Broadband time-reversing array retrofocusing in noisy environments.

Active acoustic time reversal is a technique for focusing sounds recorded in complex unknown environments back to their remote point(s) of origin. It can be accomplished with a transducer array—a time-reversing array (TRA)—that sends and receives sound. Nearly all prior work on TRA performance has involved stationary arrays. This letter describes how random array deformation influences TRA retrofocusing in shallow ocean environments. For harmonic signals, randomly drifting array elements degrade TRA performance by ∼20% when the average horizontal wavenumber times the root-mean-square horizontal element displacement approaches 0.5. TRA focusing should be less sensitive to vertical element drift.

Time reversal acoustic (TRA) focusing allows concentration of elastic energy at a location in the soil being investigated to detect landmines. The TRA process is conducted by broadcasting a wide bandwidth signal and recording the surface vibration by a Laser Doppler Vibrometer (LDV). The system impulse response from speaker to the LDV output can then be computed by cross correlating the original and recorded signals for each channel. Each transducer re-radiates the time reversal impulse response. This provides efficient focusing of the seismic wave in both space and time, thus enhancing the nonlinear effects associated with soil and landmine vibrations. Using orthogonal initial signals the suggested TRA procedure can be implemented simultaneously with multiple transmitters to increase the scanning speed. The nonlinear effects were investigated using a phase inversion method where the TRA signal is broadcast a second time with an opposite sign and the two received signals are added in post processing. The summed signal contains mainly the results of nonlinear wave interaction and tends to cancel the linear response. Small scale land mine detection experiments were conducted using a laser Doppler vibrometer and an array of speakers in the frequency band 50-500Hz. They demonstrate that the TRA system provides high concentration of elastic wave energy in the tested area. The measurements of spectral density of the TRA focused signal reveal increased spectral density in the vicinity of mine resonance frequencies. The nonlinear TRA phase inversion method shows higher contrast between mine and no mine than the linear TRA method.

Time reversal acoustics is based on the time reversal symmetry of the acoustic fields. A detailed description of the acoustic field equation showing the time reversal symmetry property of S the solution is given. A geometric structure of a metamaterial for both an electromagnetic wave and acoustic wave is given. Then, the geometric structure of the metamaterial to implement time reversal acoustics is given. Time reversal acoustics has been successfully applied to non-destructive testing, medical ultrasound imaging, and underwater acoustics. The advantage of using metamaterial in time reversal acoustics is that it supports modes which radiate spatial information of the near field of a source efficiently in the far field.

: Acoustic time reversal is of interest to the United States Navy because it is an effective means for focusing sound in unknown underwater environments. For the last nine years, the Office of Naval Research has supported theoretical and numerical investigations at the University of Michigan into the performance of time reversing arrays in the ocean. This report summarizes the research accomplishments of these investigations.

In this work a novel approach called peak detection and boundary identification (PD-BI) for period detection of heart sound (HS) signals in noisy environment is proposed. The PD method is inspired by a natural phenomenon that a horizontal soft string hits peaks of uneven ground line when falling from a high position. The BI method is based on the quasi-cyclic feature of HS. The simulation results validate the effectiveness and accuracy of PD-BI.Keywordsheart soundperiod detectionpeak detectionboundary identification

There are numerous medical applications of ultrasound radiation force (RF) which could be made more effective using the time reversal acoustics (TRA) principles. This paper gives an overview of research into physical and technical bases of RF generation in heterogeneous biological media using TRA focusing systems. A custom-designed compact multichannel TRA system for receiving, digitizing, storing, time reversing, and transmitting acoustic signals in a wide frequency range from 0.01 to 10 MHz has been developed and extensively tested in model systems and ex vivo tissues and bones. Shear strain and shear waves remotely induced in soft tissues and bones by radiation force were detected using various acoustical and optical means. Experimental studies fully confirmed the feasibility of TRA generation of RF and demonstrated several advantages over conventional means of remotely inducing shear stress in biological media. These advantages include a possibility to create highly localized (close to diffraction limit) shear stress in heterogeneous media stir focused ultrasound beam in 3-D volume using very simple hardware. [Work supported by NIH grant.]

This chapter contains sections titled: Introduction Theory of Time-Reversal Acoustics Application of TR to Medical Ultrasound Imaging Application of Time-Reversal Acoustics to Ultrasonic Nondestructive Testing Application of TRA to Landmine or Buried Object Detection Application of Time-Reversal Acoustics to Underwater Acoustics References

Among structural health monitoring techniques, Lamb waves is frequently used as diagnostic tools to detect damage in plate-like structures. Lamb waves usually excited by piezoelectric transducers essentially involve exciting the structure with high frequency guided-waves and processing the difference in structural response with respect to a baseline signal for the pristine condition to detect damage. As a kind of baseline free method, time reversal (TR) method applied to ultrasonic detection and focusing with arrays of transducers has been proposed. The decomposition of the time reversal operator (DORT) method is a selective detection and focusing technique using an array of transmit-receive transducers. Unlike body waves, the propagation of Lamb waves is complicated due to their dispersive and multimode characteristics. So Lamb wave is hard to locate different scatterers on the plate using DORT method because of the asymmetry of the time reversal transfer matrix. In this paper, a proposed approach based on the DORT method is developed to distinguish and locate two simulated damages on the aluminum plate using Lamb wave through finite element simulation on the commercial finite element code ANSYS platform. The behavior of reflected waves is analyzed by studying the eigenvalues and eigenvectors of the time reversal transfer matrix, showing that the number of significant eigenvalues is not exactly the number of damage targets and two significant eigenvalues correspond to one target. This method, which uses PZT array and operates under multi-modes pulse-echo mode, can estimate the position of the damaged zone in 2D image by numerically backpropagating selective eigenvector using the S0 and/or A0 Lamb wave propagation analytical solution.

We investigate some of the limitations of time reversal acoustics (TRA) in solid media with transducers attached to the surface. In particular, we consider the limitations due to the finite size of the transducers and elastic wave propagation. Using a theoretical approach, numerical simulations and validation from laboratory ultrasound experiments, we find that finite size transducers and the existence of longitudinal and shear waves play significant roles in perturbing the time reversal process. Despite these limitations, we show that TRA in solids is very robust, providing the means to reconstruct the main features of the source signal. The analysis of TRA retro-focusing properties in solid specimens is of foremost importance for the development of new non-destructive evaluation techniques.

In percutaneous procedures there is often a need to focus therapeutic ultrasound to a predefined area without affecting surrounding tissues. Focusing based on Time Reversal Acoustics (TRA) principles constitutes a promising approach for generating high intensity ultrasound field tailored to the shape of the predefined area. Conventional TRA technique enables ultrasound focusing only at a site, where there is an ultrasound beacon, e.g. piezo-transducer mounted at the tip of a catheter. We developed a method of steering the focus away from the beacon location. The method is based on the measurements of impulse response (IR) in several reference points and calculating virtual IRs for the points outside the reference beacon location. The IR for the point away from the beacon is constructed based on mathematical extrapolation of the measured reference IRs frequency spectra, particularly phases. The effectiveness of extrapolated TRA focusing is explored experimentally and by computer simulation. Potential applications include ultrasounda-ssisted drug delivery, artery recanalization and tumor ablation.

The pitch of voiced speech sounds provides very important information in speech analysis. Pitch estimation is a difficult task when unprevented noise exists. However experimental results have shown that even robust pitch detection techniques fail in noisy environment with periodic patterns such as noise generated by machines. Wavelet transform, with its special properties in time frequency relation, can be used to detect pitch with remarkable advantage in noise resistance. In wavelet signal analysis, the modulus of the transform have been used extensively, however, we found that the phase information is equally important especially for pitch detection. Since the phase spectrum is always intensive to noise, a more promising pitch period can be obtained from the phase diagram. Properties of the phase pattern in wavelet transform are investigated and the result is applied to construct a robust pitch detector. In our first test, the detector is employed to detect the pitches of a set of speech signals with white noise. We found that our approach clearly outperforms other non-wavelet methods with low signal-to-noise ratio. Sinusoidal noise with different frequency levels is used in the second test. Simulation results have shown that our system works quite stable in such an environment.

To address the challenge of quickly and accurately identifying the characteristic parameters of complex power quality disturbance signals in noisy environments, a parameter identification method for atomic decomposition based on two-stage search is proposed by partitioning the damped sinusoidal atom library according to different types of disturbances. In the coarse search, the fundamental component is separated from the disturbance signal, and then the rough values of the start time, frequency, phase, attenuation coefficient, and amplitude of the residual disturbance signal are quickly obtained. In the fine search, a pseudo-Newton method is used to improve the search strategy of the matching pursuit (MP) algorithm, optimizing the disturbance characteristic parameter values determined by the coarse search to obtain the best atoms matching the disturbance type. The atom library includes fundamental, quasi-fundamental, impulse, fluctuation, oscillation, and harmonic. To confirm the effectiveness of the presented approach, parameter identification is conducted on different types of composite disturbances in noisy environments. Simulation findings demonstrate that the presented approach ensures parameter identification accuracy while providing good real-time performance and noise resistance.

Event Abstract Back to Event Noise Reduction of DC Measurements based on Neuronal Network Data Processing Wolfgang Mueller1, Silvia Knappe-Grueneberg1, Rainer Koerber1, Heike Richter1, Tilmann Sander1*, Frank Wiekhorst1, Martin Burghoff1, Tino Gablenz2 and Lutz Herrmann3 1 Physikalisch-Technische Bundesanstalt, Germany 2 BioMed Jena, Germany 3 University of Applied Science Jena, Department of Medical Engineering, Germany Measurements of biomagnetic signals below 0.1 Hz are compromised by an increased noise level for two reasons. The contribution of the environmental magnetic noise is much higher than for high frequencies. And the shielding efficiency of magnetically shielded rooms is inferior. We present a new tool to improve the signal-to-noise ratio of those measurements by almost factor 100. One or more reference sensors are positioned far from the signal source of interest, so that any source input into the reference sensors is negligible. In order to pick up all magnetic noise components a 3 axis SQUID (or fluxgate) sensor is recommended as reference. In contrast to conventional gradiometer techniques no delicate sensor balancing is needed and no signal amplitude losses occur. Due to the short processing time the filter can operate in real time and is suitable for multi channel systems. All measurements start with a training period only recording the noise. Based on these data, the filter “learns” a functional relation between the noise in each sensor signal and each reference signal. After that, in the measurement phase, the noise is subtracted by using these filter functions. The deduced functions vary according to the amount of unwanted interference signals, i.e. urban activity generates low frequency signals according to the time of day. Firstly, we demonstrate the quality of our tool by detecting two well-known technical signals in noisy environment. Then, we show that this tool enables near dc MEG measurements with a conventional SQUID setup in a shielded room with only two permalloy layers. As another application we improve relaxation measurements of magnetic nanoparticles (MRX) accumulated in tissue samples in a noisy environment by using this technique. Thus, our neural network based filtering opens up new measurement possibilities in a broad range of magnetic dc phenomena. Conference: Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010. Presentation Type: Poster Presentation Topic: Instrumentation and Multi-modal Integrations: MEG, Low-field MRI,EEG, fMRI,TMS,NIRS Citation: Mueller W, Knappe-Grueneberg S, Koerber R, Richter H, Sander T, Wiekhorst F, Burghoff M, Gablenz T and Herrmann L (2010). Noise Reduction of DC Measurements based on Neuronal Network Data Processing. Front. Neurosci. Conference Abstract: Biomag 2010 - 17th International Conference on Biomagnetism . doi: 10.3389/conf.fnins.2010.06.00413 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 09 Apr 2010; Published Online: 09 Apr 2010. * Correspondence: Tilmann Sander, Physikalisch-Technische Bundesanstalt, Berlin, Germany, tilmann.sander-thoemmes@ptb.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Wolfgang Mueller Silvia Knappe-Grueneberg Rainer Koerber Heike Richter Tilmann Sander Frank Wiekhorst Martin Burghoff Tino Gablenz Lutz Herrmann Google Wolfgang Mueller Silvia Knappe-Grueneberg Rainer Koerber Heike Richter Tilmann Sander Frank Wiekhorst Martin Burghoff Tino Gablenz Lutz Herrmann Google Scholar Wolfgang Mueller Silvia Knappe-Grueneberg Rainer Koerber Heike Richter Tilmann Sander Frank Wiekhorst Martin Burghoff Tino Gablenz Lutz Herrmann PubMed Wolfgang Mueller Silvia Knappe-Grueneberg Rainer Koerber Heike Richter Tilmann Sander Frank Wiekhorst Martin Burghoff Tino Gablenz Lutz Herrmann Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

: The overall long-term goal for this project is to develop engineering tools that are useful to the Navy as it operates in uncertain, partially known, or unknown ocean environments. During the last year, this project has focused on further determining the utility of a fully-passive propagation-physics-based technique for blind deconvolution of array-recorded sounds from a remote source with emphasis on determining how sparse-array measurements might be used for this task. The long term goals of this project are: i) to determine the effectiveness of synthetic time reversal (STR) for the purposes of blind deconvolution in noisy unknown ocean sound channels, ii) to effectively apply STR to marine mammal sounds recorded in the ocean with vertical and/or horizontal arrays, and iii) to utilize the STR-estimated signals and ocean-sound-channel impulse responses to classify, localize, and/or track individual marine mammals (or other sound sources) of interest. Since early 2009 this project has focused on developing an acoustic-ray-based version of synthetic time reversal (STR), a fully-passive technique for recovering the original signal and the source-to-arrayelement impulse responses for a remote unknown sound source in an unknown underwater waveguide [1-4]. The specific objectives are to: a) determine STR performance as a function of the signal-tonoise, array size, and array element number using acoustic propagation simulations, b) verify these findings with simple airborne- or water-borne acoustic laboratory experiments involving multiple receivers and multiple ray paths, c) obtain and process underwater array recordings of remote-butcooperative sound sources, and d) obtain and process marine mammal vocalizations for the purposes of marine mammal localization, tracking, and identification. This research effort extends the prior modebased version of STR [1] to higher frequencies, smaller receiving arrays, sparse receiving arrays, and sound channels with modal dispersion

This paper presents an interdisciplinary review of the correlation properties of random wavefields. We expose several important theoretical results of various fields, ranging from time reversal in acoustics to transport theory in condensed matter physics. Using numerical simulations, we introduce the correlation process in an intuitive manner. We establish a fruitful mapping between time reversal and correlation, which enables us to transpose many known results from acoustics to seismology. We show that the multiple-scattering formalism developed in condensed matter physics provides a rigorous basis to analyze the field correlations in disordered media. We discuss extensively the various factors controlling and affecting the retrieval of the Green's function of a complex medium from the correlation of either noise or coda. Acoustic imaging of complex samples in the laboratory and seismic tomography of geologic structures give a glimpse of the promising wide range of applications of the correlation method.