Sound Channel Research Articles

Folk Music Audio Enhancement Method Combining Internet of Things and GA

The current methods of music sound enhancement mainly use the way of enhancing the intensity of the sound source, which ignores the spatial noise cancellation of the human ear, resulting in poor sound enhancement effect. Therefore, this paper proposes an audio enhancement method for folk music combining Internet of Things and GA. After acquiring folk music audio by Internet of Things technology, the postprocessing algorithm of wavelet packet decomposition combined with Wiener filter is used to suppress the noise in music. On the basis of HRTF, genetic algorithm is used to reduce the output error of left and right sound channels and realize the enhancement of music sound effect. The simulation experiment verifies the method of application research. The reverberation is established at 65 ms and lasts for a long time, which has good effect of sound enhancement.

Comparison Among All‐Sky Simulations, FY‐3E MWTS‐3 and FY‐4A AGRI Observations of the First Typhoon Malakas in 2022

AbstractThe newly launched early morning satellite Fengyun‐3E (FY‐3E) helps to form a three‐orbit constellation for better observing the first Typhoon Malakas in 2022. Together with MetOp‐B and NOAA‐20, global observations are made available 6 times daily from three temperature sounders of MWTS‐3, AMSU‐A, and ATMS onboard FY‐3E, MetOp‐B, and NOAA‐20, respectively. Having channel frequencies much less than 200 GHz, brightness temperatures (TBs) at different sounding channels are linearly related to temperatures at different altitudes. This allows Malakas's warm cores to be retrieved from MWTS‐3, AMST‐A, and ATMS TB observations. The warm‐core maxima of Malakas at 250 hPa has a single‐peaked diurnal cycle, with its maximum and minimum peaking around midnight and noon, respectively. FY‐3E MWTS‐3 observations allowed the intensity and phase of the diurnal cycle better captured. The diurnal variations of warm core retrieved from all‐sky TB simulations of the ERA5 reanalysis and National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analysis compared well with the three‐orbit constellation retrieval. All‐sky simulations of TB from the NCEP GFS analysis compared more favorably with FY‐4A AGRI TB observations than those from the ERA5 reanalysis.

Optimization of Acoustic Channels to Minimize Scattered Pressure Fields

Acoustic and elastic metamaterials have shown potential for many different applications in wave manipulation and structural isolation. Minimization of a scattered wave field has been identified as an important use for such materials, and a wide variety of designs have been proposed for this purpose. Several of these designs face practical challenges (e.g. pentamode designs, which are difficult to manufacture), and many others involve numerous tunable parameters and a large design parameter space (e.g. distributions of resonators), making global search-based optimization less tractable and raising the need for gradient-based methods. In this work, gradient-based optimization governed by partial differential equations (PDEs) is used to optimize acoustic channels in a coiled-space metastructure layer surrounding a rigid acoustic scatterer. The optimization technique is chosen to more feasibly search the design parameter space, and the acoustic channel design is chosen to exploit sound speed optimization as a proxy for channel length optimization to give practically achievable structures. Results are presented for both a single plane wave direction and multiple plane wave directions, as well as for both a single frequency and multiple frequencies. The equivalence of sound speed optimization and channel length optimization is demonstrated for a feasible design minimizing the scattered pressure field. Designs with various numbers of optimized channels are also considered.

Acoustic Float Tracking with the Kalman Smoother

Abstract Acoustically-tracked subsurface floats provide insights into ocean complexity and were first deployed over 60 years ago. A standard tracking method uses a Least-Squares algorithm to estimate float trajectories based on acoustic ranging from moored sound sources. However, infrequent or imperfect data challenge such estimates, and Least-Squares algorithms are vulnerable to non-Gaussian errors. Acoustic tracking is currently the only feasible strategy for recovering float positions in the sea ice region, a focus of this study. Acoustic records recovered from under-ice floats frequently lack continuous sound source coverage. This is because environmental factors such as surface sound channels and rough sea ice attenuate acoustic signals, while operational considerations make polar sound sources expensive and difficult to deploy. Here we present a Kalman Smoother approach that, by including some estimates of float behavior, extends tracking to situations with more challenging data sets. The Kalman Smoother constructs dynamically constrained, error-minimized float tracks and variance ellipses using all possible position data. This algorithm outperforms the Least-Squares approach and a Kalman Filter in numerical experiments. The Kalman Smoother is applied to previously-tracked floats from the southeast Pacific (DIMES experiment), and the results are compared with existing trajectories constructed using the Least- Squares algorithm. The Kalman Smoother is also used to reconstruct the trajectories of a set of previously untracked, acoustically-enabled Argo floats in the Weddell Sea.

Сравнение результатов измерений уровней подводного шума источника, полученных прямым методом и с использованием векторно-фазовой обработки

Object and purpose of research. Moving broadband source underwater noise level estimations obtained by direct and indirect methods using a receiver for the measurement of sound pressure and sound particle velocity components combined. Materials and methods. Theoretical research using computer simulation, full-scale experiments in real measurement conditions, statistical analysis of experimental results. Main results. The noise level estimations obtained by the direct method, only through the sound pressure channel, and by the indirect method with recalculating the values of sound intensity were compared based on representative samples under ideal conditions and in real water area conditions. Conclusion. Under ideal measurement conditions a statistically significant difference between the two estimation methods occurs only at frequencies below 16 Hz. The results of a full-scale experiment in real conditions show that the estimate obtained on the basis of the spatial selective sound intensity vector is characterized by greater reliability and accuracy since it is less affected by reflections from the boundaries of the water area.

Mitigation of Systematic Noise in F16 SSMIS LAS Channels Observations for Tropical Cyclone Applications

The Special Sensor Microwave Imager Sounder (SSMIS) onboard the Defense Meteorological Satellite Program (DMSP) F16, launched on 18 October 2003, was the first conical-scanning radiometer to combine the Special Sensor Microwave/Imagers (SSM/I), Special Sensor Microwave/Temperature Sounder (SSM/T), and the Special Sensor Microwave/Water Vapor Sounder (SSM/T2). Nearly 20 years of F16 SSMIS data are available to the general public, providing many opportunities to study the atmosphere at both the synoptic and decadal scales. However, data noise from complicated structures has occurred in the brightness temperature (TB) observations of lower atmospheric sounding (LAS) channels since 25 April 2013. We used a two-dimensional Fast Fourier Transform to analyze the characteristic features of data noise in cross-track and along-track directions. We found that the data noise is around 1–2 K and occurs at certain cross-track wavelengths (Δλ)noise. A latitudinal variation was found for (Δλ)noise. Due to noise interference, TB observations reflecting rain, clouds, tropical cyclone warm core, temperature, and water vapor distributions are not readily distinguishable, especially in channels above the middle troposphere (channels 4–7 and 24), whose dynamic TB range is smaller than low tropospheric channels 1–3. Examples are provided to show the impact of the proposed noise mitigation for conical-scanning TB observations to capture 3D structures of hurricanes directly. Once the noise in F16 SSMIS LAS channels from 25 April 2013to the present is eliminated, we may investigate the decadal change of many features of tropical cyclones derivable from these TB observations.

The Effects of Vertical Correlation Characteristics on Vertical Array Gain Performance in Deep Water

Spatial correlation characteristics of sound field have significant impact on array gain. Compared with the single-sensor receiving scenario, the vertical array with multiple receivers can remarkably enhance signal-to-noise ratio and detection ability. For the large-scale vertical array deployed in the incomplete deep-water sound channel, different bathymetry conditions or source-receiver ranges lead to different vertical correlation patterns, thus leading to the variation of array gain. Based on the deep-water experimental data conducted in the South China Sea, the effects of the vertical correlation on array gain performance are studied in both the flat and the uneven bottom environments. In the flat bottom environment, three representative ranges in the direct arrival zone and first shadow zone are chosen, respectively, to analyze how vertical correlation characteristics would affect the array gain performance. In the uneven bottom environment, attention is confined to those ranges that endure significant transmission loss differences when compared with the flat bottom environment. In summary, when the correlation coefficients grow smaller, it is observed that the array gains of different beamformers decrease, along with the energy drifting away from the first few eigenvalues and dispersing on many other eigenvalue elements. At the same time, the gap between the array gain results of eigenvalue beamforming and optimal quadratic beamforming would grow wider and the effective rank of signal covariance matrix would sprawl as well. The study is of great significance to analyze and enhance vertical sonar array detection performance at different ranges under various environmental conditions in deep water.

A Decoupling Algorithm Based on PID Neural Network for Multi-Channel Active Noise Control of Nonstationary Noise

It is critical to study multi-channel active noise control (ANC) systems to satisfy the requirements of noise reduction in multi-target positions. However, the complexity of the system may increase as the result of an increased number of sound channels. In addition, multi-channel coupling affects the stability of a system. In this paper, a decoupling algorithm based on the Proportion Integration Differentiation (PID) neural network and the filtered-x least-mean-square (FxLMS) for the multi-channel ANC of nonstationary noise is proposed. Due to the nonlinear characteristics of the PID neural network, the coupling problem can be solved through the algorithm. The performance of the novel proposed algorithm is verified by comparing the simulation results with the results from the traditional FxLMS algorithm and the FxLMS algorithm with matrix decoupling. The results illustrate that the convergence speed of the traditional FxLMS algorithm is similar to that of the FxLMS algorithm with matrix decoupling, while the proposed algorithm converges significantly faster than the other two algorithms. In terms of control performance, the proposed algorithm executes the best and the residual error signal has the minimum amplitude, followed by the FxLMS algorithm with matrix decoupling and the traditional FxLMS algorithm. With the advantages in convergence speed and control performance, the proposed decoupling algorithm could be suitable for multi-channel nonstationary ANC.

Assessment of the Consistency and Stability of CrIS Infrared Observations Using COSMIC-2 Radio Occultation Data over Ocean

The accuracy of brightness temperature (BT) from the Cross-track Infrared Sounder (CrIS) onboard the Suomi National Polar-orbiting Partnership (S-NPP) satellite and NOAA-20 is estimated using the Constellation Observing System for Meteorology, Ionosphere, and Climate 2 (COSMIC-2) radio occultation (RO) wet retrievals (temperature and water vapor profiles) as input to the Community Radiative Transfer Model (CRTM). The matchup criteria between RO and CrIS observations are time less than 30 min, a distance less than 50 km, and over oceans to reduce the collocation and simulation uncertainty. Based on the information provided in the CrIS and RO observations, only upper temperature sounding channels with weighting function peak height (WFPH) above 200 hPa (~12 km) from the CrIS longwave infrared (LWIR) and shortwave infrared (SWIR) bands and water vapor channels from the CrIS mid-wave infrared (MWIR) band with WFPH above 500 hPa (~6.3 km) are selected for comparison to minimize the impacts from the surface emission, cloud absorption/scattering, and atmospheric gaseous absorption. The absolute differences between CrIS observations and their CRTM simulations using RO data as input are less than 1.0 K for the majority of those selected channels. The double differences between CrIS observations on NOAA-20 and S-NPP using CRTM simulations as transfer references are very stable. They range from −0.05 K to 0.15 K for LWIR channels and −0.20 K to 0.10 K for SWIR channels during the two years from 1 October 2019 to 30 September 2021. For MWIR channels, the double differences range from −0.15 K to 0.25 K but have significant variations in both daily mean and monthly mean time series. The results provide ways to understand the qualities of RO retrieval and CrIS measurements: RO data can be used to assess the consistency and stability of CrIS observations quantitatively, and CrIS measurements have the quality to assess the quality and stability of RO retrievals.

Impact of Stratosphere on Cold Air Outbreak: Observed Evidence by CrIS on SNPP and Its Comparison with Models

A cold air outbreak (CAO) is an extreme weather phenomenon that has significant social and economic impacts over a large region of the midlatitudes. However, the dynamical mechanism of the occurrence and evolution of CAO events, particularly the role of the stratosphere, is not well understood. Through an analysis of one extreme CAO episode that occurred on 27–31 January 2019 across much of the US Midwest, this study examined its thermodynamic structure and the impact of stratospheric downward transport using the single-field-view (SFOV) satellite products (with a spatial resolution of ~14 km at nadir) from the Cross-track Infrared Sounder (CrIS) onboard Suomi National Polar-Orbiting Partnership (SNPP) in conjunction with MERRA-2 and ERA-5 reanalysis products. It is found that along the path of cold air transport, particularly near the coldest surface center, there exists a large enhancement of O3, deep tropopause folding, significant downward transport of stratospheric dry air, and a warm center above the tropopause. The upper warm center can be observed directly using the brightness temperature (BT) of CrIS stratospheric sounding channels. While similar large-scale patterns of temperature (T), relative humidity (RH), and ozone (O3) are captured from CrIS, MERRA-2, and ERA-5 products, it is found that, in the regions impacted by CAO, MERRA-2 has a thicker dry layer under the tropopause (with the difference of RH up to ~10%) and the total column ozone (TCO) from ERA-5 has a relatively large positive bias of 2.8 ± 2.8% compared to that measured by Ozone Mapping and Profiler Suite (OMPS). This study provides some observational evidence from CrIS that confirm the impact of the stratosphere on CAO through downward transport and demonstrates the value of the SFOV retrieval products for CAO dynamic transport study and model evaluation.

Image Denoising With Contourlet Transform

Abstract: Sound causes image degradation during detection, processing, signal processing, and transmission through sound channels. Noise removal is a technique for eliminating noise from photographs. Denoising refers to the process of removing noise from a picture. In this paper we have proposed a novel algorithm was built based on a variable called "contourlet transform" is a term used to describe the transformation of a contourlet This algorithm is significantly more effective. in image extraction than the wavelet technique and especially the shock wave's elimination. The parameters were set. when wavelet and contourlet are compared SNR and IEF transforms The findings show that in terms of performance, the suggested approach outperforms the wavelet

The Beaufort Sea acoustic duct's variability and its impact on acoustic propagation using the mode interaction parameter

The Beaufort duct is a subsurface sound channel formed by cold Pacific Winter Water sandwiched between warmer Pacific Summer Water and Atlantic Water in the Western Arctic Ocean. This duct traps sound waves and allows them to travel long distances without losing energy to lossy interactions with sea ice and surface waves. This study quantifies Beaufort duct variability based on Canada Basin Acoustic Propagation Experiment (CANAPE) and Coordinated Arctic Acoustic Thermometry Experiment (CAATEX) oceanographic observations. Deterministic ocean features induce coupling between acoustic modes confined to the Beaufort duct and non-ducted modes by weakening the duct or causing it to take on an asymmetric form. A non-dimensional mode interaction parameter (MIP) can be defined using the acoustic frequency and the vertical and horizontal scales of sound speed perturbation to characterize coupling strength. It identifies three important wave propagation regimes: sudden approximation, adiabatic approximation, and maximum interaction regime (Colosi and Zinicola-Lapin, 2021). When the MIP between ducted and lossy modes is more and less than 1, strong and weak acoustic variability is predicted, respectively. Variability is high when the MIP between two ducted modes surpasses 1, but modei–mode interference patterns grow more complicated. Acoustic numerical simulations are used to demonstrate various effects.

Temperature and Moisture Sounding Performance of Current and Prospective Microwave Instruments under All-Sky Conditions

We provide consistent theoretical and empirical assessments of the major driving factors of the information content and retrieval performance for current and potential future microwave (MW) sounders. For the specific instrument concepts assessed, we find that instrument noise is a major driver, impacting vertical resolution as measured by the degrees of freedom for signal as much as 50%. We also observe diminished performance in the 118 GHz temperature sounding band as compared to the 50–60 GHz band, which is largely due to the increased sensor noise in the assessed 118 GHz sensor for comparable channels—a reduction in the performance gap between 118 GHz and 50 GHz bands can be obtained with a reduction of instrument noise in the 118 GHz temperature sounding channels. As expected, scene-type also significantly impacts the vertical resolution, emphasizing the importance of separating clear, cloudy, rainy, and icy conditions when evaluating instrument performance.

The Effects of Sound Speed Profile to the Convergence Zone in Deep Water

The structure of a sound speed profile (SSP) in deep water causes refraction of sound rays and Convergence Zones (CZs) of high intensity where the rays focus at shallow depth. Study of sound field characteristics in the CZs has always been the focus of deep-water acoustics research. Many studies have been conducted on sound propagation in different parts of the oceans with different environments and, in this paper, the range and width of CZ is analyzed in the East Indian Ocean (EIO) and the South China Sea (SCS). Through the experimental data collected in different seasons with the propagation conditions change in the EIO and the SCS, we observe that the SSPs in different marine environments have a significant impact on the CZs of deep water. The sound channel mixing layer and isothermal layer have great effect on the CZ ranges. The water depths in the two experimental areas are similar, the range of the first CZ in the EIO is 7–8 km farther than that in the SCS, and the width of the CZs in the EIO is about 2–3 km narrower than that in the SCS. The surface mixed layer and the thermocline affect the CZ width but has little effect on the CZ range when the sound speed at the source and the bottom are practically the same. As the propagation conditions change along the seasons in the EIO, the range of the first CZ is almost the same, but the width of the CZs in the summer is about 2 km narrower than that in the spring. The water depth affects the CZ width but has little effect on the CZ range if the CZs can be formed. The different CZ characteristics between EIO and SCS are explained by both theoretical calculation and numerical simulation. The influence of the SSP structure and water depth on the CZ range are analyzed and the corresponding mechanism is explained. The research results are of great significance for underwater acoustic detection in deep sea.

The Acoustic Nature of “Storm Nose”, “Supercell” Vortices and Tornadoes

The process of cumulonimbus cloud Cb calvus formation in the middle latitudes of real atmosphere is analyzed in this work. Its transformation from initial lifecycle stage to “maturity” undergoes due to the formation of the waveguide called “aerial acoustic channel” in the troposphere near the level of temperature minimum that is close to 2 km altitude. This “aerial acoustic channel” can be considered as analog of “deep sound channel” that corresponds to the minimal sound speed level. Tropospheric “channel” related to the thermal inversion zone is almost unlimited horizontally. Synchronous generation of two compression waves (ascending one above Cb and descending one inside Cb) is caused by Cb calvus dome ascension. The first one can provoke the aerodynamic draft previously unexplained. The second one results in the growth of its “storm nose” and in the axial and peripheral descending mechanisms in Cb. The penetration of Cb into stratosphere results in the destruction of dynamic balance around Cb top and hence in its unloading in the descending decompression wave. Here the air cools down to the “dew point” in the place of conjugation with parental cloud – due to Snellius law it results in the formation of aerosol “vortex” as condensation front; this “vortex” has calculated value of its generatrix against vertical. Due to D. Snow’s criterion, this vortex forms either “supercell” vortex or tornado vortex.

Evaluation and Assimilation of FY-3C/D MWHS-2 Radiances in the RMAPS-ST

Currently, humidity information can be obtained from the Microwave Humidity Sounder-2 (MWHS-2) mounted on the polar-orbiting satellites FY-3C and FY-3D. However, making full use of the MWHS-2 data remains a challenge, particularly in the application of regional numerical weather models. This study is the first to include MWHS-2 radiance data in the Rapid-refresh Multi-scale Analysis and Prediction System—Short-term (RMAPS-ST) regional model. The results and impact of MWHS-2 radiance data assimilation were investigated and evaluated. It is found that MWHS-2 radiance data can be effectively assimilated in the RMAPS-ST after a series of quality control and variational bias correction. Benefits could be obtained in the reduction of background departures for each humidity sounding channel. Assimilation experiments over a period of one month were carried out, and the impacts of MWHS-2 radiances were quantitatively analyzed on the forecasts of RMAPS-ST system. The results showed that MWHS-2 saw a small but significant improvement for low-level humidity of short-range forecast, by 16.5% and 3.2% in terms of mean bias and root-mean-square error, respectively. The positive impact on short-range forecast also can be found for middle and low level temperature and wind. For quantitative precipitation forecast, the assimilation of MWHS-2 radiances increased the score skills of different rainfall levels in the first 12 h forecast by an average of 1.4%. There was a slight overall improvement in the 24-h precipitation forecast for over-estimation and false alarm of 3-h accumulated rainfall below 1.0 mm, with 0.75% and 0.36%, respectively. The addition of MWHS-2 radiance data gives a small positive impact on low-level humidity, temperature, and wind in the RMAPS-ST regional model, and it also improves short-range forecast of rainfall, particularly in the first 12 h of the forecast.

Retracted] Research on Digital Image Watermarking Algorithm Based on Scrambling and Singular Value Decomposition

At present, the digital image watermarking algorithm has not embedded the synchronization signal in the image, resulting in the poor performance of the embedded image in terms of, for example, imperceptibility, anti‐attack ability, and robustness. Therefore, a digital image watermarking algorithm based on scrambling and singular value decomposition is proposed. The digital watermark is preprocessed by dimensionality reduction and encryption; the digital image is processed in sections and embedded in the synchronization signal. The digital watermark image is embedded in the digital image watermark by using the low‐frequency energy ratio technology of sound channel. The watermark image extraction step is designed to extract the watermark image. The transmittance of the degraded image is calculated, the transmittance is refined by relying on the soft matting algorithm and guiding filter, the image contrast in the image frequency domain is enhanced, and the results are mapped to the appropriate visual range to optimize the visual brightness of the image. Finally, according to the uniqueness of singular value matrix, the distributed characteristics of image matrix data are described, the visual effect is enhanced, and the research of digital image watermarking algorithm is completed. The experimental results show that the algorithm can extract the watermark information completely, the image contrast and singular value have been significantly improved, and the algorithm has better anti‐attack performance.

Deriving Surface Reflectance From Visible/Near Infrared and Ultraviolet Satellite Observations Through the Community Radiative Transfer Model

Earth's surface reflectance is an important parameter affecting ultraviolet (UV) and visible (VIS) radiance calculations at the top of the atmosphere because many UV and VIS channels can acquire information about the surface and atmosphere. This article provides the theoretical basis for deriving the surface reflectance from satellite-measured UV and VIS observations at window and lower sounding channels with the help of the community radiative transfer model (CRTM) and collocated atmospheric profiles such as ozone, water vapor, and aerosols. Cirrus cloud may be included in the calculation as long as the observations contain enough reflected radiation from the surface. An explicit equation with three scalar parameters <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha $</tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\beta $</tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\delta $</tex-math></inline-formula> is obtained for users to calculate Lambertian surface reflectance from the observation. The expressions for the three parameters are somewhat complicated and computationally expansive. We found a simple and smart way that can exactly calculate the three parameters with quasi-linear functions. Numerical experiments using the CRTM simulations have demonstrated the algorithm accuracy for the surface reflectance retrieval better than 2.0E-14. As a case study, measured surface reflectance and the derived surface reflectance over desert from satellite UV measurements are compared. The derived surface reflectance from Suomi National Polar-orbiting Partnership. Visible Infrared Imaging Radiometer Suite (VIIRS) observations and the VIIRS reflectance product are compared as well. In addition, this methodology can also be used to calculate microwave and infrared surface emissivity with scatterings and solar radiation by adding the surface Planck radiance at the surface temperature.

Effect of acoustic diffraction phase shift on caustic structure in deep sea convergence zone

The constant phase shift &lt;inline-formula&gt;&lt;tex-math id="M1"&gt;\begin{document}$- {{\text{π}}}/{2}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220763_M1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220763_M1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; generated by a turning point is an ideal approximation under high frequency conditions. In fact, when sound waves pass through a turning point, it will travel horizontally for a certain distance in the form of inhomogeneous plane wave and then be refracted back. At this time, a functional phase shift should also be included at the turning point. In addition, a refracted ray will bring about reflection phase shift when the turning point is close to the waveguide edge. These two kinds of phase shifts which are associated with the diffraction phenomenon are called diffraction phase shifts in this paper and are more notable when the frequency goes down. In order to accurately obtain the caustic structure at low frequency, the diffraction phase shift is used to correct the classical ray theory in calculating ray skip distance, traveling time, and group velocity in this work. On this basis, a simple and explicit analytical model is proposed which is suitable for calculating the low frequency deep water caustics. The numerical study of the first upper convergence zone in the complete deep water sound channel shows that there are three caustic lines in the refracted-refracted (RR) convergence zone and four caustic lines in the refracted surface-reflected (RSR) convergence zone under the condition of high frequency hypothesis. When the diffraction phase shifts at low frequency are included, and compares with high frequency results, it is found that the horizontal beam displacement caused by the reflection phase shift will make the RR caustics horizontally move towards the source and also lead the RSR rays to generate several new caustics, while the beam displacement caused by sound propagating in the form of inhomogeneous plan wave will make the RR caustics horizontally move away from the sound source. Accompanied with the increased frequency, the diffraction effect will decrease, and the caustic structure gradually tends to the classical ray theory results. The reliability of the correction results is verified by the normal mode theory. The model proposed in this work makes up for the deficiency of classical ray theory.

Effect of Subsurface Mediterranean Water Eddies on Sound Propagation Using ROMS Output and the Bellhop Model

Ocean processes can locally modify the upper ocean density structure, leading to an attenuation or a deflection of sound signals. Among these phenomena, eddies cause significant changes in acoustic properties of the ocean; this suggests a possible characterization of eddies via acoustics. Here, we investigate the propagation of sound signals in the Northeastern Atlantic Ocean in the presence of eddies of Mediterranean Water (Meddies). Relying on a high-resolution simulation of the Atlantic Ocean in which Meddies were identified and using the Bellhop acoustic model, we investigated the differences in sound propagation in the presence and absence of Meddies. Meddies create sound channels in which the signals travel with large acoustic energy. The transmission loss decreases to 80 or 90 dB; more signals reach the synthetic receivers. Outside of these channels, the sound signals are deflected from their normal paths. Using receivers at different locations, the acoustic impact of different Meddies, or of the same Meddy at different stages of its life, are characterized in terms of angular distributions of times of arrivals and of energy at reception. Determining the influence of Meddies on acoustic wave characteristics at reception is the first step to inverting the acoustic signals received and retrieving the Meddy hydrological characteristics.