Broadband Acoustic Signals Research Articles

Tidal height and frequency dependence of acoustic velocity and attenuation in shallow gassy marine sediments

Remote prediction of gassy marine sediment properties is important for geohazard assessment. Gas bubble resonance theory suggests that gassy sediments exhibit acoustic wave velocity‐frequency and attenuation‐frequency relationships that depend on gas bubble size, gas content, and sediment elastic properties. An acoustic monitoring experiment to investigate gas bubble resonance effects was undertaken at an intertidal site at Dibden Bay, Southampton, United Kingdom. A vertical hydrophone array was positioned to straddle the top of the gassy zone identified on acoustic reflection profiles at about 1 m below the seabed. A miniboomer in the seabed above the array was used to generate broadband (600 Hz to 3000 Hz) acoustic signals every 10 min during a 24 hour period with water depths varying between 0 m (subaerial exposure) at low tide and 2.35 m at high tide. The calculated frequency spectra of compressional wave attenuation coefficient show an attenuation maximum (over 200 dB/m) that shifts in frequency from 1050 Hz at low tide to 1250 Hz at high tide, thus for the first time providing direct evidence of in situ gas bubble resonance in marine sediments. Modeling suggests that effective gas bubble radii of 11 mm to 13 mm are responsible for the attenuation maximum, supported by X‐ray computed tomography scan observations on a pressure core (which also indicate that bubble shape depends on sediment type). Modeling of bubble size fluctuations due to pressure equilibration cannot reproduce the observed frequency shift of the attenuation maximum, implying that gas diffusion and nonspherical bubbles are significant.

Reviews Of Acoustical Patents

The purpose of these acoustical patent reviews is to provide enough information for a Journal reader to decide whether to seek more information from the patent itself. Any opinions expressed here are those of the reviewers as individuals and are not legal opinions. Printed copies of United States Patents may be ordered at $3.00 each from the Commissioner of Patents and Trademarks, Washington, DC 20231. Patents are available via the Internet at http://www.uspto.gov.

Perceptually improved enhancement of encoded acoustic signals

The invention relates to encoding of broadband and narrowband acoustic source signals (x) such that the perceived sound quality of corresponding reconstructed signals is improved in comparison to the known solutions. An enhancement estimation unit (102), operating in serial or in parallel with the regular encoding / decoding means (101), perceptually enhances a reconstructed acoustic source signal by utilisation of an enhancement spectrum (C) comprising a larger number of spectral coefficients than the number of sample values in corresponding frames of the signals carrying the basic encoded representation of the acoustic source signal. The thus extended block length of the enhancement spectrum frame provides a basis for accomplishing the desired improvement of the perceived sound quality.

Inversion for sediment geoacoustic properties at the New England Bight

This article discusses inversions for bottom geoacoustic properties using broadband acoustic signals obtained from explosive sources. Two different inversion schemes for estimating the compressional wave speeds and attenuation are presented in this paper. In addition to these sediment parameters, source-receiver range is also estimated using the arrival time data. The experimental data used for the inversions are SUS charge explosions acquired on a vertical hydrophone array during the Shelf Break Primer Experiment conducted south of New England in the Middle Atlantic Bight in August 1996. The modal arrival times are extracted using a wavelet analysis. In the first inversion scheme, arrival times corresponding to various modes and frequencies from 10 to 200 Hz are used for the inversion of compressional wave speeds. A hybrid inversion scheme based on a genetic algorithm (GA) is used for the inversion. In an earlier study, Potty et al. [J. Acoust. Soc. Am. 108(3), 973-986 (2000)] have used this hybrid scheme in a range-independent environment. In the present study results of range-dependent inversions are presented. The sound speeds in the water column and bathymetry are assumed range dependent, whereas the sediment compressional wave speeds are assumed range independent. The variations in the sound speeds in the water column are represented using empirical orthogonal functions (EOFs). The replica fields corresponding to the unknown parameters were constructed using adiabatic theory. In the second inversion scheme, modal attenuation coefficients are calculated using modal amplitude ratios. The ratios of the modal amplitudes are also calculated using time-frequency diagrams. A GA-based inversion scheme is used for this search. Finally, as a cross check, the computed compressional wave speeds along with the modal arrival times were used to estimate the source-receiver range. The inverted sediment properties and ranges are seen to compare well with in situ measurements and historical data.

Effects of the propagation environment on coherence of underwater broadband acoustic signals

Acoustic and environmental measurements were collected in 550 m of water approximately 2.5 km east of San Clemente Island, CA over a 4-day period in August 2002. Continuous-wave and frequency-modulated pulses with center frequencies of 20 and 40 kHz were transmitted over a 1 km distance. Four projectors were located between 10 and 70 m depth, and five receivers were positioned between 40 and 220 m. Data from an anemometer, wave rider buoy, acoustic Doppler current profiler and towed conductivity-temperature-depth array were recorded simultaneously over the test period to characterize the marine environment. The extensive set of environmental data provided a basis for boundary condition and water column characterization. The most significant environmental dynamic was found to be tidally related water current. The environmental conditions were input into a propagation model to predict the acoustic signal structure and propagation behavior in the measured environment. Predictions of acoustic propagation parameters such as time fluctuation, amplitude fluctuation, and effect of bandwidth will be compared to results from on-site acoustic measurements. [Work supported by ONR Code 321US under the ARL Program.]

Time-frequency variability of broadband sound propagation through shallow water internal soliton

Temporal fluctuations of the sound intensity in shallow water are studied using broadband acoustic signal propagation through an internal soliton (IS) wave front. Time-frequency analysis of the received signals at two different vertical arrays is conducted based on the wavelet transform. The intensity fluctuations as a function of geophysical time are shown for different positions of the IS with respect to the acoustic propagation path. A theoretical model consisting of internal waves coupled with the acoustic normal modes and horizontal rays is presented. The correlation between the signal fluctuations and the internal soliton is analyzed using frequency dependent index of refraction. Model results are compared with the SWARM-95 experimental observation. [Work supported by ONR and RFBR Grant No. 03-05-64568.]

Geoacoustic inversion of broad-band acoustic data in shallow water on a single hydrophone

The inversion of broad-band low-frequency acoustic signals received on sparse arrays can lead to robust and efficient estimations of sea-bed properties. This paper describes a shallow-water geoacoustic inversion scheme based on the use of a model-based matched-impulse response on a single hydrophone. Results from the INTIMATE'96 experiment on the Portuguese shelf break are reviewed. In order to minimize the effects of strong time variability due to internal tides, only the time-stable waterborne bottom-surface reflected arrivals are exploited. A quasi-linear inversion algorithm is first applied to refine the geometry of the experiment. Then, inversion of bottom parameters is performed with an objective function that only makes use of the bottom-surface reflected arrivals' amplitudes. The experimental results show that broad-band transmissions (300-800 Hz) received on a single hydrophone, combined with the use of a simple eigenray code, are sufficient to correctly resolve geometrical parameters and bottom features. The analysis of the reflection coefficients both on simulated and real data helps to understand the validity of the inverted parameters and to derive the basis of an equivalent medium concept for geoacoustic inversion based on a "through-the-sensor" approach.

Acoustic detection of pneumothorax

This study aims at investigating the feasibility of using low‐frequency (<2000 Hz) acoustic methods for medical diagnosis. Several candidate methods of pneumothorax detection were tested in dogs. In the first approach, broadband acoustic signals were introduced into the trachea during end‐expiration and transmitted waves were measured at the chest surface. Pneumothorax was found to consistently decrease pulmonary acoustic transmission in the 200–1200‐Hz frequency band, while less change was observed at lower frequencies (p<0.0001). The ratio of acoustic energy between low (<220 Hz) and mid (550–770 Hz) frequency bands was significantly different in the control (healthy) and pneumothorax states (p<0.0001). The second approach measured breath sounds in the absence of an external acoustic input. Pneumothorax was found to be associated with a preferential reduction of sound amplitude in the 200‐ to 700‐Hz range, and a decrease of sound amplitude variation (in the 300 to 600‐Hz band) during the respiration cycle (p<0.01 for each). Finally, chest percussion was implemented. Pneumothorax changed the frequency and decay rate of percussive sounds. These results imply that certain medical conditions may be reliably detected using appropriate acoustic measurements and analysis. [Work supported by NIH/NHLBI ♯R44HL61108.]

Effects of the propagation environment on coherence of underwater broadband acoustic signals

Acoustic and environmental measurements were performed in 550 m of water approximately 2500 m east of San Clemente Island, California over a four-day period in August 2002. Continuous wave and frequency modulated pulses with center frequencies of 20 kHz to 40 kHz were transmitted over a 700 m distance underwater. Four projectors were located between 10 m and 70 m deep, and five receivers were positioned between 45 m and 220 m deep. Data from an anemometer, wave rider buoy, acoustic Doppler current profiler and towed conductivity–temperature–depth array were recorded simultaneously over the test period to characterize the marine environment. The extensive set of environmental data provided a basis for boundary condition and water column characterization. These conditions were input into a propagation model to predict the acoustic signal structure and propagation behavior in the measured environment. Predictions of acoustic propagation parameters will be compared to results from on-site acoustic measurements. [Work supported by ONR under Award No. N00014-02-1-0156.]

Material properties of North Atlantic cod eggs and early-stage larvae and their influence on acoustic scattering

Abstract To study the acoustic signatures of Atlantic cod (Gadus morhua) at different biological stages from eggs to early-stage larvae (&lt;37 days post-hatch), we conducted a series of experiments to estimate their sound-speed and density contrasts. A laboratory version of the “Acoustic Properties of Zooplankton” system was used. Sound speed was estimated by means of travel time between two transducers using a broadband acoustic signal (∼300–600 kHz). Density was estimated using a dual-density method in which two fluids of different densities were employed. It was found that the density contrasts of cod eggs and early-stage larvae were nearly all slightly less than unity (0.969–0.998), while the effective sound-speed contrasts were only slightly greater than unity (1.017–1.024) for eggs and yolk-sac stage larvae (&lt;5 days post-hatch), and increased significantly (&gt;1.130) for larvae older than 16 days. This change in sound-speed contrast reflected the transition of the swimbladder from an uninflated state to an inflated state. The regression relation between estimated target strength at 500 kHz and larval length in centimetres was found to be TS = 176.1 log10L − 82.4(dB). The inflation ratio of the swimbladder for early-stage larvae was an exponential function of time. The predicted period of time until full swimbladder inflation was 43.3 days.

Inferring fish orientation from broadband-acoustic echoes

Abstract A new method has been developed for inferring the orientation of fish through the use of broadband-acoustic signals. The method takes advantage of the high range resolution of these signals, once temporally compressed through cross-correlation. The temporal resolution of these compressed signals is inversely proportional to the bandwidth, thus the greater the bandwidth the higher the resolution. This process has been applied to broadband-chirp signals spanning the frequency range 40–95 kHz to obtain a range resolution of approximately 2 cm from the original, unprocessed resolution of about 50 cm. With such high resolution, individual scattering features along the fish have been resolved, especially for angles well off normal incidence. The overall duration of the compressed echo from live, individual alewife, as measured in a laboratory tank, is shown to increase monotonically with orientation angle relative to normal incidence. The increase is due to the greater range separation relative to the transducer between the echoes from the head and tail of the fish. The results of this study show that with a priori knowledge of the length of the fish, the orientation could be estimated from the duration of a single, compressed broadband echo. This method applies to individual, acoustically resolved fish. It has advantages over previous approaches because it derives the orientation from a single ping and it does not use a formal, mathematical scattering model. Design parameters for applications in the ocean are given for a range of conditions and fish size.

Acoustic signal spatial and temporal correlation variability in the ASIAEx 2001 South China Sea experiment

Broadband acoustic signal intensity fluctuations, temporal correlations, and horizontal spatial correlations have been extracted from a horizontal array data set. The relationship between the acoustic signal variability and the fluid processes that perturb the sound-speed field will be discussed. The 2.048-s linear frequency-modulated signals were received on a 450-m horizontal array. The signals were centered at 300 and 500 Hz and had respective bandwidths of 60 and 90 Hz. The signal intensity was found to fluctuate by 10 to 20 dB as a function of time and space. Horizontal correlation length varied over time from 5 to 100 acoustic wavelengths and the signal correlation time ranged from 1 to 50 min. The data were acquired in May 2001 during the Asian Seas International Acoustics Experiment (AsiaEx). The 18-km acoustic propagation path lay along a 125-m isobath at the South China Sea shelf break. The experiment site was oceanographically dynamic, with energetic internal gravity wave fields and currents that strongly impacted the acoustic propagation. [Work supported by ONR.]

Pneumothorax detection using pulmonary acoustic transmission measurements.

Pneumothorax is a common clinical condition that can be life threatening. The current standard of diagnosis includes radiographic procedures that can be costly and may not always be readily available or reliable. The objective of this study was to investigate the hypothesis that pneumothorax causes detectable pathognomonic changes in pulmonary acoustic transmission. An animal model was developed whereby 15 mongrel dogs were anaesthetised, intubated and mechanically ventilated. A thoracoscopic trocar was placed into the pleural space for the introduction of air and confirmation of a approximately 30% pneumothorax by direct visualisation. Broadband acoustic signals were introduced into the endotracheal tube, while transmitted waves were measured at the chest surface. Pneumothorax was found consistently to lower the pulmonary acoustic transmission in the 200-1200 Hz frequency band, whereas smaller transmission changes occurred at lower frequencies (p< 0.0001, sign test). The ratio of acoustic energy between low-(< 220 Hz) and high-(550-770 Hz) frequency bands was significantly different in the control and pneumothorax states (p < 0.0001, sign test). This implies that pneumothoraces can be reliably detected using pulmonary acoustic transmission measurements in the current animal model. Further studies are needed to investigate the feasibility of using this technique in humans.

Underwater high energy acoustic communications device

An underwater high energy acoustic communications device has inner shell-like structure having an outer surface connected to first ends of tuning springs and outer shell-like structure having an inner surface connected to second ends of the tuning springs to concentrically locate and space the inner and outer structures apart. Explosive units mounted on the inner surface of the outer structure each includes an explosive squib, rigid tube, and projectile. A micro-controller feeds firing signals over electrical leads to the explosive units to detonate them and fire their projectiles to strike the inner structure that emits ringing high-energy broadband acoustic signals that are propagated into surrounding seawater. A transducer connected to the micro controller receives acoustic signals from a remote source to activate the micro-controller to generates preprogrammed sequences of firing signals to propagate coded messages by the high-energy broadband, acoustic signals.

Experimental study of geo-acoustic inversion uncertainty due to ocean sound-speed fluctuations

Acoustic data measured in the ocean fluctuate due to the complex time-varying properties of the channel. When measured data are used for model-based, geo-acoustic inversion, how do acoustic fluctuations impact estimates for the seabed properties? In May 1999 SACLANT Undersea Research Center and TNO-Physics and Electronics Laboratory (FEL), conducted a shallow-water experiment on the Adventure Bank off the southwest coast of Sicily, Italy to assess the effects of a time-varying ocean on acoustic propagation and geo-acoustic inversion. A favorable area for acoustic propagation was identified which had slight internal wave activity and a weakly range-dependent bathymetry with sand-like bottom properties. Oceanographic and acoustic measurements were performed continuously over a 3-day period. Broadband (0.2–3.8 kHz) acoustic signals from a bottom-moored source were transmitted over fixed paths and received on a moored vertical hydrophone array. During the transmissions extensive environmental measurements (e.g., sound speed, current, sea-surface waveheight, etc.) were made to correlate the time-varying environmental and acoustic data. Modeled acoustic data show time variability which agrees with the measurements. Results illustrate severe problems when modeling shallow-water acoustic propagation at ranges beyond a few kilometers in the frequency band considered. Further, the acoustic fluctuations in time caused erroneous time variability in inverted seabed properties.

Horizontal refraction of acoustic signals retrieved from NPAL billboard array data

A comprehensive, long-range sound propagation experiment was carried out with the use of the billboard acoustic array of the North Pacific Acoustic Laboratory (NPAL) in 1999. The antenna consisting of five vertical line arrays was deployed near a California coast and received broadband acoustic signals transmitted from Hawaii over a distance of about 4000 km. Acoustic signals propagating over such a long distance might exhibit noticeable horizontal refraction. The paper will present results of processing the NPAL data to infer horizontal refraction angle (HRA) as a function of time. Different methods were used for determining HRA. The first approach employed cross correlation of the acoustic signals at different VLAs. Time delay corresponding to maximum of cross correlation is related to HRA assuming the angle is approximately the same for all rays (or modes). The second method used modal representation of the arriving broadband signals. The dependency of the amplitudes of acoustic modes on mode number, frequency, and arrival angle was determined independently within narrow frequency bins, and then the results were averaged over whole frequency range. This method allowed, in particular, to evaluate angular width of the arrived signal, which appeared to be of the order of a few milliradians.

Broadband transducer requirements and development for a synthetic aperture sonar

Broadband acoustic signal transmission and reception are key to enhancing the performance of underwater acoustical sensor systems to support functions like underwater acoustic communications and high resolution underwater imaging. In particular, synthetic aperture sonar (SAS) is an attractive technology for high-resolution imaging because of its ability to produce better cross range resolution than that of a real aperture array sonar of the same length. In addition, SAS systems can produce a cross range resolution that is independent of frequency and range. When the advantages of a SAS system are coupled with those gained from going to a broadband signal space, the performance enhancements can be significant. Specifically improvements in clutter rejection, object classification, and area coverage rate will be achieved. A description of the transducer requirements and expected payoffs for a SAS system designed to operate and image proud and buried objects in shallow waters will be presented. [Work supported by ONR.]

Time variability of acoustic signals in a benign shallow-water area

SACLANTCEN conducted the ADVENT’99 experiment in the Strait of Sicily, Mediterranean, to assess the time variability of broadband acoustic signals in a benign shallow-water region. Broadband acoustic signals (200–3800 Hz) were transmitted for up to 18 h over fixed paths of 2, 5, and 10 km. Dense sampling of the environment was performed including a sound-speed section every hour along the acoustic tracks. Band-averaged transmission loss is stable with time, which agrees well with modeling results. The Bartlett processor was applied to correlate the acoustic data. The correlation time varies from several hours at low frequency and short range to a few minutes at high frequency and longer ranges. Range- and time-dependent propagation modeling of the acoustic signals (200–800 Hz) shows behavior similar to the data. Particularly, the correlation time decreases abruptly from several hours to less than 1 hour at a frequency around 700 Hz for the 10-km data. This effect of the time-varying waveguide on the acoustic signals is correctly predicted by the propagation model. Severe problems using the acoustic signals coherently persist above a few hundred Hz and at ranges beyond a few kilometers, although the experiment was conducted under favorable conditions. The experimental data and the simulations illustrate the complexity of sound propagation in shallow water.

Sediment tomography in the East China Sea

Sediment tomography experiments using broadband shots are planned as part of the Asian Seas International Acoustic Experiment (ASEAEX) in the East China Sea (ECS) in the year 2001. In order to gather adequate sediment information, gravity cores were taken from the experimental location during the spring of 2000. Using the gravity core data and available historic data, a sediment geoacoustic model for the region is presented. Synthetic experiments are performed with this geoacoustic model using broadband data. Various configurations of the explosive sources are tested to arrive at a suitable experimental geometry for the tomography experiment. This inversion is based on group speed dispersion of the broadband acoustic signal and is capable of estimating range-dependent sediment properties. [Work sponsored by ONR.]

Tomographic inversion for sediment parameters in shallow water

This article discusses inversions for bottom geoacoustic properties using broadband acoustic signals obtained from explosive sources. The experimental data used for the inversions are SUS charge explosions acquired on a vertical hydrophone array during the Shelf Break Primer Experiment conducted south of New England in the Middle Atlantic Bight in August 1996. The SUS signals were analyzed for their time-frequency behavior using wavelets. The group speed dispersion curves were obtained from the wavelet scalogram of the SUS signals. A genetic algorithm (GA) was used for the inversion of sound speeds in the water column and compressional wave speeds in the sediment layers. The variations in the sound speeds in the water column were represented using empirical orthogonal functions (EOFs). A range-independent normal mode routine was used to construct the replica fields corresponding to the parameters. Comparison of group speeds for modes 1 to 9 and for a range of frequencies 8 to 200 Hz was used to arrive at the best parameter fit. An efficient hybrid optimization scheme using the GA and a Levenberg-Marquardt algorithm is presented. Linear perturbation methods were also used to "fine tune" the inversions and to obtain resolution and variance estimates. Analysis was also done to compute the degree of convergence of each of the parameters by explicitly calculating the Hessian matrices numerically. A posteriori estimation of mean and covariance was also done to obtain error estimates. Group speeds for the inverted sound speed fields provide an excellent match to the experimental data. The inverted sediment compressional speed profile compares well with in situ measurements.