Tone-burst Signal Research Articles

Use of Nonlinear Frequency Modulated Signals for the Enhancement of Subharmonic Response from Contrast Microbubbles

Ultrasound imaging with the subharmonic component from contrast microbubbles provide improved CTR (Contrast-to-Tissue Ratio), however it is susceptible to the low amplitude of the subharmonic component. In this simulation study, NLFM (Nonlinear Frequency Modulated) signals are proposed in order to enhance the subharmonic response from microbubbles. NLFM signals having fractional bandwidths of 10, 20, and 40% with up and down sweeps were used as excitation. The performance of NLFM signals were compared with the reference tone-burst and LFM (Linear Frequency Modulated) signals. The results show that the ultrasound contrast microbubbles can produce subharmonic response which is dependent on the applied signal pressure and bandwidth. It is observed that the subharmonic component of the scattered NLFM signal is 3.2dB higher than the LFM signal, whereas it is 9dB higher compared to the sinusoidal tone-burst signal. The results are also presented which show that the up and down sweeps NLFM signals performed better than the LFM signals at the same acoustic pressure and bandwidth.

Vibro-acoustic stimulating ultrasonic guided waves in long bone

Ultrasonic guided wave is sensitive to waveguide microstructure and material property, which has great potential applications in long cortical bone evaluation. Due to the multimodal dispersion effect, low-frequency guided wave is usually used to avoid multimode overlapping and simplify the signal processing. However, the traditional low-frequency ultrasound transducer is usually designed on a large-scale (around several millimeters), leading to relatively low-spatial resolution. In response to such a technique limit, an ultrasound-stimulated vibro-acoustic method is introduced to excite low-frequency ultrasonic guided waves. There are two excitation ways of the ultrasound-stimulated vibro-acoustic method, i.e., a single amplitude-modulated (AM) beam and confocal beam excitation. In the case of the single beam excitation, a high-frequency signal is modulated by using a low-frequency amplitude. In addition, low-frequency vibration can also be produced by a confocal transducer, where two beams are close to the center frequency and focus on a small region. In this way, the frequency difference between two beams can be selected to generate the arbitrary low-frequency excitation in a given bandwidth on the focus point. In this paper, we first introduce the theory of ultrasonic guided wave in the plate and the basic principle of ultrasound-stimulated acoustic emission. Second, the three-dimensional finite element method is used to simulate the phenomena of the low-frequency ultrasonic guided waves excited by the ultrasound-stimulated vibro-acoustic method. Two Gaussian-function enveloped tone-burst signals close to the center frequencies of 5 MHz are used to excite 150 kHz low-frequency guided wave in a 3 mm-thick bone plate. An ex-vivo bovine bone plate is involved in the experiments to test the feasibility of the proposed method. The axial transmission ultrasonic guided waves are recorded at eight different propagation distances. The time-frequency representation method is used to analyze the dispersive guided waves. The results indicate that both the two confocal beams and the single AM beam are capable of stimulating low-frequency ultrasonic guided waves in the bone plate. The first two fundamental guided wave modes, i.e., symmetrical S0 and asymmetrical A0 are observed in the bone plate. Similar spectrum can be obtained in the two different excitation ways. In the simulation and experiment, two wave packets can be separated in the distance-time diagram of the received signals. Good agreement can be found between the results of time-frequency representation and the theoretical group dispersion curves. This study can enhance the spatial resolution of measuring ultrasonic guided wave in long bone, and improve the flexibility of excitation with arbitrary frequency in a given bandwidth. The study can be helpful for developing the new clinical techniques of using low-frequency guided waves for long cortical bone assessment.

광음향 영상화를 위한 PMN-PZT 바늘형 수중청음기 제작

수 MHz의 초음파를 이용하는 저주파 광음향 영상장치에 적용하는 것을 목적으로 압전단결정 PMN-PZT를 사용한 바늘형 수중청음기를 설계 제작하고, 그 특성을 수신감도가 알려져 있는 상용 PVDF(Polyvinylidene Fluoride) 수중청음기와 비교하여 평가하였다. 설계한 수중청음기의 임펄스응답을 KLM 모델에 의해 시뮬레이션한 결과, <TEX>$50{\Omega}$</TEX>의 종단 임피던스에 걸리는 최대 전압을 기준으로 한 수신감도는 -261.6 dB re <TEX>$1V/{\mu}Pa$</TEX>이며, 2 ~ 12 MHz 대역에서 5 dB 이내의 비교적 평탄한 특성을 가지는 것으로 예측되었다. 제작한 수중청음기의 수신감도를 순음 펄스를 사용하여 측정한 결과, 측정 가능한 2 ~ 8 MHz 대역에서 상용의 수중청음기에 비해 평균 10.9 dB 높게 나타났으며, 그 값은 <TEX>$-255.8{\pm}2.8$</TEX> dB re <TEX>$1V/{\mu}Pa$</TEX>이었다. 나아가, 제작한 수중청음기를 기계주사형 광음향 영상장치에 적용하여 머리카락에 대한 영상을 획득하였는바, 수신된 광음향 신호가 상용의 것보다 크고, 영상 또한 우수함을 알았다. For application to several MHz photoacoustic imaging systems, a needle hydrophone was designed and fabricated by using PMN-PZT piezoelectric single crystal, and its characteristics were evaluated through comparison with a commercial PVDF(Polybinylidene Fluoride) hydrophone of which receiving sensitivity is known. The simulation using the KLM model results show that the peak receiving impulse response for <TEX>$50{\Omega}$</TEX> terminating impedance of the fabricated hydrophone is -261.6 dB re <TEX>$1V/{\mu}Pa$</TEX> and the frequency response is relatively flat over 2 ~ 12 MHz with fluctuation less than 5 dB. The measurement results using tone burst signals also show that it has higher (ave. 10.9 dB) sensitivity than the commercial hydrophone in 2 ~ 8 MHz, and the receiving sensitivity of <TEX>$-255.8{\pm}2.8$</TEX> dB re <TEX>$1V/{\mu}Pa$</TEX> was measured for the fabricated hydrophone. In addition, it is known that the photoacoustic signals and the image of a hair obtained by a mechanical scanned photoacoustic imaging system with the fabricated hydrophone were bigger and better than those obtained with the commercial hydrophone.

Analysis of nonlinear frequency mixing in 1D waveguides with a breathing crack using the spectral finite element method

A breathing crack, due to its bilinear stiffness characteristics, modifies the frequency spectrum of a propagating dual-frequency elastic wave, and gives rise to sidebands around the probing frequency. This paper presents an analytical-numerical method to investigate such nonlinear frequency mixing resulting from the modulation effects induced by a breathing crack in 1D waveguides, such as axial rods and the Euler–Bernoulli beams. A transverse edge-crack is assumed to be present in both the waveguides, and the local flexibility caused by the crack is modeled using an equivalent spring approach. A simultaneous treatment of both the waveguides, in the framework of the Fourier transform based spectral finite element method, is presented for analyzing their response to a dual frequency excitation applied in the form of a tone-burst signal. The intermittent contact between the crack surfaces is accounted for by introducing bilinear contact forces acting at the nodes of the damage spectral element. Subsequently, an iterative approach is outlined for solving the resulting system of nonlinear simultaneous equations. Applicability of the proposed method is demonstrated by considering several test cases. The existence of sidebands and the higher order harmonics is confirmed in the frequency domain response of both the waveguides under investigation. A qualitative comparison with the previous experimental observations accentuates the utility of the proposed solution method. Additionally, the influence of the two constituent frequencies in the dual frequency excitation is assessed by varying the relative strengths of their amplitudes. A brief parametric study is performed for bringing out the effects of the relative crack depth and crack location on the degree of modulation, which is quantified in terms of the modulation parameter. Results of the present investigation can find their potential use in providing an analytical-numerical support to the studies geared towards the inverse analyses based on nonlinear wave-damage interactions.

Embedded Damage Localization Subsystem Based on Elastic Wave Propagation

AbstractThis article presents an embedded signal processing subsystem constituting a part of a whole structural health monitoring system (SHM). Typical SHM system is responsible for elastic wave generation and sensing, signal acquisition, and signal processing. Signal processing subsystem was designed with the aim of localizing damage utilizing elastic wave propagation in the interrogated structure. The embedded signal processing subsystem is realized in a field programmable gate array chip, which also implements a damage localization algorithm designed for creating damage maps that can indicate elastic wave reflection sites within the investigated structure. Elastic waves are generated and received using a prototype electronic system developed specially for this purpose. Piezoelectric transducers are arranged in networks with different geometrical configurations (strip, cross, and square). Elastic waves are excited by a five‐cycle tone burst signal with carrier frequency of 220 kHz. The investigated structure is a simple isotropic panel made out of aluminum alloy. First, dispersion curves are computed on the basis of registered elastic wave signals. These are subsequently used in the damage localization process. The damage localization process utilizes the base antisymmetric A0 mode. This article presents results of experimental verification of the developed damage localization algorithm as well as results of damage localization by the embedded subsystem.

Visualization of temperature elevation due to focused ultrasound generated by tone burst wave

The temperature elevation effect due to burst wave ultrasound was investigated experimentally. To observe the temperature distribution when focused ultrasound is radiated onto an acoustic dissipative medium of gelatin, a thermochromic film was used to perform a temperature visualization method. The discolored area on the thermochromic film caused by the ultrasound increased with different speeds depending on the duty cycle of the tone burst signal. Using the image process, the high-temperature region, the discolored region, caused by the focused ultrasound was measured. It is confirmed experimentally that the high-temperature area can be controlled by adjusting the duty cycle of the burst signal.

Ultrasonic preliminary measurements of oenological malolactic fermentation parameters in red wine

In the winemaking process, the malolactic fermentation is an essential process in the production of high quality red wines which concerns the conversion of malate into lactate. In this work, the ultrasonic velocity through wine samples with different concentrations of malate and lactate was measured using the pulse echo technique with 1 MHz tone burst signals. The evolution of these concentrations during malolactic fermentation was taken into account in order to determine the ratio between concentrations of malate and lactate of the different samples. These preliminary results have revealed that the ultrasonic velocity increases during the conversion of malate to lactate. In addition, measurements have been conducted to quantify the influence of variations in turbidity and temperature on test samples. Therefore, these results show the possibility of using ultrasonic velocity measurements for on-line monitoring the malolactic fermentation of red wine and may help to improve and contribute to the development of the winemaking process.

Vibro-acoustography beam formation with reconfigurable arrays

In this work, we present a numerical study of the use of reconfigurable arrays (RCA) for vibro-acoustography (VA) beam formation. A parametric study of the aperture selection, number of channels, number of elements, focal distance, and steering parameters is presented to show the feasibility and evaluate the performance of VA imaging based on RCA. The transducer aperture was based on two concentric arrays driven by two continuous-wave or toneburst signals at slightly different frequencies. The mathematical model considers a homogeneous, isotropic, inviscid medium. The pointspread function of the system is calculated based on angular spectrum methods using the Fresnel approximation for rectangular sources. Simulations considering arrays with 50 x 50 to 200 x 200 elements with number of channels varying in the range of 32 to 128 are evaluated to identify the best configuration for VA. Advantages of two-dimensional and RCA arrays and aspects related to clinical importance of the RCA implementation in VA, such as spatial resolution, image frame rate, and commercial machine implementation, are discussed. It is concluded that RCA transducers can produce spatial resolution similar to confocal transducers and steering is possible in the elevational and azimuthal planes. Optimal settings for number of elements, number of channels, maximum steering, and focal distance are suggested for VA clinical applications. Furthermore, an optimization for beam steering based on the channel assignment is proposed for balancing the contribution of the two waves in the steered focus.

A modified time reversal method for Lamb wave based diagnostics of composite structures

This experimental study presents Lamb wave based diagnostics of damage in a composite plate using time reversal of signals for baseline-free damage detection. A modified time reversal method (MTRM) is developed which requires only one transducer to actuate signals and the other transducer acts as a sensor for any signal path. An 8-layer symmetric cross-ply carbon-epoxy composite plate is fabricated and four PZT actuators/sensors are surface bonded at the corners of the plate. A 20kHz, 9.5 cycle tone burst signal is used to generate fundamental asymmetric Lamb waves with high signal-to-noise ratio and low dispersion. Theoretical analysis and experimental measurements show that conventional time reversal method (TRM) and the MTRM yield identical signals at the end of time reversal. The composite plate is impacted incrementally with a steel ball to cause three levels of damage and MTRM is applied for prediction of damage severity. Results show that magnitudes of the two damage indices used are directly correlated to the severity of damage along any signal path. The development of MTRM paves the way for implementing time reversal of signals using a single actuator and multiple sensors, including non-contact sensor such as a laser vibrometer.

Simultaneous Measurements of Harmonic Waves at Fatigue-Cracked Interfaces

Nonlinear harmonic waves generated at cracked interfaces are investigated theoretically and experimentally. A compact tension specimen is fabricated and the amplitude of the transmitted wave is analyzed as a function of position along the fatigued crack surface. In order to measure as many nonlinear harmonic components as possible, broadband lithium niobate (LiNbO3) transducers are employed together with a calibration technique for making absolute amplitude measurements with fluid-coupled receiving transducers. Cracked interfaces are shown to generate high acoustic nonlinearities, which are manifested as harmonics in the power spectrum of the received signal. The first subharmonic f/2 and the second harmonic 2f waves are found to be dominant nonlinear components for an incident toneburst signal of frequency f. To explain the observed nonlinear behavior, a partially closed crack is modeled by planar half interfaces that can account for crack parameters, such as crack opening displacement and crack surface conditions. The simulation results show reasonable agreement with the experimental results.

Hydrophone area-averaging correction factors in nonlinearly generated ultrasonic beams

The nonlinear propagation of an ultrasonic wave can be used to produce a wavefield rich in higher frequency components that is ideally suited to the calibration, or inter-calibration, of hydrophones. These techniques usually use a tone-burst signal, limiting the measurements to harmonics of the fundamental calibration frequency. Alternatively, using a short pulse enables calibration at a continuous spectrum of frequencies. Such a technique is used at PTB in conjunction with an optical measurement technique to calibrate devices. Experimental findings indicate that the area-averaging correction factor for a hydrophone in such a field demonstrates a complex behaviour, most notably varying periodically between frequencies that are harmonics of the centre frequency of the original pulse and frequencies that lie midway between these harmonics. The beam characteristics of such nonlinearly generated fields have been investigated using a finite difference solution to the nonlinear Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation for a focused field. The simulation results are used to calculate the hydrophone area-averaging correction factors for 0.2 mm and 0.5 mm devices. The results clearly demonstrate a number of significant features observed in the experimental investigations, including the variation with frequency, drive level and hydrophone element size. An explanation for these effects is also proposed.

A time reversal damage imaging method for structure health monitoring using Lamb waves

This paper investigates the Lamb wave imaging method combining time reversal for health monitoring of a metallic plate structure. The temporal focusing effect of the time reversal Lamb waves is investigated theoretically. It demonstrates that the focusing effect is related to the frequency dependency of the time reversal operation. Numerical simulations are conducted to study the time reversal behaviour of Lamb wave modes under broadband and narrowband excitations. The results show that the reconstructed time reversed wave exhibits close similarity to the reversed narrowband tone burst signal validating the theoretical model. To enhance the similarity, the cycle number of the excited signal should be increased. Experiments combining finite element model are then conducted to study the imaging method in the presence of damage like hole in the plate structure. In this work, the time reversal technique is used for the recompression of Lamb wave signals. Damage imaging results with time reversal using broadband and narrowband excitations are compared to those without time reversal. It suggests that the narrowband excitation combined time reversal can locate and determine the size of structural damage more precisely, but the cycle number of the excited signal should be chosen reasonably.

Instantaneous crack detection under varying temperature and static loading conditions

On the basis of guided wave propagation in plate-like structures, the authors' group previously developed a reference-free crack detection technique. The previous technique requires the placement of two pairs of collocated lead zirconate titanate (PZT) transducers on the top and bottom surfaces of a structure. However, as access to both surfaces can be limited for some structural systems, such as aircraft and pipelines, an improved reference-free crack detection technique is developed in this study to overcome this limitation. The major advancements of the proposed reference-free technique over the previous one are (1) reformulation of the reference-free technique considering energy distribution among Lamb wave modes to overcome the limitations of the previous time domain approach, (2) extension to allow the use of various input waveforms including broadband chirp signals and narrowband toneburst signals, and (3) newly designed PZT transducers for single surface installation and better ruggedness. Numerical simulations and experimental tests on aluminum plates are performed to investigate the performance of the proposed technique. In particular, its robustness to varying temperature and external loading is experimentally tested. Copyright © 2010 John Wiley & Sons, Ltd.

Measurement of the frequency dependence of the sound speed and attenuation of seafloor sands from 1 to 400 kHz

Measurements of sound speed and attenuation were performed over a frequency range from 1 to 400 kHz in the sandy seafloor near Ft. Walton Beach, FL, as part of the 2004 Sediment Acoustics Experiment (SAX04). The measurements at the lowest frequencies, from 1 to 20 kHz, were performed by transmitting tone burst signals from two acoustic sources positioned at depths up to 1 m below the seafloor which were recorded on a 4 m × 4 m array of 35 hydrophones emplaced to depths of between 0.1 and 1 m. Measurements were obtained at frequencies from 40 to 200 kHz with the In Situ Sediment Acoustic Measurement System (ISSAMS), a fixed linear array of four piezoelectric probes. The probes were inserted to a depth of 0.3 m, and the outer two probes were used as transmitters, with the inner probes acting as receivers. Sound-speed measurements were also made at 100, 200, and 400 kHz on diver-collected cores using four separate pairs of ultrasonic transducers. The sound-speed measurements above 40 kHz demonstrate an essentially constant sound-speed ratio with frequency of between 1.165 and 1.175, where the sound-speed ratio is the ratio of the speed through the sand to the speed through the overlying seawater. The sound-speed ratio determined from the low-frequency array data decreased from 1.135 to 1.115 with decreasing frequency below 20 kHz. Uncertainties in the sound-speed ratio values on the order of ±0.02 do not allow meaningful tests of the ability of Biot-Stoll or Buckingham propagation models to describe the observed dispersion. The measured attenuation values show a range from 1 to 120 dB/m with frequency from 1 to 400 kHz. At frequencies above 40 kHz, the attenuation follows a linear trend parallel to the estimates from the Buckingham model, as fit to the data with two free parameters, and at the highest frequencies significantly exceeding the Biot model prediction, as parameterized with values selected from the range of measurements of sediment and pore-water properties made from throughout the SAX04 site. At frequencies below 20 kHz, the data straddle the trends from the two models.

Ultrasonic imaging and characterization of objects submerged in highly attenuating fluids.

An ultrasonic scanning tool to image objects with high-spatial resolution submerged in highly attenuating fluids is described. The attenuation coefficient of the fluids used range over two orders of magnitude. For deeper penetration into the attenuating fluids, a parametric array with a difference frequency between 200 and 500 kHz was used. The primary frequencies that generated this difference frequency varied between 2 and 8 MHz. This parametric array provides a collimated beam for imaging. Both simple tone burst and frequency chirp signals were used for the measurement. Using this apparatus, the quality and accuracy of the image as a function of frequency, fluid attenuation coefficient, and fluid type (clear and suspensions) were investigated. It was found that imaging was possible over a wide range of attenuating coefficients and fluid types.

Penetration depth for measurements with ultrasonic diffraction grating spectroscopy.

The current status of ultrasonic diffraction grating spectroscopy was described at the Acoustics 08 Conference in Paris. An important question remains: What is the penetration depth of the ultrasound for such measurements? Although the spectral order m=1 at 90 deg is of primary interest, much insight can be gained by investigating a range of angles. The first step was to measure the amplitude of the m=1 transmitted beam in the liquid as a function of the transmitted angle. Data were obtained by sending a toneburst signal to the send transducer and measuring the response of a receive transducer immersed in the liquid. The angle of the receive transducer was varied in half‐degree steps from about 50 to 90 deg. The data were compared with a formulation for the interference of N wavelets from N grooves and showed very good agreement. In addition, an experimental technique was developed to measure the penetration of ultrasound into the liquid. Experimental parameters that affect this penetration depth were also investigated. The results of these measurements will be discussed and compared with the theory.

Underwater speed of sound and ranging measurements using the sliding discrete Fourier transform.

It is often desirable to extract parametric information from a signal in order to determine its properties with high accuracy. Such parameters might be the time delay, the amplitude, the frequency, and/or the phase. These parameters are usually highly correlated with the material through which the signal has passed or from which it has been reflected. Time delay estimation is a fundamental step in source localization or beamforming applications, and has attracted considerable research attention over the past few decades in many fields. In this paper, we propose a novel time delay estimation approach based on the sliding discrete Fourier transform (DFT). The analysis window is slid sample by sample over the received single frequency tone burst signal with the DFT evaluated successively. The optimal peak spectrum and its corresponding phase are used to estimate the time delay of the signal. We use the corresponding time as the first estimate, which is improved based on the related phase. Simulated examples are given with synthetic signals. An underwater application for speed of sound measurements using this approach compared to those obtained by a commercial device is then demonstrated. Finally, the proposed method is shown to outperform standard correlator‐based approaches.

AN ENHANCED REFLECTION REMOVAL TECHNIQUE AND ITS APPLICATIONS

Electroacoustic transducers using piezoelectric materials are popular in various applications such as underwater acoustics, ultrasound, earthquakes and elastic wave propagations. Especially, they are widely used in non-destructive testing for ultrasonic or acoustic emission transducers. In general, they generate and receive waves through media to find meaningful targets or physical characteristics of materials. However, in most uses, the media are bounded with finite dimensions, therefore there are multiple transmitting paths reflected from the boundaries. Such reflections corrupt the principal path signal to be analyzed. To overcome this problem, gating technique to gate successively transmitting and receiving signals, in other words, tone-burst signal technique, is most representatively used. This basically isolates the direct signal before the arrival of reflected signals in the time domain, and therefore it is also described as time windowing or time-selective windowing techniques without loss of generality. These techniques have inherent overlap problems invoked by long pulse duration, especially slightly damped signals or low frequency waves. An enhancement technique of shortening the pulses by digital filtering is proposed and successively applied in practical uses. It can isolate the principal path signal from reflected signals. Thereafter the signal can be perfectly recovered after removing reflections.

Tone-Burst Generation of Circumferential Guided Waves by a Bulk Shear Wave Sensor and Their Wide-Range Time–Frequency Analyses

Wide range generation of the circumferential guided waves (C-GWs) propagating along the perimeter of a pipe has been carried out using a single shear wave sensor along with several tone-burst sources. The C-GWs are classified mainly into two propagation modes, circumferential shear horizontal (C-SH) and circumferential Lamb (C-Lamb) waves. The authors previously reported that a simple and selective generation of the two modes using a single shear wave sensor could be realized by changing the polarization direction of the sensor. When the polarization direction of the sensor is parallel or vertical to the axial direction of a pipe, either the C-SH or the C-Lamb wave is selectively generated. In this paper, a newly exploited signal processing method to obtain a wide-range time–frequency analysis for the C-GW is proposed using wavelet transformation along with various tone-burst signals swept in the required frequency range. Time–frequency analyses of the results for the C-SH and C-Lamb waves propagating along 30-mm outer diameter and 1-mm-thick aluminum pipe are shown in comparison with the theoretical dispersion relation of the propagation modes. Quantitative evaluations of this method are discussed based on the Reciprocity theorem and Fourier transformation.

Ultrasonic focused waveform generation using time reversal acoustic focusing system

Abstract The concept of Time Reversal Acoustics (TRA) provides an elegant possibility of both temporal and spatial concentrating of acoustic energy in highly inhomogeneous media. We explored the possibility of generating acoustical signals with arbitrary waveforms using the TRA Focusing System (TRA FS). A method has been developed to predict TRA-focused ultrasound waveforms and spatial distribution by using the measurements of transfer function of transfer function relating the signal at the TRA transmitter to that at the focusing point. The developed approach for TRA-focused signal waveform prediction from the results of direct signal measurements was tested on ten-channel TRA FS based on aluminum resonator with glued piezotransducers. The TRA FS operated in the frequency band of 100–1000 kHz. The formation of ultrasonic signals with various envelopes was demonstrated experimentally. The calculated and experimentally measured waveforms and spatial distributions were practically identical. We formed triangular, rectangular, and amplitude modulated tone burst signals with different modulation frequencies in the focal region. The level of side lobes in the generated signals was much lower than that for standard TRA focusing.