Reverberation Model Research Articles

Simulation model of sound backscattering in gas flares and evaluation of the potential accuracy of determining seep coordinates by split-beam echosounders

A simulation model of sound backscattering in gas flares is presented, designed to solve direct and inverse problems of acoustic sensing of flares using the simulation (computer) modeling method. The model is based on the concept of the discrete character of sound scattering in a gas plume, according to which the received echo signal is the sum of elementary signals, backward-scattered bubbles that form the flare. At the same time, the model of sound reverberation in a gas flare is considered as a temporary random process in the absence of multiple scattering effects. The model takes into account the distributions of gas bubbles by sizes and rise velocities, as well as the evolution of bubbles and their gas exchange with the marine environment during ascent. Based on this, the potential accuracy of deter- mining the coordinates of gaseous sources (seeps) by a split-beam echo sounder was estimated by simulation modeling. It has been established that the main parameters that determine the RMS (root- mean-square deviations) of the estimates from the actual coordinates are the transverse size of the flare near the bottom, the SNR (signal-to-noise ratio), and the volume of echo signal samples. Dependences of the RMS on the determining parameters are obtained.

Differentiable Artificial Reverberation

We propose differentiable artificial reverberation (DAR), a family of artificial reverberation (AR) models implemented in a deep learning framework. Combined with the modern deep neural networks (DNNs), the differentiable structure of DAR allows training loss gradients to be back-propagated in an end-to-end manner. Most of the AR models bottleneck training speed when implemented as is in the time domain and executed with a parallel processor like GPU due to their infinite impulse response (IIR) filter components. We tackle this by further developing a recently proposed acceleration technique, which borrows the frequency-sampling method (FSM). With the proposed DAR models, we aim to solve an artificial reverberation parameter (ARP) estimation task in a unified approach. We design an ARP estimation network applicable to both analysis-synthesis (RIR-to-ARP) and blind estimation (reverberant-speech-to-ARP) tasks. And using different DAR models only requires slightly a different decoder configuration. This way, the proposed DAR framework overcomes the previous methods' limitations of task-dependency and AR-model-dependency.

Mid-frequency sound transmission in a deep ice-covered ocean at short and medium ranges

A model of propagation and reverberation in a deep Arctic ocean and a tentative interpretation of ICEX16 data are discussed. The data subset used for analysis was collected by a NPS team in the Beaufort Sea (∼3650 m depth) on 12 March 2016 using mobile EMATT transmitters which were moving under the ice in circular patterns from Ice Camp SARGO that provided various combinations of source-receiver ranges (∼1–10 km) and depths (45–183 m). Transmitted waveforms were sequences of 60 s-long segments comprised of 57 s-long CW pulses at various frequencies (2800, 2900, and 3000 Hz) and 3 s-long 2300–3000 Hz LFM sweeps. Sound speed profiles measured during the experiment showed two shallow ducts, at 0–75 m and 75–250 m water depths. The model developed considers multi-path propagation including multiple reflections and scattering from rough interfaces. The model-based analysis allows examining the spatial and time-frequency structure of recorded arrivals and their sensitivity to variations in water stratification and acoustic parameters of ice and ocean bottom. Preliminary results of the ICEX16 data-model comparisons are presented. Suggestions are considered for environmental measurements to provide necessary inputs and ground-truth for the acoustic model. A potential for enhanced remote sensing capabilities and optimal configurations for future experiments are discussed. [Work supported by ONR.]

An introduction to Technical Committee on Underwater Acoustics

The Acoustical Society of America Technical Committee on Underwater Acoustics investigates sound propagation in underwater environments such as oceans, lakes, rivers and tanks. Sound waves can travel thousands of kilometers in water and therefore, have been widely used as an effective tool to study the ocean for both economic and defense purposes. It is an interdisciplinary, observation-driven field of study that involves physics, oceanography, marine biology, engineering, and signal processing. In this talk, several representative research areas are presented, including measurement and modeling of sound propagation and reverberation, and ocean ambient sound field monitoring and characterization. A number of large, integrated field experiments are also presented, to illustrate the manner in which observations are designed to pursue scientific questions.

A look at reverberation received on a vertical array during the TREX13 experiment

The TREX13 Target and Reverberation Experiment was conducted in shallow water off Panama City, Florida. Previously published work focused on reverberation from a fixed horizontal array, which resolves scattering in the azimuthal direction, but has very limited information about the vertical dependence of the scattering. Here, we look at the TREX13 datai—LFM pulses of various bandwidths in the 1.8–3.6 kHz frequency range—received on the vertical line arrays (VLAs) to see what information can be gleaned. The depth and time dependence of the reverberation and echo from a target are investigated. Beams are formed at various vertical angles, which should provide information about the grazing angle dependence of the scattering. A complication arises in determining the scattering strength in non-uniform environments, since the reverberation comes from all azimuths, and since the source and vertical array receivers were not co-located. In such bistatic geometry, the energy arriving at any instant comes from different ranges, azimuths, and vertical angles. However, if the source and receiver are close together, at longer times the geometry approximates the simpler monostatic situation. To help interpretation, results are compared with predictions from a range-dependent bistatic reverberation model. [Work supported by ONR, Ocean Acoustics.]

X-ray reverberation models of the disc wind in ultraluminous X-ray source NGC 5408 X−1

ABSTRACT Majority of ultraluminous X-ray sources (ULXs) are believed to be super-Eddington objects, providing a nearby prototype for studying an accretion in supercritical regime. In this work, we present the study of time-lag spectra of the ULX NGC 5408 X−1 using a reverberation mapping technique. The time-lag data were binned using two different methods: time-averaged-based and luminosity-based spectral bins. These spectra were fitted using two proposed geometric models: single and multiple photon scattering models. While both models similarly assume that a fraction of hard photons emitted from inner accretion disc could be downscattered with the super-Eddington outflowing wind becoming lagged, soft photons, they are different by the number that the hard photons scattering with the wind, i.e. single versus multiple times. In case of an averaged spectrum, both models consistently constrained the mass of ULX in the range of ∼80–500 M⊙. However, for the modelling results from the luminosity-based spectra, the confidence interval of the BH mass is significantly improved and is constrained to the range of ∼75–90 M⊙. In addition, the models suggest that the wind geometry is extended in which the photons could downscatter with the wind at the distance of ∼104–10$^{6}\, r_{\rm g}$. The results also suggest the variability of the lag spectra as a function of ULX luminosity, but the clear trend of changing accretion disc geometry with the spectral variability is not observed.

Residual feedback suppression with extended model-based postfilters

When designing closed-loop electro-acoustic systems, which can commonly be found in hearing aids or public address systems, the most challenging task is canceling and/or suppressing the feedback caused by the acoustic coupling of the transducers of such systems. In many applications, feedback cancelation based on adaptive filters is used for this purpose. However, due to computational complexity such a feedback canceler is often limited in the length of the filter’s impulse response. Consequently, a residual feedback, which is still audible and may lead to system instability, remains in most cases. In this work, we present enhancements for model-based postfilters based on a priori knowledge of the feedback path which can be used cooperatively with the adaptive filter-based feedback cancelation system to suppress residual feedback and improve the overall feedback reduction capability. For this, we adapted an existing reverberation model such that our model additionally considers the presence and the performance of the adaptive filter. We tested the effectiveness of our approach by means of both objective and subjective evaluations.

A Hard Look at Relativistic Reverberation in MCG-5-23-16 and SWIFT J2127.4+5654: Testing the Lamppost Model

Abstract X-ray reverberation mapping has emerged as a new tool to probe accretion in active galactic nuclei (AGN), providing a potentially powerful probe of accretion at the black hole scale. The lags, along with relativistic spectral signatures are often interpreted in light of the lamppost model. Focusing specifically on testing the prediction of the relativistic reverberation model, we have targeted several of the brightest Seyfert Galaxies in X-rays with different observing programs. Here, we report the results from two large campaigns with NuSTAR targeting MCG-5-23-16 and SWIFT J2127.4+5654 to test the model predictions in the 3–50 keV band. These are two of three sources that showed indications of a delayed Compton hump in early data. With triple the previously analyzed exposures, we find no evidence for relativistic reverberation in MCG-5-23-16, and the energy-dependent lags are consistent with a log-linear continuum. In SWIFT J2127.4+5654, although a continuum-only model explains the data, the relativistic reverberation model provides a significant improvement to the energy and frequency-dependent lags, but with parameters that are not consistent with the time-averaged spectrum. This adds to mounting evidence showing that the lag data is not consistent with a static lamppost model.

Horizontal Correlation of Long-Range Bottom Reverberation in Shallow Sloping Seabed

The performance of active sonar detection systems is seriously affected by the reverberation at the bottom of the waveguide in shallow water. In order to improve the performance of active sonar detection, it is necessary to understand the horizontal correlation of shallow-water bottom reverberation in active towed-array processing technology. However, the current research on the spatial correlation of reverberation is mainly based on vertical correlation, little work has been done on the horizontal correlation characteristics of long-distance seabed reverberation, and there is no support from sea test data. In this paper, the coupled mode reverberation model is applied to the horizontal correlation, and is studied according to the receiving position, time, and frequency. The simulation results show that, for the long-range bottom reverberation, the lateral correlation is greater than the longitudinal correlation in the horizontal space. By introducing the adiabatic mode solution, the mathematical model of horizontal correlation in the range-dependent waveguide with depth is derived. The numerical results show that the influence of the seabed dip angle on the horizontal correlation should be considered and that the horizontal correlation is affected obviously by the propagation effects of the sloped sea floor. Finally, the experimental data processing and analysis are given and verify the correctness of the algorithm.

Reverberation Noise Suppression in Ultrasound Channel Signals Using a 3D Fully Convolutional Neural Network.

Diffuse reverberation is ultrasound image noise caused by multiple reflections of the transmitted pulse before returning to the transducer, which degrades image quality and impedes the estimation of displacement or flow in techniques such as elastography and Doppler imaging. Diffuse reverberation appears as spatially incoherent noise in the channel signals, where it also degrades the performance of adaptive beamforming methods, sound speed estimation, and methods that require measurements from channel signals. In this paper, we propose a custom 3D fully convolutional neural network (3DCNN) to reduce diffuse reverberation noise in the channel signals. The 3DCNN was trained with channel signals from simulations of random targets that include models of reverberation and thermal noise. It was then evaluated both on phantom and in-vivo experimental data. The 3DCNN showed improvements in image quality metrics such as generalized contrast to noise ratio (GCNR), lag one coherence (LOC) contrast-to-noise ratio (CNR) and contrast for anechoic regions in both phantom and in-vivo experiments. Visually, the contrast of anechoic regions was greatly improved. The CNR was improved in some cases, however the 3DCNN appears to strongly remove uncorrelated and low amplitude signal. In images of in-vivo carotid artery and thyroid, the 3DCNN was compared to short-lag spatial coherence (SLSC) imaging and spatial prediction filtering (FXPF) and demonstrated improved contrast, GCNR, and LOC, while FXPF only improved contrast and SLSC only improved CNR.

A variable corona for GRS 1915+105

ABSTRACT Most models of the low-frequency quasi-periodic oscillations (QPOs) in low-mass X-ray binaries (LMXBs) explain the dynamical properties of those QPOs. On the other hand, in recent years reverberation models that assume a lamp-post geometry have been successful in explaining the energy-dependent time lags of the broad-band noise component in stellar mass black holes and active galactic nuclei. We have recently shown that Comptonization can explain the spectral-timing properties of the kilo-hertz (kHz) QPOs observed in neutron star (NS) LMXBs. It is therefore worth exploring whether the same family of models would be as successful in explaining the low-frequency QPOs. In this work, we use a Comptonization model to study the frequency dependence of the phase lags of the type-C QPO in the BH LMXB GRS 1915+105. The phase lags of the QPO in GRS 1915+105 make a transition from hard to soft at a QPO frequency of around 1.8 Hz. Our model shows that at high QPO frequencies a large corona of ∼100–150 Rg covers most of the accretion disc and makes it $100{{\ \rm per\ cent}}$ feedback dominated, thus producing soft lags. As the observed QPO frequency decreases, the corona gradually shrinks down to around 3–17 Rg, and at 1.8 Hz feedback on to the disc becomes inefficient leading to hard lags. We discuss how changes in the accretion geometry affect the timing properties of the type-C QPO.

Disk, Corona, Jet Connection in the Intermediate State of MAXI J1820+070 Revealed by NICER Spectral-timing Analysis

Abstract We analyze five epochs of Neutron star Interior Composition Explorer (NICER) data of the black hole X-ray binary MAXI J1820+070 during the bright hard-to-soft state transition in its 2018 outburst with both reflection spectroscopy and Fourier-resolved timing analysis. We confirm the previous discovery of reverberation lags in the hard state, and find that the frequency range where the (soft) reverberation lag dominates decreases with the reverberation lag amplitude increasing during the transition, suggesting an increasing X-ray emitting region, possibly due to an expanding corona. By jointly fitting the lag-energy spectra in a number of broad frequency ranges with the reverberation model reltrans, we find the increase in reverberation lag is best described by an increase in the X-ray coronal height. This result, along with the finding that the corona contracts in the hard state, suggests a close relationship between spatial extent of the X-ray corona and the radio jet. We find the corona expansion (as probed by reverberation) precedes a radio flare by ∼5 days, which may suggest that the hard-to-soft transition is marked by the corona expanding vertically and launching a jet knot that propagates along the jet stream at relativistic velocities.

Surface reverberation based on small-slope approximation in deep water

For the reverberation experiments with a near surface source or receiver, surface reverberation arriving first after source signal is not influenced by bottom reverberation due to time-delay difference of acoustic multipath in deep water. A surface reverberation model is proposed in the paper. The Green’s function of sound propagation is described by the ray theory, and first-order small-slope approximation is employed for the surface scattering from full angle. The effect of bubbles scattering is also considered to get the surface reverberation theory. The reverberation model is verified by comparing the simulation results with the experimental data. The measured data show that the decaying rate of surface reverberation intensity decreases with the frequency increasing. Numerical calculation demonstrates that the frequency dependence is caused by the positive correlation between scattering strength and frequency, and the surface reverberation at low frequency for low sea state is dominated by the scattering from rough air-sea interface. Moreover, the reverberation data from experiment show that the surface reverberation is not sensitive to the change of receiving depth. A method of inversion for the two surface-wave spectral parameters in the model is achieved based on the reverberation model. The inversion results verify that the spectral parameters of rough surface can be obtained from surface reverberation data on the premise of the wind speed parameter. As a result, the scattering properties of rough interface will be obtained.

A Depth-Bistatic Bottom Reverberation Model and Comparison with Data from an Active Triplet Towed Array Experiment

In this paper, a depth-bistatic bottom reverberation model that employs the ray theory is presented. The model can be applied to an active towed array in the ocean. The reverberation time series are modeled under the depth-bistatic assumption and their Doppler shift is calculated based on the actual source–receiver geometry. This model can handle N × 2D range-dependent bathymetry, the geometry of a triplet array, and the Doppler motion of the source, targets, and receiver. The model predictions are compared with the mid-frequency reverberation data measured by an active triplet towed array during August 2015 in the East Sea, Korea. These data are collected with a variable depth source at mid-frequency and the triplet line array in a deep-water environment. Model predictions of the beam time series and its spectrogram are in good agreement with the measurement. In particular, we discuss the effects of the source and receiver depths on the reverberation in deep water observed in both the measured and modeled results.

X-ray variability analysis of a large series of XMM–Newton +NuSTAR observations of NGC 3227

ABSTRACT We present a series of X-ray variability results from a long XMM–Newton + NuSTAR campaign on the bright, variable AGN NGC 3227. We present an analysis of the light curves, showing that the source displays typically softer-when-brighter behaviour, although also undergoes significant spectral hardening during one observation which we interpret as due to an occultation event by a cloud of absorbing gas. We spectrally decompose the data and show that the bulk of the variability is continuum-driven and, through rms variability analysis, strongly enhanced in the soft band. We show that the source largely conforms to linear rms-flux behaviour and we compute X-ray power spectra, detecting moderate evidence for a bend in the power spectrum, consistent with existing scaling relations. Additionally, we compute X-ray Fourier time lags using both the XMM–Newton and – through maximum-likelihood methods – NuSTAR data, revealing a strong low-frequency hard lag and evidence for a soft lag at higher frequencies, which we discuss in terms of reverberation models.

Testing The Lamp-Post and Wind Reverberation Models with XMM-Newton Observations of NGC 5506.

The lamp-post geometry is often used to model X-ray data of accreting black holes. Despite its simple assumptions, it has proven to be powerful in inferring fundamental black hole properties such as the spin. Early results of X-ray reverberations showed support for such a simple picture, though wind-reverberation models have also been shown to explain the observed delays. Here, we analyze new and old XMM-Newton observations of the variable Seyfert-1 galaxy NGC 5506 to test these models. The source shows an emission line feature around 6.7 keV that is delayed relative to harder and softer energy bands. The spectral feature can be modeled with either a weakly relativistic disk line or by scattering in distant material. By modeling both the spectral and timing signatures, we find that the reflection fraction needed to explain the lags is larger than observed in the time-averaged spectrum, ruling out both a static lamp-post and simple wind reverberation models.

Sparse spatial spectral estimation with heavy sea bottom reverberation in the fractional fourier domain

In shallow water, the target bearing estimation performance of active detection methods can be degraded, when the underwater scene is complicated by the presence of heavy sea bottom reverberation. To address this problem, this paper proposes an improved approach. Based on the principle of fractional Fourier transform (FrFT) on a linear frequency modulation signal, reverberation model for active detection in shallow water is established in the fractional Fourier (FrF) domain. Based on the characteristics of high peak-signal-to-reverberation ratio (PSRR), an approach of sparse spatial spectral estimation based on singular value decomposition is proposed. To verify the performances of reverberation suppression and high resolution of the proposed approach, target bearing estimation simulations are conducted with simulated heavy sea bottom reverberation, and the effect of the signal-to-noise ratio (SNR) on the detection probability of the proposed method is provided. An experiment was conducted in a harbor with heavy sea bottom reverberation, and the proposed approach was applied to the experimental data. Compared with conventional methods, the proposed method not only achieves the target bearing with high resolution, but also suppresses the reverberation considerably. Furthermore, the average value of the spatial spectrum of the sea bottom reverberation is reduced by a factor of 5.3.

Reverberation mapping of the disc wind in ultraluminous X-ray source NGC 5408 X-1

Recent observational evidences suggested that ultraluminous X-ray sources (ULXs) which are in supercritical accretion rate might release the outflowing wind and change the geometry to slim disc. However, the exact structure of the disc is still unclear. In this work, we develop a simple reverberation model to analyse the X-ray time lags of the ULX NGC 5408 X-1, in order to determine the geometry of the disc and the outflowing wind. The data of NGC 5408 X-1 from the XMM-Newton archive are binned into three groups based on the instrument count rates, i.e. low, medium and high count rates. The lag profiles are then extracted from each group. Fitting the models to the lag data suggests that the wind-launching radius tends to increase with the count rates, which is in agreement with the super-Eddington framework that prefers a stellar mass black hole (sMBH). The wind-launching radius for the sMBH is constrained to be in the order of ∼ 105rg. We also report a degeneracy between the black hole mass and the wind launching radius. A more realistic geometry and model might be required to explain the time lags in NGC 5408 X-1.

A Space-Time Reverberation Model for Moving Target Detection

In recent years, moving target detection methods based on low-rank and sparse matrix decomposition have been developed, and they have achieved good results. However, there is not enough interpretation to support the assumption that there is a high correlation among the reverberations after each transmitting pulse. In order to explain the correlation of reverberations, a new reverberation model is proposed from the perspective of scattering cells in this paper. The scattering cells are the subarea divided from the detection area. The energy fluctuation of a scattering cell with time and the influence of the neighboring cells are considered. Key parameters of the model were analyzed by numerical analysis, and the applicability of the model was verified by experimental analysis. The results showed that the model can be used for several simulations to evaluate the performance of moving target detection methods.

Reverberation Intensity Decaying in Range-Dependent Waveguide

In this paper, an analytic range-independent reverberation model based on the first-order perturbation theory is extended to range-dependent waveguide. This model considers the effect of bottom composite roughness: small-scale bottom rough surface provides dominating energy for reverberation, whereas large-scale roughness has the effect of forward and back propagation. For slowly varying bottom and short signal pulse, analytic small-scale roughness backscattering theory is adapted in range-dependent waveguides. A parabolic equation is used to calculate Green functions in range-dependent waveguides, and the orthogonal property of local normal modes is employed to estimate the modal spectrum of PE field. Synthetic tests demonstrate that the proposed reverberation model works well, and it can also predict the reverberation of range-independent waveguide as a special case.