Modeling acoustic propagation of airgun array pulses recorded on tagged sperm whales (Physeter macrocephalus)

In 2002 and 2003, tagged sperm whales (Physeter macrocephalus) were experimentally exposed to airgun pulses in the Gulf of Mexico, with the tags providing acoustic recordings at measured ranges and depths. Ray trace and parabolic equation (PE) models provided information about sound propagation paths and accurately predicted time of arrival differences between multipath arrivals. With adequate environmental information, a broadband acoustic PE model predicted the relative levels of multipath arrivals recorded on the tagged whales. However, lack of array source signature data limited modeling of absolute received levels. Airguns produce energy primarily below 250 Hz, with spectrum levels about 20-40 dB lower at 1 kHz. Some arrivals recorded near the surface in 2002 had energy predominantly above 500 Hz; a surface duct in the 2002 sound speed profile helps explain this effect, and the beampattern of the source array also indicates an increased proportion of high-frequency sound at near-horizontal launch angles. These findings indicate that airguns sometimes expose animals to measurable sound energy above 250 Hz, and demonstrate the influences of source and environmental parameters on characteristics of received airgun pulses. The study also illustrates that on-axis source levels and simple geometric spreading inadequately describe airgun pulse propagation and the extent of exposure zones.     

Male sperm whale acoustic behavior observed from multipaths at a single hydrophone

Sperm whales generate transient sounds (clicks) when foraging. These clicks have been described as echolocation sounds, a result of having measured the source level and the directionality of these signals and having extrapolated results from biosonar tests made on some small odontocetes. The authors propose a passive acoustic technique requiring only one hydrophone to investigate the acoustic behavior of free-ranging sperm whales. They estimate whale pitch angles from the multipath distribution of click energy. They emphasize the close bond between the sperm whale's physical and acoustic activity, leading to the hypothesis that sperm whales might, like some small odontocetes, control click level and rhythm. An echolocation model estimating the range of the sperm whale's targets from the interclick interval is computed and tested during different stages of the whale's dive. Such a hypothesis on the echolocation process would indicate that sperm whales echolocate their prey layer when initiating their dives and follow a methodic technique when foraging.     

A novel method for the measurement of acoustic speed

Traditional methods for measuring acoustic speed require knowledge of either the specimen thickness or the distances between the transducers and the specimen. In general, the accuracy in measuring these quantities determines the accuracy of the experimental technique for measuring speed. This problem is particularly acute in measuring sound speed in biological specimens. A new method for measuring acoustic speed of materials, which eliminates the need for determining these quantities, has been developed. The technique, which necessitates the use of only one transducer, requires measurement of four times of flight of a sound pulse and the knowledge of the speed of sound in a reference fluid medium in which the specimen is placed. Ultrasonic speed in stainless steel and Plexiglas was measured using this method to verify its validity. Results on measurements on porcine liver, myocardium, and soft fat are also reported.     

Acoustic and diving behavior of sperm whales (Physeter macrocephalus) during natural and depredation foraging in the Gulf of Alaska

Sperm whales have depredated black cod (Anoplopoma fimbria) from demersal longlines in the Gulf of Alaska for decades, but the behavior has recently spread in intensity and geographic coverage. Over a three-year period 11 bioacoustic tags were attached to adult sperm whales off Southeast Alaska during both natural and depredation foraging conditions. Measurements of the animals' dive profiles and their acoustic behavior under both behavioral modes were examined for statistically significant differences. Two rough categories of depredation are identified: "deep" and "shallow." "Deep depredating" whales consistently surface within 500 m of a hauling fishing vessel, have maximum dive depths greater than 200 m, and display significantly different acoustic behavior than naturally foraging whales, with shorter inter-click intervals, occasional bouts of high "creak" rates, and fewer dives without creaks. "Shallow depredating" whales conduct dives that are much shorter, shallower, and more acoustically active than both the natural and deep depredating behaviors, with median creak rates three times that of natural levels. These results suggest that depredation efforts might be measured remotely with passive acoustic monitoring at close ranges.     

Off-axis effects on the multipulse structure of sperm whale usual clicks with implications for sound production

Sperm whales (Physeter macrocephalus) produce multipulsed clicks with their hypertrophied nasal complex. The currently accepted view of the sound generation process is based on the click structure measured directly in front of, or behind, the whale where regular interpulse intervals (IPIs) are found between successive pulses in the click. Most sperm whales, however, are recorded with the whale in an unknown orientation with respect to the hydrophone where the multipulse structure and the IPI do not conform to a regular pulse pattern. By combining far-field recordings of usual clicks with acoustic and orientation information measured by a tag on the clicking whale, we analyzed clicks from known aspects to the whale. We show that a geometric model based on the bent horn theory for sound production can explain the varying off-axis multipulse structure. Some of the sound energy that is reflected off the frontal sac radiates directly into the water creating an intermediate pulse p1/2 seen in off-axis recordings. The powerful p1 sonar pulse exits the front of the junk as predicted by the bent-horn model, showing that the junk of the sperm whale nasal complex is both anatomically and functionally homologous to the melon of smaller toothed whales.     

Calculations of internal-wave-induced fluctuations in ocean-acoustic propagation

Variability in the ocean sound-speed field on time scales of a few hours and horizontal spatial scales of a few kilometers is often dominated by the random, anisotropic fluctuations caused by the internal-wave field. Results have been compiled from analytical approaches and from numerical simulations using the parabolic approximation into an efficient set of algorithms for calculating approximations to internal-wave effects on temporal and spatial coherences, coherent bandwidths, and regimes of acoustic fluctuation behavior. These approximate formulas account for the background, deterministic, sound-speed profile and the anisotropy of the internal-wave field, and they also allow for the incorporation of experimentally determined profiles of sound speed, buoyancy frequency, and sound-speed variance. The algorithms start from the geometrical-acoustics approximation, in which the field transmitted from a source can be described completely in terms of rays whose characteristics are determined by the sound speed as a function of position. Ordinary integrals along these rays provide approximations to acoustic-fluctuation quantities due to the statistical effects of internal waves, including diffraction. The results from the algorithms are compared with numerical simulations and with experimental results for long-range propagation in the deep ocean.     

Localization of marine mammals near Hawaii using an acoustic propagation model

Humpback whale songs were recorded on six widely spaced receivers of the Pacific Missile Range Facility (PMRF) hydrophone network near Hawaii during March of 2001. These recordings were used to test a new approach to localizing the whales that exploits the time-difference of arrival (time lag) of their calls as measured between receiver pairs in the PMRF network. The usual technique for estimating source position uses the intersection of hyperbolic curves of constant time lag, but a drawback of this approach is its assumption of a constant wave speed and straight-line propagation to associate acoustic travel time with range. In contrast to hyperbolic fixing, the algorithm described here uses an acoustic propagation model to account for waveguide and multipath effects when estimating travel time from hypothesized source positions. A comparison between predicted and measured time lags forms an ambiguity surface, or visual representation of the most probable whale position in a horizontal plane around the array. This is an important benefit because it allows for automated peak extraction to provide a location estimate. Examples of whale localizations using real and simulated data in algorithms of increasing complexity are provided.     

Characteristics of gunshot sound displays by North Atlantic right whales in the Bay of Fundy

North Atlantic right whales (Eubalaena glacialis) produce a loud, broadband signal referred to as the gunshot sound. These distinctive sounds may be suitable for passive acoustic monitoring and detection of right whales; however, little is known about the prevalence of these sounds in important right whale habitats, such as the Bay of Fundy. This study investigates the timing and distribution of gunshot sound production on the summer feeding grounds using an array of five marine acoustic recording units deployed in the Bay of Fundy, Canada in mid-summer 2004 and 2005. Gunshot sounds were common, detected on 37 of 38 recording days. Stereotyped gunshot bouts averaged 1.5 h, with some bouts exceeding 7 h in duration with up to seven individuals producing gunshots at any one time. Bouts were more commonly detected in the late afternoon and evening than during the morning hours. Locations of gunshots in bouts indicated that whales producing the sounds were either stationary or showed directional travel, suggesting gunshots have different communication functions depending on behavioral context. These results indicate that gunshots are a common right whale sound produced during the summer months and are an important component in the acoustic communication system of this endangered species.     

海洋-声学耦合模式捕捉水声环境不确定性

考虑到海洋环境的时空变化对水声环境不确定性的影响,建立了海洋-声学耦合数值模式,实现了并行计算。该模式将声学计算纳入到运动的海洋中,从而实现了对水声环境的动态预报和估计。同时,采用集合预报方法对典型断面的温度垂直结构、实验海区声速剖面和传播损失进行预报,并给出了声速剖面的预报误差、不同深度与频率下,传播损失90%的概率区间以及声速、传播损失、声呐作用距离的不确定性直方图。结果反映了海洋时空变化对水声环境不确定性的影响,量化了水声环境中不确定性的大小。实验结果表明该方法可以刻画海洋动态变化引起的水声环境不确定性,并对其进行了量化和描述。      

Simulation of the acoustic field emitted from medical linear transducer in a heterogeneous tissue

A numerical model is developed to simulate the acoustic field in heterogeneous tissue from a medical linear transducer.The coupled full-wave equation for nonlinear ultrasound is solved using a staggered-grid finite difference time domain method.The distribution of acoustic pressure and power in human abdominal wall with heterogeneities in sound speed,density,and nonlinear parameter are obtained.Compared with homogeneous medium,when sound speed in tissue is uniform and density unchanged,the acoustic energy decreases only1.8 dB in the focal region;when density in tissue is uniform and sound speed unchanged,the energy decreases 3.8 dB in the focal region,which is almost the same as heterogeneous tissue.Thus,the primary factor of the aberration of focused beam is the heterogeneous distribution of the tissue sound speed.     

非均匀组织医学超声发射声场仿真

为了分析医学超声在非均匀组织中的分布特性,建立了超声发射声场的计算模型。采用交错网格有限差分法对耦合超声非线性方程进行数值求解,获得了声速、密度及非线性参数非均匀分布情况下人体腹壁组织内的超声声场分布数据。同均匀介质相比:当声速均匀而密度非均匀时,声束仍聚焦良好,焦点处声能下降了1.8 dB;当密度均匀而声速非均匀时,声束发散严重,焦点处声能下降了3.8 dB,下降程度与非均匀组织接近。组织声速在空间分布的非均匀性是导致聚焦声束能量分布畸变的主要原因。      

Measuring the off-axis angle and the rotational movements of phonating sperm whales using a single hydrophone

The common use of the bent-horn model of the sperm whale sound generator describes sperm whale clicks as the pulse series {p0, p1, p2, p3,...}. Clicks, however, deviate from this standard when recorded using off-axis hydrophones. The existence of additional pulses within the {p0, p1, p2, p3, ...} series can be explained still using the bent-horn model. Multiple reflections on the whale's frontal and distal sacs of the p0 pulse lead to additional sets of pulses detectable using a farfield, off-axis hydrophone. The travel times of some of these additional pulses depend on the whale's orientation. The authors propose a method to estimate the off-axis angle of sperm whale clicks. They also propose a method to determine the nature of the movement (if it is pitch, yaw, or roll) of phonating sperm whales. The application of both methods requires the measurement of the travel time differences between pulses composing a sperm whale click. They lead, using a simple apparatus consisting of a single hydrophone at an unknown depth, to new measurements of the underwater movements of sperm whales. Using these methods shows that sperm whales would methodically scan seawater while searching for prey, by making periodic pitch and yaw movements in sync with their acoustic activity.     

Capturing uncertainties of the underwater acoustic environment based on an ocean-acoustic coupled model

Considering the uncertain effects of temporal and spatial changes in the marine environment on the underwater acoustic environment,we established an ocean-acoustic coupled numerical model and performed a parallel calculation.This model incorporated acoustic calculations into the dynamic ocean,thereby achieving a dynamic forecasting and assessment of the acoustic environment.Furthermore,we adopted the ensemble prediction method to predict the vertical structure of temperature in a classic cross-section,the sound speed of the cross-section of the investigated sea area,and transmission losses.We gave the prediction errors of the sound speed profile as well as the 90%probability interval of transmission losses and the uncertainty histograms of the sound speeds,transmission losses,and sonar ranges at different depths and frequencies.The results reflected the influence of marine temporal and spacial variations on the uncertainties of the underwater acoustic environment,and the results also quantified the uncertainties of the underwater acoustic environment parameters.The experimental results indicate that the method used in this study is able to delineate and quantify the uncertainties of the underwater acoustic environment caused by marine dynamic changes.     

On the utilization of acoustic diffraction in monitoring cetaceans

An active acoustic technique for monitoring the whales is proposed. The technique allows one to monitor the whales’ crossing of a conventional borderline extending for several tens of kilometers in a shallow-water area. The potentialities of the technique are demonstrated in the framework of a numerical experiment by solving the problem of diffraction by model scatterers in an acoustic waveguide. The scatterers are selected in the form of soft spheroids with dimensions characteristic of various kinds of cetaceans.     

Sound speed in air-filled sand and 03ine shallow-layer sandy sediments

Based on the author's theory for acoustic propagation in granular media and by employing the extinction theorem, the sound speed formulae in these media were derived. The numerical computations of sound speed in 03ine sediments and air-filled sand were carried out, and the results demonstrated that under the normal atmosphere the sound speed in air- filled sand is lower than the sound speed in the air. The numerical results also indicated that the influence of scattering interaction between the grains upon the sound speed in the 03ine shallow-layer sandy sediments has to be taken into account; however, the influence of the viscous-wave interaction can be neglected. The theoretical results obtained from the rigid-granular model seem to match the measured data better than from the elastic-granular model, even the latter model fits better for the real situation, indicating further measurements are necessary in order to gain an insight into this problem thoroughly. Through an analysis of experimental data published in journals a conclusion can be drawn that the theory of granular media is suited to deal with the problems of the sound propagation in air-filled sand better than the theory of porous media.     

Internal-wave time evolution effect on ocean acoustic rays

A range-dependent field of sound speed in the ocean, c(x,z), caused by internal waves, can give rise to instabilities in acoustic ray paths. Past work has shown the importance of the background, range-independent, sound-speed profile; the ray initial conditions; the source-receiver geometry (depths and range); and the strength of the internal waves. However, in the past the time evolution of the internal waves has been ignored on the grounds that the speed of internal waves is much slower than the speed of the acoustic wave. It is shown here by numerical simulation that two rays with identical initial conditions, traveling through an ocean with the same background profile and the same random realization of internal waves, but with the internal waves frozen in one case and evolving in the other, travel significantly different trajectories. The dependence of this "frozen-unfrozen" difference on the initial ray launch angle, the background profile, and the strength of the internal-wave spectrum, is investigated. The launch-angle difference that generates similar arrival-depth differences to those induced by internal-wave time evolution is on the order of 100 microrad. The pattern of differences is measured here by the arrival depth at the final range of 1000 km. The observed pattern as a function of launch angle, change in the background profile, and change in internal-wave strength is found to be nearly the same for "frozen-unfrozen" change as for a slight change in launch angle.     

Numerical model to calculate microphysical influences on sound wave propagation in forests

There is a growing need for increasingly accurate and reliable numerical models to predict micrometeorological and turbulence conditions in complex sound wave propagation environments. In this paper, a prototype finite-difference computer model is developed to calculate the microphysical influences on sound speed in forested areas. Several numerical tests are conducted to assess model code capabilities using micrometeorological field data collected in June 2006. For the current analysis, the model domain and total number of grid points are greatly increased from earlier reported versions and the finite-difference numerical schemes for advection are modified to permit different inflow and outflow boundaries. Preliminary results for three cases are encouraging. Micrometeorological profiles and calculated fields of sound speed are presented. Some initial approximations of short-range acoustic transmission loss for the experimental test site are also discussed.     

Three-dimensional localization of sperm whales using a single hydrophone

A three-dimensional localization method for tracking sperm whales with as few as one sensor is demonstrated. Based on ray-trace acoustic propagation modeling, the technique exploits multipath arrival information from recorded sperm whale clicks and can account for waveguide propagation physics like interaction with range-dependent bathymetry and ray refraction. It also does not require ray identification (i.e., direct, surface reflected) while utilizing individual ray arrival information, simplifying automation efforts. The algorithm compares the arrival pattern from a sperm whale click to range-, depth-, and azimuth-dependent modeled arrival patterns in order to estimate whale location. With sufficient knowledge of azimuthally dependent bathymetry, a three-dimensional track of whale motion can be obtained using data from a single hydrophone. Tracking is demonstrated using data from acoustic recorders attached to fishing anchor lines off southeast Alaska as part of efforts to study sperm whale depredation of fishing operations. Several tracks of whale activity using real data from one or two hydrophones have been created, and three are provided to demonstrate the method, including one simultaneous visual and acoustic localization of a sperm whale actively clicking while surfaced. The tracks also suggest that whales' foraging is shallower in the presence of a longline haul than without.     

空腔流动的大涡模拟及气动噪声控制

大涡模拟(LES)和三维的Ffowcs Williams-Hawkings声学比拟方法相结合,研究空腔过流的一种噪声控制措施.空腔的底板/后墙使用多孔壁板,因此流体可以穿透空腔壁面,多孔效果使用Darcy压力-速度关系模拟.声源流场由LES计算,声辐射和远声场由声学比拟获得.结果表明,这种措施有效地减弱了空腔内的压力脉动和远场声辐射,低频脉动Rossiter模数对应的波动幅值被有效抑制,声源中偶极子占优项大幅度减小,从而抑制了声辐射.     

Sperm whales (Physeter catodon L. 1758) do not react to sounds from detonators

A number of observations show that sperm whales (Physeter catodon L. 1758) react to various man-made pulses with moderate source levels. The behavioral responses are described to vary from silence to fear. Click rates of five submerged male sperm whales were measured during the discharge of eight detonators off Andenes, northern Norway. In addition, the behavioral response of a surfaced specimen was observed. Click rates of the submerged whales and the behavior of the surfaced specimen did not change during the discharges with received sound levels of some 180 dB re 1 microPa peRMS. The apparent lack of response to the discharges could be due to similarity between sperm whale clicks and detonations. Accordingly, it can be speculated that the discharges may have been perceived as isolated clicks from conspecifics.