean environment modeled as a water layer overlying a solid half-space. A series of spectral transfer functions relating the source pressure field induced by the wave action to the underwater acoustic noise field and that of the microseisms this generates in the seabed are defined and calculated. The effect of bottom reflections on the transfer function is examined and an estimate of the contribution of the Rayleigh wave component of the microseism signal received onshore is also made.

Channel Modeling and Threshold Signal Processing in Underwater Acoustics: An Analytical Overview

An overview of underwater acoustic channel modeling and threshold signal processing is presented, which emphasizes the inhomogeneous, random, and non-Ganssian nature of the generalized channel, combined with appropriate weak-signal detection and estimation. Principal attention is given to the formal structuring of the scattered and ambient acoustic noise fields, as well as that of the desired signal, including both fading and Doppler "smear" phenomena. The role of general receiving arrays is noted, as well as their impact on spatial and temporal signal processing and beam forming, as indicated by various performance measures in detection and estimation. The emphasis here is on limiting optimum threshold systems, with some attention to suboptimum cases. Specific first-order probability density functions (pdf's) for the non-Ganssian components of typical underwater acoustic noise environments are included along with their field covariances. Several examples incorporating these pdf's are given, to illustrate the applications and general methods involved. The fundamental role of the detector structure in determining the associated optimum estimators is noted: the estimators arc specific linear or nonlinear functionals of the original optimum detector algorithm, depending on the criterion (i.e., minimization of the chosen error or cost function) selected. Results for both coherent and incoherent modes of reception are presented, reflecting the fact that frequently signal epoch is not known initially at the receiver. To supplement the general discussion, a selected list of references is included, to provide direct access to specific detailed problems, techniques, and results, for which the present paper is only a guide.

Wave–wave interactions, microseisms, and infrasonic ambient noise in the ocean

Underwater ambient noise is known to be wind dependent. Several mechanisms have been proposed to explain the nature of the transfer of energy from the wind to the acoustic noise field. Examples include wind and wave turbulence and nonlinear interactions between surface waves. This study examines these wind-related mechanisms at the low end of the acoustic spectrum. Data from a long-term investigation of ocean waves and the associated microseism response recorded ashore have provided evidence helpful in identifying the active processes. It is concluded that the noise field below 5 Hz is controlled by nonlinear wave–wave interactions and that existing theories account adequately for the effects observed.