Long range transmission loss of broadband seismic pulses in the Arctic under ice-free conditions

In 2008 the Louis S. St-Laurent (LSSL) surveyed deep Arctic waters using a three-airgun seismic source. Signals from the seismic survey were detected between 400 km and 1300 km range on a directional autonomous acoustic recorder deployed in water 53 m deep off the Alaskan North Slope. Observations of received signal levels between 10-450 Hz versus LSSL range roughly fit a cylindrical transmission loss model plus 0.01 dB/km attenuation in deep ice-free waters, and fit previous empirical models in ice-covered waters. The transition between ice-free and ice-covered propagation conditions shifted 200 km closer to the recorder during the survey.     

Seismic properties of Asian elephant (Elephas maximus) vocalizations and locomotion

Seismic and acoustic data were recorded simultaneously from Asian elephants (Elephas maximus) during periods of vocalizations and locomotion. Acoustic and seismic signals from rumbles were highly correlated at near and far distances and were in phase near the elephant and were out of phase at an increased distance from the elephant. Data analyses indicated that elephant generated signals associated with rumbles and "foot stomps" propagated at different velocities in the two media, the acoustic signals traveling at 309 m/s and the seismic signals at 248-264 m/s. Both types of signals had predominant frequencies in the range of 20 Hz. Seismic signal amplitudes considerably above background noise were recorded at 40 m from the generating elephants for both the rumble and the stomp. Seismic propagation models suggest that seismic waveforms from vocalizations are potentially detectable by instruments at distances of up to 16 km, and up to 32 km for locomotion generated signals. Thus, if detectable by elephants, these seismic signals could be useful for long distance communication.     

Long-range multi-carrier acoustic communication in deep water using a towed horizontal array

During a recent long-range acoustic communication experiment carried out in deep water, multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) communication signals were transmitted with a 50 Hz bandwidth (225-275 Hz) at various source-receiver ranges from 100 to 700 km. The experiment consisted of two mobile components: (1) a source towed slowly at a speed of 2-3 knots at ～75 m depth and (2) a horizontal line array towed at 3.5 knots at a depth of ～200 m. In addition to beamforming, an interleaver gain is exploited to compensate for low signal-to-noise ratio at the expense of data rate while providing diversity in the frequency domain. Error-free performance is shown at effective data rates of 15 and 7.5 bits/s at ranges of 550 km and 700 km, respectively, by combining interleaved repetitions with low-density parity-check coding after beamforming, demonstrating the feasibility of multi-carrier OFDM communications in deep water using a towed horizontal array.     

Diversity combining for long-range acoustic communication in deep water

A recent experiment showed that coherent long-range acoustic communication is feasible in deep water over a ～550?km range between a source towed slowly at ～75?m depth and a horizontal line array towed at 3.5 knots at ～200?m depth. This letter further demonstrates that diversity combining mitigates channel fading and increases the output signal-to-noise ratio. Using sparse channel-estimate-based equalization, three transmissions are combined successfully to decode a 40?Hz bandwidth (230-270?Hz) 8 phase-shift-keying communication signal, achieving an effective data rate of 17 bits/s at ～550?km range.     

Analytical Study on the Rate of Sound Transmission Loss in Single Row Honeycomb Sandwich Panel Using a Numerical Method

Honeycomb structures have recently, replaced with conventional homogeneous materials. Given the fact that sandwich panels containing a honeycomb core are able to adjust geometric parameters, including internal angles, they are suitable for acoustic control applications. The main objective of this study was to obtain a transmission loss curve in a specific honeycomb frequency range along with same overall dimensions and weight. In this study, a finite element model (FEM) in ABAQUS software was used to simulate honeycomb panels, evaluate resonant frequencies, and for acoustic analysis. This model was used to obtain acoustic pressure and then to calculate the sound transmission loss (STL) in MATLAB software. Vibration and acoustic analysis of panels were performed in the frequency range of 1 to 1000 Hz. The models analyzed in this design includes 14-single row-honeycomb designs with angles of −45°, −30°, −15°, 0°, +15°, +30°, +45°. The results showed that a-single row and −45°cell angle honeycomb panel in the frequency range of 1 to 1000 Hz had the highest STL as well as the highest number of frequency modes (90 mods). Furthermore, the panel had the highest STL regarding the area under the STL curve (dB∙Hz). The panels containing more frequency mods, have a higher transmission loss. Moreover, the sound transmission loss is more sensitive to the cell angle variable (θ). In other studies, the STL was more sensitive to the number of honeycomb cells in the horizontal and vertical directions, as well as the angle of cells.     

Acoustic propagation through anisotropic internal wave fields: transmission loss, cross-range coherence, and horizontal refraction

Results of a computer simulation study are presented for acoustic propagation in a shallow water, anisotropic ocean environment. The water column is characterized by random volume fluctuations in the sound speed field that are induced by internal gravity waves, and this variability is superimposed on a dominant summer thermocline. Both the internal wave field and resulting sound speed perturbations are represented in three-dimensional (3D) space and evolve in time. The isopycnal displacements consist of two components: a spatially diffuse, horizontally isotropic component and a spatially localized contribution from an undular bore (i.e., a solitary wave packet or solibore) that exhibits horizontal (azimuthal) anisotropy. An acoustic field is propagated through this waveguide using a 3D parabolic equation code based on differential operators representing wide-angle coverage in elevation and narrow-angle coverage in azimuth. Transmission loss is evaluated both for fixed time snapshots of the environment and as a function of time over an ordered set of snapshots which represent the time-evolving sound speed distribution. Horizontal acoustic coherence, also known as transverse or cross-range coherence, is estimated for horizontally separated points in the direction normal to the source-receiver orientation. Both transmission loss and spatial coherence are computed at acoustic frequencies 200 and 400 Hz for ranges extending to 10 km, a cross-range of 1 km, and a water depth of 68 m. Azimuthal filtering of the propagated field occurs for this environment, with the strongest variations appearing when propagation is parallel to the solitary wave depressions of the thermocline. A large anisotropic degradation in horizontal coherence occurs under the same conditions. Horizontal refraction of the acoustic wave front is responsible for the degradation, as demonstrated by an energy gradient analysis of in-plane and out-of-plane energy transfer. The solitary wave packet is interpreted as a nonstationary oceanographic waveguide within the water column, preferentially funneling acoustic energy between the thermocline depressions.     

North Pacific right whale up-call source levels and propagation distance on the southeastern Bering Sea shelf

Call source levels, transmission loss, and ambient noise levels were estimated for North Pacific right whale (Eubalaena japonica) up-calls recorded in the southeastern Bering Sea in autumn of 2000 and 2001. Distances to calling animals, needed to estimate source levels, were based on two independent techniques: (1) arrival-time differences on three or more hydrophones and (2) shallow-water dispersion of normal modes on a single receiver. Average root-mean-square (rms) call source levels estimated by the two techniques were 178 and 176 dB re 1 μPa at 1 m, respectively, over the up-call frequency band, which was determined per call and averaged 90 to 170 Hz. Peak-to-peak source levels were 14 to 22 dB greater than rms levels. Transmission loss was approximately 15?log(10)(range), intermediate between cylindrical and spherical spreading. Ambient ocean noise within the up-call band varied from 72 to 91 dB re 1 μPa(2)/Hz. Under average noise conditions, call spectrograms were detectable for whales at distances up to 100 km, but propagation and detection distance may vary depending on environmental parameters and anthropogenic noise. Obtaining distances to animals and acoustic detection range is a step toward using long-term passive acoustic recordings to estimate abundance for this critically endangered whale population.     

Underwater radiated noise from modern commercial ships

Underwater radiated noise measurements for seven types of modern commercial ships during normal operating conditions are presented. Calibrated acoustic data (<1000 Hz) from an autonomous seafloor-mounted acoustic recorder were combined with ship passage information from the Automatic Identification System. This approach allowed for detailed measurements (i.e., source level, sound exposure level, and transmission range) on ships of opportunity. A key result was different acoustic levels and spectral shapes observed from different ship-types. A 54 kGT container ship had the highest broadband source level at 188 dB re 1 μPa@1m; a 26 kGT chemical tanker had the lowest at 177 dB re 1 μPa@1m. Bulk carriers had higher source levels near 100 Hz, while container ship and tanker noise was predominantly below 40 Hz. Simple models to predict source levels of modern merchant ships as a group from particular ship characteristics (e.g., length, gross tonnage, and speed) were not possible given individual ship-type differences. Furthermore, ship noise was observed to radiate asymmetrically. Stern aspect noise levels are 5 to 10 dB higher than bow aspect noise levels. Collectively, these results emphasize the importance of including modern ship-types in quantifying shipping noise for predictive models of global, regional, and local marine environments.     

Acoustic normal mode fluctuation statistics in the 1995 SWARM internal wave scattering experiment

In order to understand the fluctuations imposed upon low frequency (50 to 500 Hz) acoustic signals due to coastal internal waves, a large multilaboratory, multidisciplinary experiment was performed in the Mid-Atlantic Bight in the summer of 1995. This experiment featured the most complete set of environmental measurements (especially physical oceanography and geology) made to date in support of a coastal acoustics study. This support enabled the correlation of acoustic fluctuations to clearly observed ocean processes, especially those associated with the internal wave field. More specifically, a 16 element WHOI vertical line array (WVLA) was moored in 70 m of water off the New Jersey coast. Tomography sources of 224 Hz and 400 Hz were moored 32 km directly shoreward of this array, such that an acoustic path was constructed that was anti-parallel to the primary, onshore propagation direction for shelf generated internal wave solitons. These nonlinear internal waves, produced in packets as the tide shifts from ebb to flood, produce strong semidiurnal effects on the acoustic signals at our measurement location. Specifically, the internal waves in the acoustic waveguide cause significant coupling of energy between the propagating acoustic modes, resulting in broadband fluctuations in modal intensity, travel-time, and temporal coherence. The strong correlations between the environmental parameters and the internal wave field include an interesting sensitivity of the spread of an acoustic pulse to solitons near the receiver.     

Comparison between ocean-acoustic fluctuations in parabolic-equation simulations and estimates from integral approximations

Line-integral approximations to the acoustic path integral have been used to estimate the magnitude of the fluctuations in an acoustic signal traveling through an ocean filled with internal waves. These approximations for the root-mean-square (rms) fluctuation and the bias of travel time, rms fluctuation in a vertical arrival angle, and the spreading of the acoustic pulse are compared here to estimates from simulations that use the parabolic equation (PE). PE propagations at 250 Hz with a maximum range of 1000 km were performed. The model environment consisted of one of two sound-speed profiles perturbed by internal waves conforming to the Garrett-Munk (GM) spectral model with strengths of 0.5, 1, and 2 times the GM reference energy level. Integral-approximation (IA) estimates of rms travel-time fluctuations were within statistical uncertainty at 1000 km for the SLICE89 profile, and in disagreement by between 20% and 60% for the Canonical profile. Bias estimates were accurate for the first few hundred kilometers of propagation, but became a strong function of time front ID beyond, with some agreeing with the PE results and others very much larger. The IA structure functions of travel time with depth are predicted to be quadratic with the form theta(2)vc0(-2)deltaz(2), where deltaz is vertical separation, c0 is a reference sound speed, and thetav is the rms fluctuation in an arrival angle. At 1000 km, the PE results were close to quadratic at small deltaz, with values of thetav in disagreement with those of the integral approximation by factors of order 2. Pulse spreads in the PE results were much smaller than predicted by the IA estimates. Results imply that acoustic tomography of internal waves at ranges up to 1000 km can use the IA estimate of travel-time variance with reasonable reliability.     

Evidence of Doppler-shifted Bragg scattering in the vertical plane by ocean surface waves

A set of narrowband tones (280, 370, 535, and 695 Hz) were transmitted by an acoustic source mounted on the ocean floor in 10 m deep water and received by a 64-element hydrophone line array lying on the ocean bottom 1.25 km away. Beamformer output in the vertical plane for the received acoustic tones shows evidence of Doppler-shifted Bragg scattering of the transmitted acoustic signals by the ocean surface waves. The received, scattered signals show dependence on the ocean surface wave frequencies and wavenumber vectors, as well as on acoustic frequencies and acoustic mode wavenumbers. Sidebands in the beamformer output are offset in frequency by amounts corresponding to ocean surface wave frequencies. Deviations in vertical arrival angle from specular reflection agree with those predicted by the Bragg condition through first-order perturbation theory using measured directional surface wave spectra and acoustic modes measured by the horizontal hydrophone array.     

Monitoring the acoustic field of seismic survey pulses in the near-coastal zone

The paper describes monitoring of seismic survey parameters conducted during 4-D seismic surveying at the Pil??tun-Astokh hydrocarbon deposit on the northeastern shelf of Sakhalin Island at the boundary of the near-coastal Pil??tun gray whale feeding area. Acoustic measurements were performed in the frequency range of 2?C15000 Hz using 12 autonomous underwater acoustic recorders, of which three were deployed at the 10-m isobath and nine were equipped with digital radiotelemetry channels and deployed at the 20-m isobath along a line extending 20 km. In real time, the shore-based radio receiver station received over the digital radiotelemetry channels acoustic data from nine observation points, measured in the frequency range of 2?C2000 Hz. The influence of the hydrology and bathymetry on the seismic survey pulse propagation to the near-coastal zone is studied experimentally and theoretically.     

Acoustic pressure losses in woven screen regenerators

In thermoacoustic travelling-wave engines and other Stirling cycle devices, good performance depends on the material of a regenerator being in intimate contact with the gas inside it, so that each particle of gas oscillates in temperature following the adjacent material as it is acoustically displaced. This requires that the passages are small enough for temperature waves to penetrate across the gas path with the frequencies of interest. One type of ‘regenerator’ that is commonly used for this purpose is composed of multiple layers of woven stainless steel mesh, laid on top of one another in random registration. Associated with the thermal penetration is a viscous loss of pressure and this must be quantified if efficient engines are to be designed.In the literature, reliance has been placed on the correlation of steady-flow loss data for these meshes, but for the coarser ones operating at frequencies greater than 28 Hz, the assumption of quasi steady-flow is dubious and direct acoustic measurements must be made. This paper reports acoustic pressure loss data for meshes with 34 and 75 wires per inch taken in two configurations of impedance tube, and finds that the dependence on velocity is the same as in steady-flow, but that there is indeed some enhancement of loss for frequencies above 40 Hz. (Separation of the mesh layers is probably responsible for the anomalously low loss coefficients that were recorded in one set of data.) It is shown that the acoustic pressure losses can be correlated in terms that give the acoustic impedance more directly than the friction factor correlations.     

Parametric acoustic antenna in the atmosphere

We consider the principles of development of a parametric antenna in the atmosphere. It is shown that to excite an infrasound at a frequency lower than 1 Hz in the atmosphere, one can use an acoustic system emitting simultaneously two waves whose frequencies differ by the infrasound frequency. In the pump-wave frequency range 20–500 Hz, the main contribution to infrasound emission is given by the region located at altitudes from 30 to 75 km, in which the oscillatory velocity in the pump wave is maximum. We propose an algorithm for calculation of the infrasound amplitude in the E and F layers of the ionosphere. Realization of a parametric antenna in the atmosphere makes it possible to broaden the capabilities of experiments based on the modification of the ionosphere by an infrasound. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 7, pp. 570–576, July 2006.     

空气中声源激发浅海水下声场传播特性的实验研究

为了获得空气中远距离声源激发水下声场的精细结构,2013年3月,声场声信息国家重点实验室在南海海域进行了一次空气中声源激发水下声场的实验。采用汽笛作为空气声源,海底放置水听器作为接收,在实验过程中,发射船由距离水听器2.4 km处行驶至9.8 km。本文对该次实验数据进行分析,获得了收发距离远达9.8 km、频率分别为128 Hz和256 Hz的声传播损失曲线,该曲线随传播距离变化存在清晰的震荡结构.利用波数积分方法计算实验环境下的水下声场理论值,并对获得的声场传播特性进行了较好的物理解释。      

Blue and fin whale call source levels and propagation range in the Southern Ocean

Blue (Balaenoptera musculus) and fin whales (B. physalus) produce high-intensity, low-frequency calls, which probably function for communication during mating and feeding. The source levels of blue and fin whale calls off the Western Antarctic Peninsula were calculated using recordings made with calibrated, bottom-moored hydrophones. Blue whales were located up to a range of 200 km using hyperbolic localization and time difference of arrival. The distance to fin whales, estimated using multipath arrivals of their calls, was up to 56 km. The error in range measurements was 3.8 km using hyperbolic localization, and 3.4 km using multipath arrivals. Both species produced high-intensity calls; the average blue whale call source level was 189+/-3 dB re:1 microPa-1 m over 25-29 Hz, and the average fin whale call source level was 189+/-4 dB re:1 microPa-1 m over 15-28 Hz. Blue and fin whale populations in the Southern Ocean have remained at low numbers for decades since they became protected; using source level and detection range from passive acoustic recordings can help in calculating the relative density of calling whales.     

Radio-acoustic sounding of the ionosphere

We present the results of first experiments on radio-acoustic sounding of ionosphere at the altitudes from 70 to 85 km. The sounding was performed in autumn 2006, using a horn acoustic emitter and a radar on the basis of the “Sura” facility. The emitter had an acoustic power of about 1 kW and operated in the chirp-modulation regime with frequency variation from 15.9 to 18.4 Hz. The radar transmitter operated in the pulse regime at a frequency of 9 MHz and had an average power of 30 kW. The power of the radio signal scattered from a sound wave in the ionosphere did not exceed 10⁻¹⁶ W, and the measured values of the temperature in the scattering region ranged from 190 to 225 K. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 2, pp. 128–133, February 2009.     

Experimental analysis on air-to-water sound transmission loss in shallow water

The research of propagation characteristics of air-to-water sound transmission is of great importance to the detection of aerial targets from underwater.In order to study the propagation characteristics of air-to-water sound transmission in shallow water,State Key Laboratory of Acoustics,Institute of Acoustics,conducted an experiment in the South China Sea in March,2013.During the experiment,multi-frequency signals transmitted by a hooter hung on a research ship were received by an underwater hydrophone,and the distance between the hooter and the hydrophone was from 2.4 km to 9.8 km approximately.Through analyzing experimental data in this work,the experimental air-to-water transmission loss at frequencies128 Hz and 256 Hz is estimated up to 9.8 km in range,and its oscillation structure is evident.The wave-number integration approach is used to simulate theoretical air-to-water transmission losses,which are in good agreement with experimental values and to explain the experimental air-to-water sound transmission characteristics.     

Spatial extension of the acoustic time reversal region

According to the data of a full-scale shallow-water experiment (in the Barents Sea, at sea depths of about 120 m), a considerable gain in the signal-to-noise ratio is obtained for an acoustic signal received from a source at a distance of 12 km when matching with the medium is performed by the signal from the same source at a distance of 10.5 km. To interpret this experimental fact, a numerical simulation is performed to determine the size of the region of signal focusing due to the time reversal of waves in an ideal waveguide with a soft bottom. It is shown that, for narrowband signals, within a distance of ±5 km along the path from the point of emission of the reversed signal, a regular interference pattern whose maxima are comparable with the principal maximum is observed throughout the whole waveguide depth. For a spectrum width from 100 to 300 Hz, only the principle maximum with an extension of about 100 m is observed at a single depth.     

warping变换提取单模态反演海底衰减系数

为了获得浅海海底地声模型参数,利用warping变换方法分离出单模态简正波.对于接收深度固定、定深爆炸声源情况,以简正波理论为基础定义了距离归一化的简正波传播损失,并且其随传播的距离呈线性关系,故可通过此变化规律得到声压值实部的衰减因子,进而可求得海底地声模型参数:海底衰减系数.为验证此方法的有效性,仿真了warping变换提取单模态简正波的过程,同时将warping变换提取的单模态简正波与数值计算的结果进行比较验证;并针对某次黄海试验数据进行了处理,得到在150—550 Hz频带范围内海底衰减随频率的变化规律为α=0.581f_k~(1.86)(dB/m).通过与其他学者在相同海域试验结果的对比验证,变化规律基本相同.此外不同模态间反演相同频点的衰减系数接近也较好地支撑了结果.