Reverberation Model Research Articles

Model-data comparisons of range-dependent shallow-water reverberation including both boundary and volume scattering

A range-dependent shallow-water reverberation model using adiabatic normal modes has been previously developed [Ellis et al., ISURC Conference, La Spezia, 2008] to handle bottom scattering and clutter echoes in a range-dependent environment. It has now been extended to handle volume scattering from the water column and volume scattering from the sub-bottom. Beam time series similar to that from a horizontal line array can be produced. Comparisons can then be made directly with data, and area scattering maps created. Of particular interest will be data obtained on the triplet line array during the 2013 TREX experiment in the Gulf of Mexico off Panama City, Florida. Predictions will be compared with data for average reverberation from a number of pings, as well as ping-to-ping variability on a particular run. [Work supported in part by U.S. Office of Naval Research, Code 32.]

Simultaneous spectral and reverberation modelling of relativistic reflection in Mrk 335

We present an X-ray spectral and timing model to investigate the broad and variable iron line seen in the high flux state of Mrk 335. The model consists of a variable X-ray source positioned along the rotation axis of the black hole that illuminates the accretion disc producing a back-scattered, ionized reflection spectrum. We compute time lags including full dilution effects and perform simultaneous fitting of the 2-10 keV spectrum and the frequency-dependent time lags of 2.5-4 vs. 4-6.5 keV bands. The best-fitting parameters are consistent with a black hole mass of approximately 1.3 x 10^7 M_sun, disc inclination of 45 degrees and the photon index of the direct continuum of 2.4. The iron abundance is 0.5 and the ionization parameter is 10^3 erg cm / s at the innermost part of the disc and decreases further out. The X-ray source height is very small, approximately 2 r_g. Furthermore, we fit the Fe L lags simultaneously with the 0.3-10 keV spectrum. The key parameters are comparable to those previously obtained. We also report the differences below 2 keV using the xillver and reflionx models which could affect the interpretation of the soft excess. While simultaneously fitting spectroscopic and timing data can break the degeneracy between the source height and the black hole mass, we find that the measurements of the source height and the central mass significantly depend on the ionization state of the disc and are possibly model-dependent.

Efficient modeling of range-dependent ray convergence effects in propagation and reverberation.

In an earlier paper [Harrison (2013). J. Acoust. Soc. Am. 133, 3777-3789] the computationally efficient energy flux approach to modeling sound propagation was modified to include focusing, ray convergence, and caustic-like behavior. The derivation started with the coherent normal mode sum but retained only terms that interfered on a scale of a ray cycle distance. Here, by starting with the adiabatic mode sum, the formulation is extended to a slowly varying range-dependent environment and applied to the target-echo and reverberation model, Artemis. Some examples are given.

Experimental Validation of the Reverberation Effect in Room Electromagnetics

The delay power spectrum is widely used in both communication and localization communities for characterizing the temporal dispersion of the radio channel. Experimental investigations of in-room radio environments indicate that the delay power spectrum exhibits an exponentially decaying tail. This tail can be characterized with Sabine's or Eyring's reverberation models, which were initially developed in acoustics. So far, these models were only fitted to data collected from radio measurements, but no thorough validation of their prediction ability in electromagnetics has been performed yet. This paper provides a contribution to fill this gap. We follow Sabine's original experimental approach, which consists in comparing model predictions to experimental observations in a room, while varying its mean absorption coefficient and total room surface. We find that Eyring's model provides a more accurate prediction of the parameters characterizing the decaying tail, such as the reverberation time, than Sabine's model. We further use the reverberation models to predict the parameters of a recently proposed model of a distance-dependent delay power spectrum. This model enables us to predict the path loss, mean delay, and root mean square (rms) delay spread versus transmitter-receiver distance. We observe good agreement between predictions and experimental results.

Using iron line reverberation and spectroscopy to distinguish Kerr and non-Kerr black holes

The iron Kα line commonly observed in the X-ray spectrum of both stellar-mass and supermassive black hole candidates is produced by the illumination of a cold accretion disk by a hot corona. In this framework, the activation of a new flaring region in the hot corona imprints a time variation on the iron line spectrum. Future X-ray facilities with high time resolution and large effective areas may be able to measure the so-called 2-dimensional transfer function; that is, the iron line profile detected by a distant observer as a function of time in response to an instantaneous flare from the X-ray primary source. This work is a preliminary study to determine if and how such a technique can provide more information about the spacetime geometry around the compact object than the already possible measurements of the time-integrated iron line profile. Within our simplified model, we find that a measurement of iron line reverberation can improve constraints appreciably given a sufficiently strong signal, though that most of the information is present in the time-integrated spectrum. Our aim is to test the Kerr metric. We find that current X-ray facilities and data are unable to provide strong tests of the Kerr nature of supermassive black hole candidates. We consider an optimistic case of 105 iron line photons from a next-generation data set. With such data, the reverberation model improves upon the spectral constraint by an order of magnitude.

Improving headphone spatialization for stereo music

Music is mixed and mastered for playback on near- and far-field speakers, which presents a problem to the growing population of listeners who listen to music primarily on headphones. Playing legacy stereo mixes on headphones places the stereo image inside the listener's head and makes the image appear ultra-wide. While this is helpful for separating the center and stereo content, it has a detrimental effect on the spatialization of the music. It makes headphone listening unnatural and uncomfortable, although listeners have learned to accept it. In this work, we develop a system that processes stereo signals to provide a better sound image to headphone listeners. The sound image is improved by adding the necessary cues to the signal so as to externalize the perceived soundstage by making it similar to the soundstage experienced inside a mixing studio. In order to work with all headphones, our system tries to maintain the mastering equalization curve of the original stereo content. We employ head-related transfer functions, de-correlation, models of reverberation, and 1/3-octave-band equalization to realize our system. The results of pilot subjective evaluations suggest that our system makes music more natural and comfortable to listen to, although with some loss of quality.

A few Canadian contributions to underwater acoustics

An historical perspective on four Canadian initiatives in underwater acoustics will be presented: Towed Variable Depth Sonar (TVDS); the Chapman-Harris model for reverberation; seabed-interaction in shallow water; and Arctic acoustics. The poor performance of hull-mounted sonars in Canadian waters during WWII prompted TVDS development in 1947. The resulting prototype (CAST/1X) was tested against a UK developmental system in 1958. Subsequently, the UK purchased the Canadian system and the US re-packaged their sonars according to the Canadian design. An understanding of the effects of back-scattering from the sea surface and bottom was required to model sonar system performance. Experiments began circa 1960 leading to a new model for sea surface back-scattering, and revealing the ocean volume as an additional factor. The conditions governing propagation and noise in shallow waters remained mysterious in the 1970s, until the significant role of the seabed emerged through theoretical developments (e.g., loss due to shear waves) and experiments conducted over a variety of seabed types. Political, sovereignty and energy issues prompted Canada to investigate under-ice acoustics in the Arctic beginning circa 1958. The efforts ultimately resulted in the ability to undertake under-ice acoustic and geophysical surveys using long endurance Autonomous Underwater Vehicles (AUVs).

Finite Element Modeling of Acoustic Scattering From Fluid and Elastic Rough Interfaces

Quantifying acoustic scattering from rough interfaces is critical for reverberation modeling, acoustic sediment characterization, and propagation modeling. In this study, a finite element (FE) scattering model is developed. The model computes the plane wave scattering strength for an ensemble of rough power-law surfaces for ocean bottoms described as fluid and elastic. The FE model is compared with two models based on approximations to the Helmholtz–Kirchhoff integral: the Kirchhoff approximation (KA) and the perturbation theory (PT). In the case of a fluid-like bottom, the KA and FE models agree except at small grazing angles. The PT and FE models deviate near specular especially at small angles. For the elastic case, the PT predicts the FE results well except at the intromission angle of the shear wave. The KA deviates for angles that are below the critical angle of the compressional wave. At the shear wave intromission angle, the FE model shows a more plausible solution likely due to multiple scattering events that are not accounted for in PT for the modeled roughness.

Low-Frequency Normal Mode Reverberation Model

본 논문에서는 천해의 거리 독립환경에서 저주파수 대역의 잔향음 준위를 계산하는 정상모드 잔향음 모델 개발을 제안하였다. 정상모드를 이용하여 음원에서 산란체, 산란체에서 수신기까지의 음파 전달을 계산하였다. 산란 면적에서 상하로 전달되는 음파는 각 모드를 이용하여 고려하였다. 산란함수는 기존에 제시된 해저면 조건을 고려한 람베르트 공식(Lambert's law)과 반사각 부근에서는 Kirchhoff 근사법에 의한 가우시안 형태를 조합하였다. 제안 모델을 검증하기 위해 미국 ONR(Office of Naval Research)이 지원한 제 I회 잔향음 모델링 워크샵(Reverberation Modeling Workshop I, RMW I)의 XI, XV번 문제를 기준으로 상대비교를 수행하였다. In this paper, a normal mode reverberation model for a range-independent environment of shallow water is proposed to calculate the reverberation level in the low-frequency range. Normal mode is used to calculate the acoustic energy propagating from the source to the scattering area and from the scattering area to the receiver. Each mode is decomposed into up and down going waves to consider scattering strength at the scattering area. The scattering functional form combines Lambert's law with a Gaussian-like term near the specular direction based on Kirchhoff approximation considering bottom condition. For verification of the suggested model, the result is relatively compared to several solutions of the problem XI and XV in the Reverberation Modeling Workshop I sponsored by the US Office of Naval Research.

Possibility of False Target Signals Induced by Reverberation Due to Internal Waves in Shallow Water

It is investigated that there exists the possibility of the false target signals induced by reverberation in an active sonar system due to the internal waves in shallow water. The rays down-refracted from the internal waves may generate strong bottom-reverberation signals, which can result in false target signals. Sound waves emitted from a source propagate 3-dimensionally. Therefore, the study of internal waves on the reverberation should be studied for azimuthal direction as well as 2-dimensional (r-z) plane. Internal-wave modelling was conducted, based on solitons which were predicted with the various conditions such as, the range of source-soliton, horizontal widths of soliton. Variable depth sonar (VDS) was assumed as a source, of which the depth was located in the minimum sound speed layer in a simulation environment. Finally, the simulation on the reverberation level with time was made based on ray-based reverberation model, and the results implied that several false-target signals could be displayed on the PPI(Plan Position Indicator) scope simultaneously with range from source to soliton, and the horizontal width of soliton.

Nonparametric Bayesian Dereverberation of Power Spectrograms Based on Infinite-Order Autoregressive Processes

This paper describes a monaural audio dereverberation method that operates in the power spectrogram domain. The method is robust to different kinds of source signals such as speech or music. Moreover, it requires little manual intervention, including the complexity of room acoustics. The method is based on a non-conjugate Bayesian model of the power spectrogram. It extends the idea of multi-channel linear prediction to the power spectrogram domain, and formulates a model of reverberation as a non-negative, infinite-order autoregressive process. To this end, the power spectrogram is interpreted as a histogram count data, which allows a nonparametric Bayesian model to be used as the prior for the autoregressive process, allowing the effective number of active components to grow, without bound, with the complexity of data. In order to determine the marginal posterior distribution, a convergent algorithm, inspired by the variational Bayes method, is formulated. It employs the minorization-maximization technique to arrive at an iterative, convergent algorithm that approximates the marginal posterior distribution. Both objective and subjective evaluations show advantage over other methods based on the power spectrum. We also apply the method to a music information retrieval task and demonstrate its effectiveness.

Measurements of ocean bottom low-angle backscattering by single-mode reverberation.

The characteristics of scattering due to interface roughness are usually described by the backscattering matrix for reverberation modeling. The backscattering matrix based on the Bass perturbation theory has significant differences from that based on the empirical scattering rule (Lambert's rule), especially at low grazing angles. In a waveguide environment with a point source, it is very difficult to extract the quantitative characteristics of the backscattering matrix at low grazing angles from the experimental data because of the difficulties in acquiring low-grazing-angle scattering data and separating the scattering data between different modes (grazing angles). In contrast, the use of single-mode excitations as sources in shallow-water waveguides enables acquisition of good quality low-grazing-angle scattering data. In this paper, the characteristics of the backscattering matrix were obtained from different single-mode reverberation experiments in shallow-water. The experiments were carried out at different sites during different seasons off the coasts of China. Model-data comparisons were made and the results showed that at low grazing angles (2°-5°), the backscattering matrices based on the Bass perturbation theory were in good agreement with the experimental data, but the backscattering matrices based on Lambert's rule were not.

Requirements, technology, and science drivers of applied reverberation modeling

The historical development of reverberation modeling is a story driven by both supporting and sometimes conflicting features of application requirements, measurement and computing capability, and scientific understanding. This paper presents an overview of how underwater reverberation modeling technology has responded to application needs and how this process has helped the community to identify and resolve related science issues. Emphasis is on the areas of System Design and Acquisition Support, Deployment and Operational Support, and Training Support. Gaps in our scientific knowledge are identified and recent advances are described that help push forward our collective understanding of how to predict and mitigate reverberation.

Reverberation models as an aid to interpret data and extract environmental information

Reverberation measurements obtained with towed arrays are a valuable tool to extract information about the ocean environment. Preston pioneered the use of polar plots to display reverberation and superimpose the beam time series on bathymetry maps. As part of Rapid Environmental Assessment (REA) exercises Ellis and Preston [J. Marine Syst. 78, S359–S371, S372–S381] have used directional reverberation measurements to detect uncharted bottom features, and to extract environmental information using model-data comparisons. One enthusiast declared “This is like doing 100 simultaneous transmission loss runs and having the results available immediately.” Though that was clearly an exaggeration and the results are not precise, the approach provides valuable information to direct more accurate and detailed surveys. The early work used range-independent (flat bottom) models for the model-data comparisons, while current work includes a range-dependent model based on adiabatic normal modes. A model has been developed which calculates reverberation from range-dependent bottom bathymetry, echoes from targets and discrete clutter objects, then outputs beam time series directly comparable with measured ones. Recent work has identified interesting effects in sea bottom sand dunes in the TREX experiments. This paper will provide an overview of the earlier work, and examples from the recent TREX experiment.

Reverberation and biological clutter in continental shelf waveguides

Seafloor reverberation in continental shelf waveguides is the primary limiting factor in active sensing of biological clutter in the ocean for noise unlimited scenarios. The detection range of clutter is determined by the ratio of the intensity of scattered returns from clutter versus the seafloor in a resolution cell of an active sensing system. We have developed a Rayleigh-Born volume scattering model for seafloor scattering in an ocean waveguide. The model has been tested with data collected from a number of Ocean Acoustic Waveguide Remote Sensing (OAWRS) experiments in distinct US Northeast coast continental shelf environments, and has shown to provide accurate estimates of seafloor reverberation over wide areas for various source frequencies. We estimate scattered returns from fish clutter by combining ocean-acoustic waveguide propagation modeling that has been calibrated in a variety of continental shelf environments for OAWRS applications with a model for fish target strength. Our modeling of seafloor reverberation and scattered returns from fish clutter is able to explain and elucidate OAWRS measurements along the US Northeast coast.

Multistatic performance prediction for Doppler-sensitive waveforms in a shallow-water environment

Navies worldwide are now operationally capable of exploiting multistatic sonar technology. One of the purported advantages of multistatics when detecting directional targets should be the increased probability of receiving a strong reflection at one of the multistatic receivers. However, it is not yet clear (or intuitive) how best to deploy multistatic-capable assets to achieve particular mission objectives. The Performance Assessment for Tactical Systems (PATS) software was recently developed by Maritime Way Scientific under contract to Defence Research and Development Canada as a research tool to assist in exploring different approaches to multistatic performance modelling. Beginning with a user-defined environment and sensor layout, PATS uses transmission loss and reverberation model results to calculate signal excess at each grid point in the model domain. Monte Carlo simulations using many realizations of target tracks allow for the calculation of the cumulative probability of detection as a means to assess performance. Results will be presented comparing the shallow-water performance of monostatic and multistatic sensors using frequency-modulated and Doppler-sensitive waveforms as well as omnidirectional and directional targets in a variety of realistic military scenarios.

Environmental measurements collected during TREX13 to support acoustic modeling

The major goal of TREX13 (Target and Reverberation EXperiment 2013) was to quantitatively investigate reverberation with sufficient environmental measurements to support full modeling of reverberation data. The collection of environmental data to support reverberation modeling is usually limited by the large ranges (10s of km) involved, the temporal and spatial variability of the environment and the time variation of towed source/receiver locations within this environment. In order to overcome these difficulties, TREX13 was carried out in a 20 m deep shelf environment using horizontal line arrays mounted on the seafloor. The water depth and well controlled array geometry allowed environmental characterization to be focused on the main beam of the array, i.e., along a track roughly 5 km long and 500 m wide. This talk presents an overview of the efforts made to characterize the sea surface, water column, seafloor, and subbottom along this track to support the modeling of acoustic data collected over the course of the experiment. [Work supported by ONR Ocean Acoustics.]

Modeling and Analysis of Monostatic Seafloor Reverberation from Bottom Consisting of Two Slopes

An incoherent reverberation from a continental shelf was simulated using a propagation model based on ray theory combined with several scattering strength formulas. The concept of an N × 2D reverberation model is used to consider azimuthal and radial dependent bottom. For verification of the reverberation model, the result is compared to consensus solutions of problem XI in Reverberation Modeling Workshop I (RMW I). Subsequently, the model is applied to one of data-inspired problems in Reverberation Modeling Workshop II (RMW II). Scattering strength formulas based on the Kirchhoff approximation (KA) and small slope approximation (SSA) are adapted to be applicable to a von Karman roughness spectral form. The monostatic reverberations on the continental shelf are observed at 200 Hz and 1600 Hz for various scattering cross-sections. The results with SSA scattering cross-section are examined to analyze the features of continental shelf reverberation depending on the sound speed profile and bottom slope, and they are found to affect both the pattern and level of reverberations.

Distant-talking speaker identification by generalized spectral subtraction-based dereverberation and its efficient computation

Abstract Previously, a dereverberation method based on generalized spectral subtraction (GSS) using multi-channel least mean-squares (MCLMS) has been proposed. The results of speech recognition experiments showed that this method achieved a significant improvement over conventional methods. In this paper, we apply this method to distant-talking (far-field) speaker recognition. However, for far-field speech, the GSS-based dereverberation method using clean speech models degrades the speaker recognition performance. This may be because GSS-based dereverberation causes some distortion between clean speech and dereverberant speech. In this paper, we address this problem by training speaker models using dereverberant speech obtained by suppressing reverberation from arbitrary artificial reverberant speech. Furthermore, we propose an efficient computational method for a combination of the likelihood of dereverberant speech using multiple compensation parameter sets. This addresses the problem of determining optimal compensation parameters for GSS. We report the results of a speaker recognition experiment performed on large-scale far-field speech with different reverberant environments to the training environments. The proposed GSS-based dereverberation method achieves a recognition rate of 92.2%, which compares well with conventional cepstral mean normalization with delay-and-sum beamforming using a clean speech model (49.0%) and a reverberant speech model (88.4%). We also compare the proposed method with another dereverberation technique, multi-step linear prediction-based spectral subtraction (MSLP-GSS). The proposed method achieves a better recognition rate than the 90.6% of MSLP-GSS. The use of multiple compensation parameters further improves the speech recognition performance, giving our approach a recognition rate of 93.6%. We implement this method in a real environment using the optimal compensation parameters estimated from an artificial environment. The results show a recognition rate of 87.8% compared with 72.5% for delay-and-sum beamforming using a reverberant speech model.

Computation and accuracy trade-offs in applied reverberation modeling

Reverberation modeling and prediction are a research area of intense interest, often involving large, high resolution databases and advanced numerical techniques, and stretching modern computers to their limits. Nevertheless, applications of underwater acoustic systems often need estimates of reverberation sooner than they can be provided by the research community. What results is that applied models for reverberation, in order to meet the pressing needs with available computer memory and allowable computation times, often employ approximations or shortcuts that fall short of current research modeling standards. This paper traces several of the approximations used in reverberation models in the past in order to meet system design and deployment needs. The results show an inherent conflict between basic research and applied interests. The traditional DOD paradigm is that basic research results transition to the applied world. History suggests the opposite, however: Applied research ignores the torpedoes and moves full speed ahead where needed, then forges the chain used to pull reluctant basic research in the needed direction.