Principle Of Acoustic Reciprocity Research Articles

Lamb wave tomography for defect localization using wideband dispersion reversal method

Tomography imaging of defects using ultrasonic Lamb waves has attracted much attention in nondestructive testing of plates. However, there are two challenges for robust localization of defects: frequency sensitivity to defects, and multi-mode interference on damage index (DI) extraction. Aiming for that, a wideband dispersion reversal (WDR) method optimized tomography is developed. Pre-dispersive wideband excitations of a certain Lamb wave mode are generated based on the configuration of transducers, and reconstruction independent component analysis is used for wave mode separation. According to the acoustic reciprocity principle, self-compensation phenomenon of mode signals can be recorded on the intact path, where the signal energy concentrates at a fix self-compensation point in the time–frequency domain, yet such compensation effect would be impaired encountering defects. Thus, a time–frequency index can be used as the DI, which is calculated based on the weighted Euclidean distance from the self-compensation point to the pixel points in the time–frequency domain. Experimental results show the WDR optimized tomography has a robust performance for magnet-simulated defects imaging without excitation frequency optimization.

Mechanical models as ground truth for vowel resonance analysis

It has long been established that LPC analysis results in formant estimates that are not accurate representations of the resonances; they are biased towards the nearest harmonic, and this bias worsens as F0 rises to 200 Hz or more. Manual measurement of formants with the reassigned spectrogram (RS) has been shown to be more accurate, but the ground truth is needed in order to test automatic measurement methods. Here, “vocal tract” models were 3D-printed to allow resonances to be excited replicably. Using the principle of acoustic reciprocity, source signals were played over an external loudspeaker, and the output filtered by the model was recorded via a microphone located in the model’s “glottis.” A white noise source signal was used to determine the resonances. Sawtooth and impulse train source signals (F0s 83–400 Hz) were filtered by the physical model and then analyzed with both LPC and RS. LPC formants were biased towards the nearest harmonic, resulting in errors from 10 Hz at low F0 to 250 at 300 Hz. RS errors ranged from 0 to 20 Hz and were not correlated with F0. These results indicate physical models are useful for creating known resonances for validating accuracy in our measurements.

Sound Localization through Multi-Scattering and Gradient-Based Optimization

A gradient-based optimization (GBO) method is presented for acoustic lens design and sound localization. GBO uses a semi-analytical optimization combined with the principle of acoustic reciprocity. The idea differs from earlier inverse designs that use topology optimization tools and generic algorithms. We first derive a formula for the gradients of the pressure at the focal point with respect to positions of a set of cylindrical scatterers. The analytic form of the gradients enhances modeling capability when combined with optimization algorithms and parallel computing. The GBO algorithm maximizes the sound amplification at the focal point and enhances the sound localization by evaluating pressure derivatives with respect to the cylinder positions and then perturbatively optimizing the position of each cylinder in the lens while incorporating multiple scattering between the cylindrical scatterers. The results of the GBO of the uni- and multi-directional broadband acoustic lens designs are presented including several performance measures for the frequency dependence and the incidence angle. A multi-directional broadband acoustic lens is designed to localize the sound and to focus acoustic incident waves received from multiple directions onto a predetermined localization region or focal point. The method is illustrated for configurations of sound hard and sound soft cylinders as well as clusters of elastic thin shells in water.

Study of high-frequency acoustic current tomography

<p indent="0mm">The sea-land interface is the most concentrated region of human marine development activities, and online monitoring of river current is very important for material flux and ecosystem evolution. In this study, the high-frequency acoustic current tomography of sea-land interface was studied. Based on the principle of acoustic reciprocity, the acoustic inversion algorithm of current was investigated. The experiment was carried out on the Jiulongjiang sea-land interface in Zhangzhou. The high-frequency transducer, with a center frequency of <sc>60 kHz</sc> and bandwidth of <sc>40–80 kHz,</sc> was used. According to the sound speed profile and underwater topographic survey, ray acoustic simulation was performed to reveal high-frequency sound propagation. The experimental results showed that the acoustic signal has high-signal amplitude and peak value of cross-correlation, which can be used to reconstruct the current of the river. Real-time monitoring of the river current during the typhoon “haibeisi” was achieved. The results showed that high-frequency acoustic tomography can be applied to the study of hydrodynamics of sea-land interface, hydrological measurement, and monitoring of extreme events, such as flood peak and heavy rainfall.

Blood Flow Imaging in the Neonatal Brain Using Angular Coherence Power Doppler.

Using ultrasound to image small vessels in the neonatal brain can be difficult in the presence of strong clutter from the surrounding tissue and with a neonate motion during the scan. We propose a coherence-based beamforming method, namely the short-lag angular coherence (SLAC) beamforming that suppresses incoherent noise and motion artifacts in Ultrafast data, and we demonstrate its applicability to improve detection of blood flow in the neonatal brain. Instead of estimating spatial coherence across the receive elements, SLAC utilizes the principle of acoustic reciprocity to estimate angular coherence from the beamsummed signals from different plane-wave transmits, which makes it computationally efficient and amenable to advanced beamforming techniques, such as f-k migration. The SLAC images of a simulated speckle phantom show similar edge resolution and texture size as the matching B-mode images, and reduced random noise in the background. We apply SLAC power Doppler (PD) to free-hand imaging of neonatal brain vasculature with long Doppler ensembles and show that: 1) it improves visualization of small vessels in the cortex compared to conventional PD and 2) it can be used for tracking of blood flow in the brain over time, meaning it could potentially improve the quality of free-hand functional ultrasound.

Critical examination of ultrasonic transducer characteristics and calibration methods

ABSTRACTThis study systematically determined the transmission and receiving sensitivities of over twenty transducers. Four types of sensitivities were evaluated for both transmission and receiving sensitivities. These are found to be different from each other and the reversibility or reciprocity conditions exist only in exceptional cases. Using their observed behavior as the basis, we critically examined the calibration methods developed to characterize them, including those based on laser interferometry and the acoustic reciprocity principle. Serious flaws in some of the reciprocity methods are uncovered, which can be rectified by using the Hill--Adams method. Four procedures emerged as workable calibration methods for contact ultrasonic and acoustic emission transducers. However, current experimental uncertainties limit the upper frequency to 2 MHz.

Numerical and experimental study of Near-Field Acoustic Radiation Modes of a baffled spherical cap

In this paper, the Acoustic Radiation Modes (ARMs) of spherical structures are studied using the Pressure-Velocity (PV) method. This method yields the active and reactive modes, which refer to the radiated and non-radiated sound power components respectively, without restrictions on the observation distance. The accuracy of the method is verified through the analysis of Near Field ARMs (NFARMs) of a sphere in spherical coordinates compared to the analytical solution. Differences are analyzed between the NFARMs and the Far Field ARMs (FFARMs) of a baffled spherical cap, as well as between the active and reactive parts of the sound power radiated when varying the frequency and the observation distance to the source. It was found that the radiation efficiency of the active ARMs is independent of the observation distance, while that of reactive ARMs decreases sharply when retreating from the source. Experiments were performed using the acoustic reciprocity principle to measure the NFARMs and FFARMs of a 3D-printed spherical cap radiating in a hemi-anechoic room. Experimental results provided a reliable validation of the numerical results.

Comparison of measured acoustic power results gained by using three different methods on an ultrasonic low-frequency device

The theme of this work is characterization of an ultrasonic low-frequency device, driven at an excitation frequency of around 25 kHz at different electrical excitation levels by using three different methods as proposed in IEC 61847 and IEC 61088 standards. The first method is based on the electromechanical characterization of the device. It consists of measuring the input electrical impedance around the excitation frequency in the unloaded and loaded conditions at a low level excitation voltage of 1 V. The equivalent RLC electrical circuit parameters of an unloaded and loaded device are determined in an anechoic tank and in a vessel at different immersion depths and tip positions in a complex geometry. The electroacoustic efficiency factor of the method is determined by knowing the real part of the radiation resistance and mechanical loss resistance which are transformed into an equivalent RLC electrical circuit of the transducer. The second method consists of measuring the spatial pressure distribution of an ultrasonic device near pressure release boundary in an anechoic tank. The acoustic reciprocity principle is used to determine the derived acoustic power of an equivalent point source in the form of radially oscillating sphere at the excitation frequency. The third method is based on the measurement of power dissipated in a restricted volume of water by using a calorimetric method. Some of the suggested methods are complicated to apply in the high energy ultrasonic devices whose size is much lower than the wavelength in the loading medium due to the occurrence of strong cavitation activity and influence of the sonotrode tip position in the complex standing wave field. However, the measured acoustic power found by using the three suggested methods is compared by means of the electroacoustic efficiency factor defined for each considered method. In the electromechanical characterization, which is made at low electrical excitation levels (applied electrical power of 1 mW at the series resonance frequency), the calculated maximum electroacoustic efficiency factor is around 48% when the influence of standing waves pattern on the radiation resistance is small. It is approximately the same as the one obtained by measuring the derived acoustic power in an anechoic tank (43%) without cavitation activity in front of the tip. When a strong cavitation activity is present in the loading medium, the bubble cloud has a significant influence on the derived acoustic power which is then dispersed in a broad frequency range and the electroacoustic efficiency factor of the method decreases down to 2%. A significant growth of the input electrical impedance magnitude at the excitation frequency is observed when the cavitation activity is present in front of the tip and when it is compared with the impedance magnitude measured at lower excitation levels without cavitation. The power dissipated in the loading medium almost linearly depends on the applied electrical power, with saturation at higher excitation levels. In the linear operating mode the electroacoustic efficiency factor of the calorimetric method (48%) is comparable with the efficiency factors of two other methods. In the nonlinear operating mode, it is larger (71%) due to a significant amount of heat energy released during the cavitation process.

Reconstruction of Vectorial Acoustic Sources in Time-Domain Tomography

A new theory is proposed for the reconstruction of curl-free vector field, whose divergence serves as acoustic source. The theory is applied to reconstruct vector acoustic sources from the scalar acoustic signals measured on a surface enclosing the source area. It is shown that, under certain conditions, the scalar acoustic measurements can be vectorized according to the known measurement geometry and subsequently be used to reconstruct the original vector field. Theoretically, this method extends the application domain of the existing acoustic reciprocity principle from a scalar field to a vector field, indicating that the stimulating vectorial source and the transmitted acoustic pressure vector (acoustic pressure vectorized according to certain measurement geometry) are interchangeable. Computer simulation studies were conducted to evaluate the proposed theory, and the numerical results suggest that reconstruction of a vector field using the proposed theory is not sensitive to variation in the detecting distance. The present theory may be applied to magnetoacoustic tomography with magnetic induction (MAT-MI) for reconstructing current distribution from acoustic measurements. A simulation on MAT-MI shows that, compared to existing methods, the present method can give an accurate estimation on the source current distribution and a better conductivity reconstruction.

Measuring derived acoustic power of an ultrasound surgical device in the linear and nonlinear operating modes

Objective and motivation The method for measuring derived acoustic power of an ultrasound point source in the form of a sonotrode tip has been considered in the free acoustic field, according to the IEC 61847 standard. The main objective of this work is measuring averaged pressure magnitude spatial distribution of an sonotrode tip in the free acoustic field conditions at different electrical excitation levels and calculation of the derived acoustic power at excitation frequency ( f 0 ≈ 25 kHz). Finding the derived acoustic power of an ultrasonic surgical device in the strong cavitation regime of working, even in the considered laboratory conditions (anechoic pool), will enable better understanding of the biological effects on the tissue produced during operation with the considered device. Experimental method The pressure magnitude spatial distribution is measured using B&K 8103 hydrophone connected with a B&K 2626 conditioning amplifier, digital storage oscilloscope LeCroy Waverunner 474, where pressure waveforms in the field points are recorded. Using MATLAB with DSP processing toolbox, averaged power spectrum density of recorded pressure signals in different field positions is calculated. The measured pressure magnitude spatial distributions are fitted with the appropriate theoretical models. Theoretical approaches In the linear operating mode, using the acoustic reciprocity principle, the sonotrode tip is theoretically described as radially oscillating sphere (ROS) and transversely oscillating sphere (TOS) in the vicinity of pressure release boundary. The measured pressure magnitude spatial distribution is fitted with theoretical curves, describing the pressure field of the considered theoretical models. The velocity and displacement magnitudes with derived acoustic power of equivalent theoretical sources are found, and the electroacoustic efficiency factor is calculated. When the transmitter is excited at higher electrical power levels, the displacement magnitude of sonotrode tip is increased, and nonlinear behaviour in loading medium appears, with strong cavitation activity produced hydrodynamically. The presence of harmonics, subharmonics and ultraharmonics as a consequence of stable cavitation is evident in the averaged power spectral density. The cavitation noise with continuous frequency components is present as a consequence of transient cavitation. The averaged pressure magnitude at the frequency components of interest (discrete and continous) in the field points is found by calculating average power spectral density of the recorded pressure waveform signal using the welch method. The frequency band of interest where average power spectral density is calculated is in the range from 15 Hz up to 120 kHz due to measurement system restrictions. The novelty in the approach is the application of the acoustic reciprocity principle on the nonlinear system (sonotrode tip and bubble cloud) to find neccessary acoustic power of the equivalent acoustic source to produce the measured pressure magnitude in the field points at the frequency components of interest. Results In the nonlinear operating mode, the ROS model for the considered sonotrode tip is chosen due to the better agreement between measurement results and theoretical considerations. At higher excitation levels, it is shown that the averaged pressure magnitude spatial distribution of discrete frequency components, produced due to stable cavitation, can be fitted in the far field with the inverse distance law. The reduced electroacoustic efficiency factor, calculated at excitation frequency component as ratio of derived acoustic power with applied electrical power, is reduced from 40% in the linear to 3% in the strong nonlinear operating mode. The derived acoustic power at other frequency components (subharmonic, harmonic and ultraharmonic) is negligible in comparison with the derived acoustic power at excitation frequency. Discussion and conclusions The sonotrode tip and loading medium are shown in the strong cavitation regime as the coupled nonlinear dynamical system radiating acoustic power at frequency components appearing in the spectrum. The bubble cloud in the strong nonlinear operating mode decreases the derived acoustic power significantly at the excitation frequency.

The identification of tyre induced vehicle interior noise

Sound transmission into a vehicle is classified as either airborne or structure borne sound. From the point view of noise control, the reduction of noise transferred by different paths requires different solutions. Coherence function analysis is often used to identify transmission paths. However it can be difficult to separate the airborne from structure borne components. The principle of acoustic reciprocity offers a convenient method for overcoming this difficulty. The principal states that the transfer function between an acoustic volume velocity source and an acoustic receiver is independent of a reversal of the position of source and receiver. The work done on this study involves exciting a stationary tyre and measuring the surface velocity of the tyre at a number of discrete points. The acoustic transfer functions between each point on the tyre and a receiver point are measured reciprocally. Two sets of measurements are then combined to yield a measure of the sound pressure due to a point force on the tyre via the acoustic transmission path only. This technique also provides information on the relative contributions of various regions of the tyre wall to the resultant noise. Also the sound radiation characteristics, the horn effect, and resonance at the wheel housing are identified through the reciprocal measurement.

DEVELOPMENT OF A BLOCKED PRESSURE CRITERION FOR APPLICATION OF THE PRINCIPLE OF ACOUSTIC RECIPROCITY

The principle of acoustic reciprocity can be applied in practice to estimate the sound radiation of a vibrating surface, from an inverse measurement of the Green function. This principle is especially useful in evaluating and ranking the sound radiation from individual vibrating components of a complex mechanical system. Usually, a blocked pressure assumption is needed in order to simplify the formulation of the reciprocity technique. This paper examines the validity of the blocked pressure hypothesis in the context of the reciprocity method, and suggests a “blocked pressure criterion” to rigorously quantify the impact of this hypothesis on the sound radiation prediction. This criterion is tested numerically on a system consisting of two coplanar, simply supported thin baffled plates that are mechanically uncoupled. It is shown that the numerical results support the conclusions obtained from the blocked pressure criterion.

Nonlinear acoustic scattering from a gassy poroelastic seabed

A model for difference frequency backscatter from trapped bubbles in sandy sediments was developed. A nonlinear volume scattering coefficient was computed via a technique similar to that of Ostrovsky and Sutin [“Nonlinear sound scattering from subsurface bubble layers,” in Natural Physical Sources of Underwater Sound, edited by B. R. Kerman (Kluwer, Dordrecht, 1993), pp. 363–373], which treats the case of bubbles surrounded by water. Biot’s poroelastic theory is incorporated to model the acoustics of the sediment. Biot fast and slow waves are included by modeling the pore fluid as a superposition of two acoustic fluids with effective densities that differ from the pore fluid’s actual density and account for its confinement within sediment pores. The principle of acoustic reciprocity is employed to develop an expression for the backscattering strength. Model behavior is consistent with expectations, based on the known behavior of bubbles in simpler fluid media.

Simulations of Acoustic Imprints of Meddies in the Iberian Basin: Toward Acoustic Detection of Meddies

Abstract Ocean currents’ effect on long-range sound propagation, though considerable in many cases, is difficult to separate from much stronger effects due to sound speed inhomogeneities, as flow velocity is usually much smaller than typical variations in the sound speed. Dramatic improvement can be achieved in reciprocal transmission experiments when sound signals propagate in opposite directions between two transceivers (source–receiver pairs). The presence of a current results in the breaking of the principle of acoustic reciprocity, thus making it possible to use nonreciprocity of acoustic field as an indicator of water movement. In this paper, reciprocal acoustic transmissions through a submesoscale interthermocline lens of Mediterranean Water (meddy) in the Atlantic are considered theoretically as a possible tool for meddies detection. A simple model of acoustic ray-travel-time nonreciprocity due to a meddy is proposed. The analytic estimates obtained from the model show that the influence of rotary...

Application of a reciprocity technique for measurement of acoustic transfer functions to the prediction of road vehicle pass-by noise

A technique based on the principle of acoustic reciprocity has been developed for measuring acoustic transfer functions of sound radiation from vibrating surfaces of the engine, power train, exhaust system and tires. In many realistic cases of sound radiation, reciprocal measurements of transfer functions are usually more convenient and mostly more accurate than the corresponding direct measurements. This paper explain the basic acoustic reciprocity and experimental setup and illustrate its benefits by application to measurements of transfer functions of sound radiation from the engine of a fully trimmed vehicle.

ON ACOUSTIC INTERACTION BETWEEN TWO THIN ELASTIC PLATES THROUGH AN ANGULAR WELDED JOINT

Contact conditions are deduced for a pair of thin plates in welded contact at an angle and under fluid loading. The conditions take into account the interaction between the plates through not only flexural but also longitudinal motion, and include the possibility of an applied force and moment acting at the junction. The two-dimensional conditions are shown to be consistent with the principle of acoustic reciprocity for the configuration, and they guarantee unique solutions if certain constraints are satisfied by the plate longitudinal impedances. The conditions also have implications for the junction admittance matrix. Simple examples are presented of the application of the contact conditions to acoustic interaction with plate configurations that are almost flat.

Reciprocity Technique for Analysis of Sound Radiation from Vibrating Components of Vehicles. 1st Report. Measurement of Acoustic Transfer Functions.

A technique based on the principle of acoustic reciprocity has been developed for measuring acoustic transfer functions for sound radiation from vibrating components of vehicles. With this technique, frequency response functions of sound pressure on the surfaces of components are measured with loudspeakers installed at receiver points. In this paper we will first explain the acoustic reciprocity principle for coupled structural-acoustic systems, realization of a simple sound source, measurement of effective volume velocity of a sound source and signal processing. Empirical examples will then be given of applications of this technique to the analysis of sound radiation from the engine of a passenger car. The results indicate that the reciprocity technique is often more accurate than the conventional acoustic excitation method which uses loudspeakers installed on the surfaces of components.

Nonlinear acoustic scattering from trapped gas bubbles in sandy sediments

A nonlinear backscattering model for trapped bubbles in sandy sediments was developed. A difference frequency scattering coefficient is computed via a technique similar to that of Ostrovsky and Sutin [NonlinearSoundScatteringfromSubsurfaceBubbleLayers, NaturalPhysicalSourcesofUnderwaterSound, edited by B. R. Kerman, pp. 363–373 (1993)], which treats the case of bubbles surrounded by water. The Biot theory is incorporated to model the acoustics of a sandy sediment. Biot fast and slow waves are included by modeling the pore fluid as a superposition of two acoustic fluids with effective densities that differ from the pore fluids’ actual density and account for its confinement within sediment pores. The principle of acoustic reciprocity is employed to develop an expression for the backscattering strength. [Work supported by ONR under management by APL/UW.]

A model for high-frequency acoustic backscatter from gas bubbles in sandy sediments at shallow grazing angles

A model for acoustic backscatter from trapped bubbles in sandy sediments was developed. The model combines a Biot acoustic penetration model with a resonance scattering mechanism from trapped bubbles. The bubble size distribution is assumed to mirror the size distribution of the fluid pores that exist between sand grains. An estimate of the pore size distribution is constructed from the grain size distribution, based on the known pore structure between dense random packings of hard spheres. The principle of acoustic reciprocity is employed to compute backscattered acoustic pressure in terms of the incident pressure and the scattering cross section of a bubble distribution. The model is applied to data from experiments recently taken at sea. It is concluded that trapped gas is often a likely cause of observed backscatter from sandy sediments. Very small amounts of gas appear to be sufficient to produce significant backscatter.

On: “Acoustic reciprocity—A paradox” by E. Eisner (GEOPHYSICS, v. 48, p. 1132–1134) or “Eisner’s paradox resolved”

Eisner has concocted a clever geometrical arrangement that appears to contradict the principle of acoustic reciprocity. In essence his example consists of a truncated opaque ellipsoidal reflector with the source and receiver situated at the foci as shown in Figure 1. In the limit of geometrical optics it is evident that only half the energy emitted by a source at 2 will contribute to the reflected arrival at 1 while a substantially larger fraction of the energy emitted by a source at 1 will appear at 2. Naively it seems as if the geometrical amplitude of the reflected arrival at 2 should exceed that at 1, in apparent violation of the reciprocity theorem.