Table of contents

This is a tutorial paper on the basics and applications of the finite-difference time-domain (FDTD) method. Two types of discretization of the linear governing equations, the scalar-type FDTD method and the vector-type one, are first discussed. Then the basic concept of the compact explicit-FDTD (CE-FDTD) method is described. By considering the relationship between the cutoff frequency and the computer resources, it is shown that the interpolated wide band scheme requires the least computer resources among the derivative schemes of the CE-FDTD method. The discretization of the arbitrary shaped sound field by voxels and its boundary conditions, and the implementation of the density variation are also described. The sound field rendering and its real time renderer "Silicon concert hall" are introduced.

This paper discusses the applicability of dielectric materials with high acoustic impedance Z_a for use in A₁ Lamb mode solidly mounted resonator (SMR) with large electromechanical coupling coefficient k² . The study first shows that the use of metal as a reflector creates parasitic capacitance C_p, which reduces k² significantly. Then, A₁ Lamb mode SMRs are designed using HfN, HfO₂, WN, WO₃ etc., and achievable performances are compared. When either HfN, HfO₂, WN, or WO₃ is employed, relative large k² up to 25% is achievable. For further k² enhancement, a hybrid reflector configuration is also examined, wherein HfN is applied only to the top high Z_a layer and W is applied to the others. The result indicates that C_p caused by the W layers is still significant, and k² becomes worse in total.

The formation of amyloid fibrils of various amyloidogenic proteins is dramatically enhanced by ultrasound irradiation. To apply this phenomenon to the study of protein aggregation science and diagnosis of neurodegenerative diseases, a multichannel ultrasound irradiation system with individually adjustable ultrasound irradiation conditions is necessary. Here, we develop a sonochemical reaction system, where an ultrasonic transducer is placed in each well of a 96-well microplate to perform ultrasonic irradiation of sample solutions under various conditions with high reproducibility, and applied it to study the amyloid fibril formation of amyloid β, α-synuclein, β2-microglobulin, and lysozyme. The results clearly show that our instrument is superior to the conventional shaking method in terms of the degree of acceleration and reproducibility of fibril formation reaction. The acceleration degree is controllable by controlling the driving voltage applied to each transducer. We have thus succeeded in developing a useful tool for the study of amyloid fibril formation in various proteins.

Spatial distribution of sonochemiluminescence (SCL) from an argon-saturated luminol solution was measured in a focused sound field at 1 MHz in a standing-wave configuration. The SCL distribution was confined to pre-focal region at acoustic powers lower than 0.9 W, and was not located at the focus but at a few mm pre-focal side at a threshold for SCL inception. The threshold pressure amplitude for SCL inception was 3.6 atm at the focus, which value was obtained with a background-oriented schlieren method. The method is based on the broadening of multiple slits due to an optical deflection caused by ultrasound, and the broadening width measured provides an acoustic pressure amplitude. A qualitative image of the focused sound field was also obtained.

We study the lattice thermal conductivity of isotope diamond superlattices consisting of ¹²C and ¹³C diamond layers at various superlattice periods. It is found that the thermal conductivity of a superlattice is significantly deduced from that of pure diamond because of the reduction of the phonon group velocity near the folded Brillouin zone. The results show that asymmetric superlattices with a different number of layers of ¹²C and ¹³C diamonds exhibit higher thermal conductivity than symmetric superlattices even with the same superlattice period, and we find that this can be explained by the trade-off between the effects of phonon specific heat and phonon group velocity. Furthermore, impurities and imperfect superlattice structures are also found to significantly reduce the thermal conductivity, suggesting that these effects can be exploited to control the thermal conductivity over a wide range.

This paper discusses the influence of displacement and patterning of phase shifters for piston mode operation of the temperature compensated surface acoustic wave (SAW) resonator on SiO₂/LiNbO₃ structure. As the phase shifters, Cu metals placed on the top surface of SiO₂ are considered. First, the conventional Cu stripes are chosen, and their displacement is considered from interdigital transducer (IDT) aperture edges. It is shown that achievable transverse mode suppression is almost identical when the stripe shape is adjusted for each case. Next, Cu dots are considered as patterned phase shifters. It is shown comparable transverse mode suppression is possible also for this case. However, relatively strong SAW lateral leakage occurs when they are placed above IDT fingers. These results indicate that location and pattern can be added as design parameters for the phase shifters on SiO₂. It is favorable for further enhancement of total device performances.

This paper discusses applicability of periodically slotted electrodes for realization of wideband transversely coupled double-mode resonator filters using lithium niobate thin plates. First, two-dimensional analysis is carried out, and it is shown that the periodic structure is effective to control the frequency separation between two resonance modes, and synthesis of the fractional bandwidth larger than 24% is achievable. Next, three-dimensional analysis is performed for suppression of spurious resonances. It is shown that mass loading at the aperture edges is effective for piston mode operation, and transverse modes can be well suppressed. It is also pointed out that the bottom electrode should cover only the aperture region and be removed from the busbar and gap regions for suppression of unwanted resonances. With these proper edge treatments, spurious-free and wide passbands can be synthesized.

The dynamics of magnetization is important in spintronics, where the coupling between phonon and magnon attracts much attention. In this work, we study the angular dependence of the coupling between longitudinal-wave phonon and magnon. We investigated the magnetization dynamics using the time-resolved magneto-optical Kerr effect, which allows measuring spin-wave resonances and the magnetic echo signal. The frequency, mode number, and amplitude of the spin-wave resonance change with the out-of-plane angle of the external magnetic field. The amplitude of the magnetic echo signal caused by the strain pulse also changes with the angle. We calculate these angular dependences based on the Landau–Lifshitz–Gilbert equation and find that the angles of the external field and magnetic moment are important factors for the phonon–magnon coupling when phonon propagates in the thickness direction under the out-of-plane magnetic field.

This study evaluates the accuracy of demodulated sound measurements using a condenser microphone in the near field of a parametric loudspeaker system. Microphones with different sensitivities placed at incidence angles of 0° and 90° were used to measure demodulation frequency components without special acoustic filters. The measured components were compared with theoretical predictions. The results show that the measured sound pressure using microphones placed at 0° was up to several tens of decibels larger than the theoretical predictions and significantly inaccurate in the near field. This was due to the nonlinear response of the microphone, which had high sensitivity at primary sound frequencies, inducing spurious signals. This result suggests that using a microphone with low sensitivity at primary sound frequencies placed at an appropriate angle that reduces sensitivity improves parametric sound measurement accuracy.

The enrichment characteristics of amino acids by ultrasonic atomization were investigated. Samples were aqueous solutions of L-phenylalanine and L-tyrosine. The ratio of amino acid concentration in the mist to that in the solution was defined as the enrichment factor. As the flow rate of carrier gas became higher, the collection mass of mist increased and the enrichment factor decreased. The enrichment factor depended on the solution pH. The enrichment factor increased with decreasing amino acid concentration in the solution and enhanced by the addition of ultrafine bubbles.

We analytically studied the Fano resonance in a simple coupled oscillator system. We demonstrate directly from the equation of motion that the resonance profile observed in this system is generally described by the Fano formula with a complex Fano parameter. The analytical expressions are derived for resonance frequency, resonance width, and Fano parameter, moreover conditions under which the Fano parameter becomes a real number are also examined. These expressions, derived for the simple system, are expected to be helpful for considering various other physical systems because the Fano resonance is a general wave phenomenon.

The singular value decomposition (SVD) based clutter filter is commonly applied to beamformed signals for the visualization of echo signals from flowing blood cells. In this paper, the SVD-based clutter filter is applied to signals directly acquired from ultrasonic elements before beamforming to be compared with the conventional strategy by evaluating contrast and standard deviation (SD) in the filtered images. As a result, the contrast was improved from 10.7 ± 3.6 dB to 18.3 ± 4.6 dB, and the SD was slightly improved from 3.78 ± 0.69 dB to 3.07 ± 0.74 dB in the measurement of a right jugular vein.

This paper describes the implementation of an autofocus function for a laser beam in a high-speed, phase-sensitive laser probe system for RF surface/bulk acoustic wave devices. This implementation can compensate for defocus caused during continuous measurements that take dozens of hours. After a brief explanation of the system used in this work, a detailed discussion is given on the employed evaluation function indicating focus status, which is a key factor determining autofocus reliability. It is shown that the sum of the energies of Laplacians is suitable as an evaluation function, which can be calculated by an image of the probing laser spot captured by a built-in CCD camera. Then, the implementation of the autofocus function in the current system is detailed. It is confirmed that this function can adjust the focus within almost $\pm 20\,\mu {\rm{m}}$ defocus conditions. Finally, it is confirmed how the implemented autofocus function works effectively to keep just-in-focus under disturbance.

The effect of the liquid crystal (LC) layer thickness on the optical characteristics of an ultrasound LC lens was explored. Three LC lenses with differing LC layer thicknesses (100, 200, and 300 μm) were fabricated, and the optical focal lengths were measured by an optical microscope with a varying driving voltage. For the lens with a 200 μm thick LC layer, a larger change in the focal length was observed for a smaller driving voltage compared with that of the other two lenses, indicating that the LC layer thickness is appropriate for a variable-focus lens.

Piezoelectricity of YbAlN films has recently been shown to be almost as high as that of ScAlN films. YbAlN film surface acoustic wave (SAW) resonators are expected to have a high coupling factor. We theoretically investigated the propagation characteristics of first-mode Rayleigh SAWs (RSAWs) on Yb_0.33Al_0.67N film/high-velocity Si, sapphire, AlN, SiC, BN, and diamond substrates. The first-mode RSAWs on the YbAlN layered structures had high coupling factors, higher than those on ScAlN layered structures. An enhancement of the effective coupling factor of the first-mode RSAWs was observed in polarity inverted YbAlN film/BN or diamond substrate structures.

In contrast enhancement ultrasound (CEUS), the vasculature image can be formed from nonlinear echoes arising from microbubbles in a blood flow. The use of binary-coded pulse compression is promising for improving the contrast of CEUS images by suppressing background noise. However, the amplitudes of nonlinear echoes can be reduced, and sidelobes by nonlinear echoes can occur depending on the binary code. Optimal Golay codes with slight nonlinear-echo reduction and nonlinear sidelobe have been proposed. In this study, CEUS images obtained by optimal Golay pulse compression are evaluated through experiments using Sonazoid microbubbles flowing in a tissue-mimicking phantom.

The analysis of local polar clusters formed by random fields in ferroelectrics is of technical and fundamental importance in understanding piezoelectricity. The temperature and electric field dependences of elastic properties and the ferroelectric phase transition have been investigated in (100)-oriented BaTiO₃ single crystals by micro-Brillouin scattering. In the field cooling process, LA phonons are scattered by polar clusters. As a result, the LA phonon frequency increases as compared with that of the zero-field cooling process. Under the application of an external electric field along the [100] direction in the tetragonal ferroelectric phase, a complete 90° domain switching is accomplished. Under the electric field, abrupt changes in the frequency shift and FWHM of the LA phonons in the paraelectric cubic phase indicate the occurrence of a phase transition from the cubic to the tetragonal phase. An E–T phase diagram has been proposed from the field-induced phase transition.

Multi-user underwater acoustic communication is expected to support efficient and reliable underwater exploration by networking devices in oceans. However, crosstalk occurs when multiple devices emit signals simultaneously. To address this issue, we previously proposed an aplanatic compound eye lens that had the potential to achieve space-division multiple access, but the lens had a complex shape. In this paper, we designed a simple-shape lens consisting of a concave meniscus and having a hemispherical incident surface. Multi-user communication using lenses was simulated. The results showed that the proposed hemispherical lens had almost the same communication quality as the existing aplanatic compound eye lens.

This study investigates how the translational velocity of phospholipid-coated bubbles caused by acoustic radiation force depends on their size. The translations of bubbles with mean radii of 0.9–5 μm were experimentally evaluated at five ultrasound frequency conditions (3.5, 5, 7.5, 10, and 15 MHz). We compared experimental data with theoretical prediction using a viscoelastic interfacial rheological model and a model suitable for high amplitude oscillation. The results suggested that the translation of bubbles could be enhanced for a mean radius of 1–3 μm but echo intensity could not.

A simple photoacoustic method was used to evaluate bovine cortical bone samples. This study aimed to evaluate the effects of glycation in bones due to non-physiological crosslinks on the properties of collagen. The average amplitude of the ultrasonic waves generated by the near-infrared pulsed laser irradiation was smaller in the glycated bone sample than in the reference (normal) sample. The results indicate that glycation due to diabetes might affect the photoacoustic properties of the bone. Ultrasonic waves with a small amplitude were also generated in the bone because the bone sample was not perfectly opaque to the light used.

This paper compares three different piston mode designs for temperature-compensated surface acoustic wave (TC-SAW) resonators using SiO₂/LiNbO₃ structure. It was shown that in rough approximation, phase shift given by extra elements for the piston mode operation is determined by the total mass of extra elements and SiO₂ which overlaid on piston region. The hammer head design without additional metal layers does not work properly due to the weaker impact of the extra elements when the SiO₂ layer is thick. On the other hand, piston mode designs using metal dots or stripes are effective to suppress the transverse mode resonances even when the SiO₂ layer is thick. Although larger metal thickness is preferable for the wideband operation, it also makes the split of main resonance. Thus, the optimal metal thickness can be found from this trade-off, and then the optimal metal width can be found to achieve good transverse mode suppression.

An experimental method is proposed to determine the frequency-dependent complex shear viscosity of liquids based on the quartz crystal microbalance with dissipation method. An AT-cut quartz transducer without metal electrodes is immersed in a sample liquid and the transducer is electrically coupled to the circuit through the dielectric response of the sample itself. After correcting for the apparent change in the resonance properties due to the dielectric coupling of the sample, our method is able to determine the viscosity of liquids of high polarity and low viscosity at frequencies as high as 3 GHz. The method was then applied to ethylene glycol and the viscoelastic relaxation in the GHz regime was observed. Furthermore, it was also applied to room-temperature ionic liquids to show that the dielectric correction of the resonance properties is valid for conductive liquids.

As a health monitoring tool of bolts in infrastructures, we propose a non-contact evaluation method for the axial force of a bolt. Deformation of the bolt head is measured as an electrical capacitance variation detected as a frequency shift of a simple circuit composed of a quartz crystal resonator and coils. The measurement was carried out via magnetic field coupling between the coil installed on the bolt head and another coil connected to the measurement instrument. Since the method requires no active electronic circuit or battery for the bolt, low cost and high durability can be expected. First, the circuit was analyzed and optimized using an equivalent circuit model. Then, the feasibility of the proposed method was experimentally studied using a prototype. It was demonstrated that the method enabled non-contact axial force estimation in which the dependence on the distance between the coils is sufficiently small for detecting bolt looseness.

The elastic modulus of tissue as a useful biomarker of disease detection can be quantitatively evaluated based on shear wave speed (SWS) measurements in shear wave elastography. Although the longitudinal wave speed (LWS) is also expected to be a promising biomarker for disease detection, the elasticity is not always dominant because the LWS is affected by the bulk modulus. In other words, LWS and SWS may reflect different tissue properties. Therefore, in this study, based on the improvement in LWS measurement, the relationship between the composition of a phantom mixed with agar and glycerol and ultrasonically measured LWS and SWS was investigated. The LWS had a good sensitivity in detecting glycerol, while the SWS had a good sensitivity in detecting agar. The calculated Poisson's ratio had a better sensitivity in detecting agar. In conclusion, a simultaneous measurement of LWS and SWS may help identify the tissue composition.

The influence of an external sound applied to a loop-tube type thermoacoustic system on the energy conversion efficiency is experimentally examined. The investigation is carried out by studying the effect of a loudspeaker (SP) set as an external sound source. As a result, it is found that the location of the SP affects the sound field in the system and that the amount of energy generated increases or decreases. The increasing or decreasing effect differs depending on the location of the SP. Furthermore, it is confirmed that, provided the SP is located near the particle velocity node, the sound energy can be increased by more than the input power to the SP without changing the sound field in the tube. From these results it can be confirmed that, similar to a straight-tube type thermoacoustic system, the energy conversion efficiency can be enhanced by locating the SP at a suitable position even in a loop-tube type system without end surfaces.

In the early stage of atherosclerosis, the luminal surface of the arterial wall becomes rough. Methods for distinguishing between the reflected and backscattered components in the ultrasonic echo from the arterial wall have the potential to be used as a method for assessment of the roughness of the arterial wall. In this study, we proposed a method to distinguish between the reflected and backscattered components using a technique based on plane wave compounding. This method was evaluated by experiments using planar phantoms with rough surfaces made of polyurethane rubber. The coefficient of variation calculated from the mean value of the reflection component and the standard deviation of the backscattering component was proportional to the roughness of the rubber phantom. This result shows the potential usefulness of this method for analyzing the surface roughness of the arterial wall.

High-frame-rate ultrasound imaging with plane wave transmissions is a predominant method of blood flow imaging, and methods for estimation of blood flow velocity vectors have been developed based on high-frame-rate imaging. On the other hand, in imaging of soft tissues, such as arterial walls and atherosclerotic plaques, high-frame-rate imaging sometimes suffers from high-level clutters. Even in observation of the arterial wall with a focused transmit beam, it would be highly beneficial if blood flow velocity vectors could be estimated simultaneously. We conducted a preliminary study on the estimation of blood flow velocity vectors based on a multi-angle Doppler method with focused transmit beam and parallel receive beamforming. It was shown that the lowest estimation error was achieved at a steering angle of 25° by simulation. Moreover, velocity vectors with typical velocity magnitudes and directions could be obtained by the proposed method in in vivo measurement of a carotid artery.

Acoustic self-bending beams for airborne ultrasound are highly expected to expand the capabilities of existing acoustic applications. Our previous study has implemented amplitude control based on the Airy function to achieve an ultrasonic self-bending beam in air. However, the amplitude control lacks sound pressure convergence into the main lobe. To improve the convergence, we propose using a reflector to achieve phase modulation. In this study, we used the Airy function and developed the reflector that incorporates different heights to control spatial phase distribution. Using the proposed reflector demonstrated a self-bending beam for an airborne ultrasound experimentally. In addition, compared to the amplitude modulation, the phase modulation using the proposed reflector improved convergence efficiency of sound pressure level by more than 6 dB at distances of 0.7 m and 1.0 m from the emitter.

We developed an audible sound source with horizontal omnidirectional patterns using facing ultrasonic transducer arrays. The arrays emitted sound with different ultrasonic frequencies from each side, and an audible sound with a differential frequency is generated between input ultrasonic signals. In particular, we designed and created a new array that can control the number of transducers driven in the array. We evaluated the frequency–amplitude characteristics and directivity when the transducers in the array were driven in an annular shape. There is an optimum array shape and number of transducers that can be driven for a specific distance between arrays.

In scanning acoustic microscopy (SAM), the image quality depends on several factors such as noise level, resolution, and interaction of the waves with sample boundaries. The theoretical equations for the reflection coefficient and transmission coefficient are suitable for plane boundaries but fail for curved/rough boundaries. We presented a finite element method-based modeling for the loss coefficients in SAM. A focused and unfocused lens with a flat object, furthermore a focused lens with a curved object was selected for loss coefficients calculation. The loss calculation in terms of energy for defining the acoustic reflection and transmission losses and its dependence on the radius of curvature of the test object has also been presented.

Controlled tablet disintegration is useful for chemical consistency checks. This study monitored the swelling of 54 analgesia tablets from two different batches, during 13-6-MHz brightness-mode sonication and simultaneous video recording. The tablets were placed on an acoustic reflector inside a container and sonicated from the top. Sonication shortened the displacement half-life by 17%–27%. During tablet swelling, their speed of sound increased linearly, confirming the linearity of this process. Diagnostic ultrasound significantly decreased tablet disintegration times, supporting the ultrasound-microbubble interaction hypothesis.

This paper discusses the applicability of double busbar design to surface acoustic wave (SAW) devices employing low-cut lithium tantalate (LT) with a multi-layered structure. This design offers good energy confinement, scattering loss suppression and transverse mode suppression for a wide frequency range. In addition, the effectiveness of manipulating the slowness curve shape for transverse mode suppression is demonstrated. First, three different lateral edge designs are applied to the layered SAW configuration on low-cut LT, and their performances are compared using the periodic three-dimensional finite-element method powered by the hierarchical cascading technique. Then, the discussion is extended to the influence of the SAW slowness shape to the transverse mode suppression.

Sonochemistry is an effective method for the initiation or enhancement of the chemical reactions by ultrasound in a wide range of applications. In this study, the efficiency of a sonochemistry transducer, defined as the ratio of ultrasonic power to electrical power, was investigated for different materials and the thicknesses of the vibration plate in the frequency range 22 kHz–2 MHz. The ultrasonic power was measured by calorimetry. To eliminate the influence of reflected waves, the transducer was attached to the side of a cylindrical vessel. The transducer with a stainless steel vibration plate was more efficient than those with vibration plates of acrylonitrile butadiene styrene plastic or chloroprene rubber. The efficiencies of the transducers also increased with decreasing thickness of the vibration plates. Langevin-type transducers were less efficient than the disk-type transducers.

Underwater acoustic (UWA) communication is an essential technology that supports underwater surveys by transmitting images and movies wirelessly. However, UWA communication is still challenging due to the large delay and Doppler spreads in UWA channels. Doppler-resilient orthogonal signal division multiplexing (D-OSDM) under water has been found effective in such channels, but Doppler modeling errors prevent further improvement in communication quality. In this paper, we clarify the effect of Doppler modeling errors on the communication quality of D-OSDM and propose the use of a window function in D-OSDM to address the issues that such errors raise. We also evaluate the communication quality of D-OSDM using the window function through simulations and experiments. The experimental results show a 56% increase in the number of error-free data blocks, indicating that windowing can improve communication quality.

Visualization of dermal circulation is important in the field of skin healthcare. We have developed a three-dimensional (3D) photoacoustic (PA) imaging system using a spherically curved array transducer that can visualize the microscale circulation in the skin layers, but limited anatomical information was available around the microvasculature. To provide such anatomical information, this study was aimed at devising a high-quality and high-speed ultrasound (US) imaging framework, particularly, for the spherical array transducer. We tested three synthetic transmit aperture (STA) methods, all-elements, outer-track, and inner-track, for transmission by evaluating the spatial resolution and uniformity of 3D images of point and copper-wire targets. The results demonstrated that the all-elements and outer-track STA methods could provide uniform and clear 3D images. In addition, the outer-track STA could be performed with fewer transmissions than the all-elements STA, and it will be useful for realizing real-time, high-resolution 3D PA/US imaging.

Noncontact acoustic inspection methods using acoustic irradiation-induced vibration and laser Doppler vibrometer that can perform defect exploration from a distance are being studied. This method has the feature that it can measure a wide range of measurement objects such as composite materials used for aerospace as well as concrete structures such as tunnels and bridges without contact. From the experimental results, it was found that the increase in noise level due to the decrease in the return light of the laser due to the condition of the measurement surface causes a decrease in the estimation accuracy of defect exploration. Therefore, it has been clarified that the detection accuracy of the defect position can be improved by devising a resonance judgement process for discriminating the signal and noise due to the resonance of the defect portion.

In this paper, moving sound source and receiver with an arbitrary trajectory are implemented in the three-dimensional compact explicit finite-difference time-domain method. To implement a moving sound source, a driving method in which the grid points around the source position are driven by the source distribution function is proposed. It is confirmed that the Gaussian distribution driving is suitable for the analysis of the moving sound sources. For a moving receiver, the sound pressure at the receiver is interpolated from the sound pressures of the adjacent eight grid points. The formulations and the numerical experiments are made for the three-dimensional sound field, and the accuracy of the proposed method is discussed. It is confirmed that the proposed method can be applied accurately to the moving sound source and receiver including the Doppler effect.

The objective of this study is to achieve vehicle self-localization using a single acoustic ranging sensor in a multipath environment. For this purpose, we proposed a measurement method of multiple time-of-flight (ToF) using an acoustic ranging sensor and a self-localization method using the ToFs. The proposed method predicts the ToFs based on the wall position and the predicted self-location and corrects the self-location by comparing it with the actual ToFs. Sound waves radiated indoors are reflected multiple times by every wall, ceiling, and floor. Therefore, the observed signal contains multiple reflected waves. Since the conventional method only considers a single reflection, self-localization becomes challenging in a multiple reflection environment. We showed that the estimation accuracy can be improved by utilizing the multiple reflections of sound waves in three-dimensional space and modeling them. The experiments confirm that the average location error of the proposed method is 0.084 m.

Acoustic underwater propulsion systems based on an ultrasonic transducer have been studied. In previous research, the self-propelled acoustic swimmer using thickness-vibration-mode transducer is evaluated widely. The thickness-vibration-mode transducer is excited in the thickness and radial direction. Because the acoustic propulsion system is based on the acoustic driving force, the vibration in the radial direction is hard to provide the propulsion thrust. In this study, a cylindrical transducer, the pure longitudinal vibrator, is evaluated for the acoustic underwater propulsion system. A prototype swimmer with multiple transducers is designed and fabricated. The admittance characteristics of the cylindrical transducer are investigated in air and in water. The zero speed propulsion and no load speed are measured in water. Multi-degrees-of-freedom swimmer with the multiple cylindrical transducers is evaluated. Because of the small size, high power density, simple structure and multi-degrees-of-freedom, self-propelled acoustic swimmer is suitable for applications such as inspection and repairment robots in a liquid environment.

We used ultrasonic shear waves for nondestructive defect detection in a billet using transmitted waves. We utilized the deviation the time-of-flight (TOF) obtained by cross-correlation of transmitted waves of a defect-free reference plane and that of a measurement plane containing a defect. We compared the performance of longitudinal waves and shear waves at different wavelengths in detecting the diameter of a circular defect in two-dimensional (2D) simulation and the TOF for a cylindrical defect while changing the vibration direction of shear waves in three-dimensional (3D) simulation. Shear waves detected defects better than longitudinal waves in the 2D simulation, especially at wavelengths of 1.4–2.4 mm. In the 3D simulation, the maximum TOF was larger when the vibration direction was perpendicular to the defect's major axis than when it was parallel in the measurement using shear waves. This suggests a defect's shape can be estimated by measurement using shear waves.

Minimum variance (MV) beamformers have been introduced in medical ultrasound imaging to improve image quality. In most cases, the MV beamformers have been investigated in terms of resolution improvement. However, the contrast-to-noise ratio (CNR) is also a clinically important metrics and gathers attention recently. In this study, we examined the diagonal loading parameter σ in MV beamforming and determined its appropriate value by evaluating image quality evaluation metrics including CNR. In order to further improve the image quality, a method for determining the value of σ based on the difference in statistical properties of received ultrasonic echo signals was also investigated. The phantom experimental results showed that the proposed method achieved a better CNR than the conventional MV beamformer while keeping resolution significantly better than that in delay-and-sum beamforming.

This paper describes a high-frequency bulk acoustic wave resonator (BAWR) with a solidly-mounted (SM) structure using single crystal LiTaO₃ (LT) thin plates. A Bragg reflector solidly supports the LT thin plate, which is fragile if self-suspended. The two kinds of BAWRs use a strip-type thickness shear mode in 0.56 μm thick X37°Y LT and X127°Y LT. The Bragg reflector is made of 5 pairs of Al and Ta films, i.e. 10 layers in total. The X37°Y LT SM structure BAWRs exhibited a resonance frequency (f_r) of 3.250 GHz, an anti-resonance frequency (f_a) of 3.463 GHz, a bandwidth (BW) of 6.6%, and an impedance (Z) ratio of 48 dB. The X127°Y LT BAWR has a similar characteristic with f_r of 3.153 GHz, f_a of 3.367 GHz, a BW of 6.8%, and a Z ratio of 46 dB. The X37°Y LT BAWR showed an advantage of 2 dB compared with the X127°Y LT one. The Al film acts as a low acoustic impedance film, but an acoustic impedance layer combination of Al and Ta films is not suitable and that of SiO₂ and W films suitable among the four combinations, Al/Ta. SiO₂/Ta, Al/W, and SiO₂/W films. Although the acoustic impedance layer combination, the acoustic film quality, the structure, electrode design, and fabrication process of the fabricated BAWRs were not optimized yet, this result suggests the high potential of this device.

For the early diagnosis of atherosclerosis, our group developed an ultrasound probe that can simultaneously measure blood pressure and vessel diameter at the same position. However, because the developed probe requires the blood vessel to be deformed by pushing to measure the blood pressure, it affects the estimation of the vessel's elastic modulus. In the present study, we derived a series of equations to estimate the elastic modulus of a blood vessel considering the pushing pressure applied by the ultrasound probe and the resultant deformation of the blood vessel. The validity of the proposed method was verified by numerical calculations, and then the method was applied to in vivo measurements. The proposed method resulted in fewer variations in the elastic modulus estimates with different pushing pressures compared with the conventional method.

We propose a dark-field evanescent imaging method to visualize surface/subsurface micro defects with a high signal-to-noise ratio (SNR). This method utilizes the mode-converted longitudinal evanescent field (MCLEF) generated at defects by the incidence of a shear (S) wave. When an incident S wave only has the in-plane displacement on the top surface of a specimen, the 2D scan of a laser Doppler vibrometer, that can only measure out-of-plane displacements, can selectively probe the MCLEF with out-of-plane displacements. Note that the MCLEF can be generated even at a defect that is much smaller than the diffraction limit. In this paper, after describing the principle of the proposed method, we prove the concept in a specimen with a hole by finite element (FE) simulation and experiments. Further FE simulations demonstrate its super-resolution imaging capability for holes of different sizes and higher SNR than a conventional method for various defect geometries.

This paper reports the effectiveness of a novel imaging system, piezoelectric and laser ultrasonic system (PLUS), for the three-dimensional (3D) imaging of fatigue cracks with a high-resolution. The PLUS combines a piezoelectric transmitter and the two-dimensional (2D) mechanical scanning of a laser Doppler vibrometer, enabling the 2D matrix array with an ultra-multiple number of receiving points for 3D phased array imaging. After describing the principle and 3D imaging algorithm of PLUS, we show the fundamental 3D imaging capability of the PLUS in a flat-bottom-hole specimen with varying the number of receiving points under a fixed large receiving aperture. We then demonstrate that the PLUS with 4275 receiving points (i.e. 75 × 57) achieves high-resolution 3D imaging of a fatigue crack with a high signal-to-noise ratio, providing the outline of the fatigue crack geometry. We also discuss the effectiveness of the ultra-multiple receiving points for suppressing grating lobes and random noise.

In this report, we proposed the installation of a conical phase adjuster (CPA) in a thermoacoustic prime mover as a method for reducing the onset temperature and investigated the effect of the installation position of a CPA on the onset temperature using stability analysis. The onset temperature of CPA also was investigated experimentally by changing the installation position of CPA. As a result, when CPA was installed at 1000 mm from the high-temperature end of the stack, the onset temperature was 195 K lower than the onset temperature without CPA, that is, the installation of the CPA in a loop-tube-type thermoacoustic prime mover reduced the onset temperature by 29%. Comparing onset temperatures of the PA had installed in a system, CPA is considered to have the same effect as a PA because the onset temperature tendency of the CPA to reduce at the installation position agrees with that of the PA.

Noninvasive measurement of the degree of red blood cell (RBC) aggregation is useful for evaluating blood properties. In the present paper, we proposed a method to estimate the size of RBC aggregates without using the power spectrum of the posterior wall by introducing a reference scattering spectrum. The reference power spectra were calculated using the power spectrum measured for an ultrafine wire with a hemispherical tip. They were applied to the size estimation of microparticles simulating RBC aggregates. The estimated sizes were close to the true values, which shows that the calculated reference power spectra were suitable for accurate size estimation. The proposed method was also applied to in vivo measurements, and the estimated sizes between at rest and in RBCs aggregated by avascularization were successfully differentiated. This demonstrates that the proposed method will be useful for estimating the size of RBC aggregates.

To achieve fine visualization of the peripheral microvascular networks, we have developed a photoacoustic (PA) microscope equipped with a four-channel annular array transducer. The quality of PA images processed with delay-and-sum (DAS) method is degraded by off-axis signals. Thus, to achieve higher image quality for the PA microscope, this study evaluated the efficacy of the five coherence factor weighting methods: coherence factor, sign coherence factor, phase coherence factor, circular coherence factor, and vector coherence factor. Using PA signals acquired from a 100 μm microtube and the skin microvessels, we generated PA images with DAS and one of the weighting methods, and quantitatively evaluated the image quality by calculating the sharpness, contrast ratio, and contrast-to-noise ratio. The results showed the phase coherence factor and the vector coherence factor methods were more effective to clearly visualize the microvascular structure, in terms of vessel sharpening and noise suppression performances, than the other methods.

Controlling sound fields is a key technology for noise removal, acoustic lenses, energy harvesting, etc. This study investigated the control of sound field by a periodic layered structure. At first, we formulated the wave propagation in a periodic layered structure and proved that the wave fields constructed by the periodic boundary conditions are limited to plane wave modes with discretely different propagation directions. Numerical calculations clarified that the desired plane wave mode can be obtained in the transmitted wave through an intermediate thin-plate stacked region in a periodic layered structure, in which Lamb waves travel in each plate at different phase velocities and create phase difference at the exit of the intermediate thin-plate region. Further numerical investigations revealed that tuning frequency and the length of the thin-plate region provides wave field more dominantly with a single wanted plane wave mode.

In a previous study, an annular-array transducer was employed to characterize homogeneous scattering phantoms and excised rat livers using backscatter envelope statistics and frequency domain analysis. A sound field correction method was also applied to take into account the average attenuation of the entire scattering medium. Here, we further generalized the evaluation of backscatter coefficient (BSC) using the annular array in order to study skin tissues with a complicated structure. In layered phantoms composed of two types of media with different scattering characteristics, the BSC was evaluated by the usual attenuation correction method, which revealed an expected large difference from the predicted BSC. In order to improve the BSC estimate, a correction method that applied the attenuation of each layer as a reference combined with a method that corrects based on the attenuation of the analysis position were applied. It was found that the method using the average attenuation of each layer is the most effective. This correction method is well adapted to the extended depth of field provided by an annular array.

A scanning airborne ultrasound source technique was developed to overcome the riskiness of laser ultrasound, which uses an ultrasound source that has a fixed sound wave focusing point and thus requires mechanical motion for sound source scanning. Therefore, the measurement time becomes longer. To solve this problem, we have proposed a method of simultaneously exciting many measurement points in the target using focused ultrasound sources of different frequencies. In this paper, we investigated the visualization of defects in a thin metal plate by the scanning elastic wave source technique using an airborne ultrasound source driven at two frequencies. When the testing was performed using two frequencies, either frequency visualized the defects.

For on-site analysis of surface materials on the Moon, planets, and small bodies and for the monitoring of air quality in crewed spacecraft, we have developed a portable gas chromatograph (GC) equipped with a ball surface acoustic wave (SAW) sensor. In this study, we fabricated a 10 cm cube GC that implements the forward flush method using two metal micro-electro-mechanical-system columns coated with different stationary phases in microchannels fabricated by wet etching and diffusion bonding of stainless-steel plates. Using this GC, we succeeded in analyzing 10 kinds of gas within 10 min. In addition, for the application of the ball SAW GC on the ground, we also developed a palm-sized GC with a single metal capillary column and used it in the analysis of the headspace gas of sake. We showed that the ratio of peak areas differed among odorants depending on the brand and brewing process of sake.

The propagation and resonance properties of longitudinal leaky surface acoustic waves (LLSAWs) on bonded structures consisting of a quartz (Qz) thin plate and a Qz support substrate with different Euler angles were investigated theoretically. By using both an X-cut Qz thin plate and a Qz support substrate with optimal Euler angles, we obtained LLSAWs with a larger coupling factor, a smaller attenuation, and a lower temperature coefficient of frequency than those on a single Qz substrate. Furthermore, from the resonance properties simulated by the finite element method, the bonded structures were found to exhibit a large admittance ratio and a high quality factor, which could not be obtained when using a single Qz substrate; the bandwidth, however, was as small as 0.016%–0.086%.

A phase-sensitive 2D motion estimator is useful for measurement of minute tissue motion. However, the effect of conditions for emission of ultrasonic waves on the accuracy of such an estimator has not been investigated thoroughly. In the present study, the accuracy of the phase-sensitive 2D motion estimator was evaluated under a variety of transmission conditions. Although plane wave imaging with a single emission per frame achieved an extremely high temporal resolution of 10417 Hz, the accuracy in estimation of lateral velocities was worse than compound-based method or focused-beam method. By contrast, the accuracy in estimation of axial velocities hardly depended on the transmission conditions. Also, the phase-sensitive 2D motion estimator was combined with the block matching method to estimate displacements larger than the ultrasonic wavelength. Furthermore, the results show that the correlation coefficient in block matching has potential to be used for evaluation of the reliability of the estimated velocity.

ScAlN films are currently being investigated for their potential use in surface acoustic wave (SAW) devices for next-generation mobile networks because of their high piezoelectricity. This paper describes the numerical simulation of SAW propagation in c-axis-tilted ScAlN films on silicon substrates and a fabrication technique for preparing c-axis-tilted ScAlN films on silicon substrates. The electromechanical coupling coefficient K² of SAW propagating in the ScAlN film/silicon substrate increased due to the c-axis tilt angle. The maximum K² value is approximately 3.90%. This value is 2.6 times the maximum K² value of the c-axis-oriented ScAlN film/silicon substrate structure. The c-axis-tilted ScAlN films with an Sc concentration of 40% were prepared on a silicon substrate via RF magnetron sputtering based on the self-shadowing effect, and the maximum c-axis tilt angle was 57.4°. These results indicate that this device structure has the potential for SAW device applications with well-established micromachining technology derived from silicon substrates.

The influence of the reflected waves at the bonding boundary on the resonance waveform and temperature characteristics was investigated using $\alpha$-quartz (QZ). The double-layered resonator specimen was fabricated using 129.55°Y- and 0°Y-cut QZ substrates with the thickness ratio x = 0.520. The temperature characteristic of the 1st and 2nd resonance modes at the range from 100 °C to 300 °C was deviated from the calculated values estimated by the equations considering thickness ratio and electric flux density ratio proposed in the previous work. Considering the phase matching conditions, it was clarified that the reflected waves at the bonding boundary are in phase in the lower acoustic impedance substrate and out of phase in the other substrate for the 1st mode, so the temperature characteristics are mainly dominated by that of the lower acoustic impedance substrate. In contrast, these relationships are reversed for the 2nd mode.

The effect of 200 kHz ultrasound on scorodite synthesis at 70 °C and 3 h reaction conditions was investigated using sulfuric acidic solutions of various pH (3.0, 2.0, 1.5, 1.0, and 0.0). In contrast to the case of only O₂ gas flow without ultrasound irradiation, oxidizing radicals generated by ultrasound irradiation promote Fe(II) oxidation in solution and precursor, allowing scorodite to synthesize with high crystallinity (>99%), which relates to low solubility, even in strong acid solution at pH 1.0. During synthesis, the particle shape was decided to be polyhedral or spindle type depending on the pH of 0.0–3.0. The spindle-shaped scorodite was probably formed by the reduction in precursor amount produced during the initial stage of synthesis. Furthermore, porous maghemite obtained by alkali treatment of scorodite showed initial discharge capacities of 146 mAh g⁻¹ (polyhedron) and 167 mAh g⁻¹ (spindle), indicating that its potential use as a cathode material for lithium-ion batteries.

In this paper, the adaptive time reversal (ATR) technique combined with a single channel decision feedback equalizer (DFE) is applied to an at-sea experiment of the underwater acoustic multiple-input/multiple-output (MIMO) communication utilizing the signal bandwidth of 4.5–8.5 kHz over the distance of 12.5 km. This paper focuses on the demonstration of the effectiveness of the spatial division multiplexing MIMO using ATR-DFE schemes in time-varying real sea environment, and how much the communication rate could be achieved, with changing the signal parameters. We report that the communication quality is dependent on the duration from the channel probing for ATR process to signal reception, and we found that the output signal-to-noise ratio of the demodulated results highly correlated to the temporal coherence of the acoustic channel. Finally, the relationship between the effective data rate and the bit error rate is evaluated. As a result, the achievable data rate in the experiment of this study would be a remarkable considering the distance of the communication.

The hardening of (Bi_0.5Na_0.5)_0.85Ba_0.15TiO₃ (BNBT15) piezoelectric ceramics was investigated by adding raw materials with Bi_0.5Na_0.5MnO₃ (BNM). BNBT15-BNM exhibited a single phase of BNBT15. BNM acts as a sintering aid for BNBT15 to produce domain pinning, and produces tetragonality based on BaTiO₃ for increased stability. BNBT15-BNM hardens piezoelectric material with low Mn content, increasing the coercive field and mechanical quality factor. The mechanical quality factor of BNBT15-BNM (0.75 wt%) exceeded 1200. In high-power conditions, BNBT15-BNM (0.75 wt%) exhibited a vibration velocity twice that of hard-PZT. The quality factor gradually decreased with a high vibration velocity. The equivalent stiffness slightly decreased with strain, and the mechanical nonlinearity was much less than that of hard-PZT. BNBT15-BNM (0.75 wt%) has superior high-power properties, and is expected to be a candidate material for use in lead-free piezoelectric ceramics in high-power applications.

The rapid development of mobile communications requires high performance surface acoustic wave (SAW) filters. ScAlN film SAW resonators have a high coupling factor (K²) and high phase velocity, enabling them to function as effective SAW filters. We theoretically investigated high-order mode Rayleigh SAWs (RSAWs) on single-layered or polarity inverted two-layered ScAlN film/AlN or BN substrate structures to find the optimal structure to achieve a higher K² and higher frequency. The K² and phase velocity for the single-layered ScAlN film/BN substrate structure were higher than those for the corresponding AlN substrate structure. The K² was enhanced in the polarity inverted ScAlN film/AlN or BN substrate structure. Finite element method analysis revealed that the effective coupling factor K_eff² for the 2nd to 4th mode RSAWs on polarity inverted multilayered ScAlN films was enhanced by setting the boundary of the polarity inverted structure at the all positions where the particle displacements were concentrated.

A LiNbO₃ (LN) based sol-gel composite could be suitable for high temperature ultrasonic transducer application at 700 °C, however, poling requires high temperature above 550 °C and it shows relatively low signal strength. In order to realize the polarization temperature reduction of an LN-based sol-gel composite ultrasonic transducer, alumina (AO) and strontium doped titanium oxide (TO) were chosen as sol-gel phase material, and LN/AO and LN/TO films were fabricated onto Inconel substrates. In each polling process, corona discharge after heat treatment in the furnace at 400 °C and at 200 °C was executed for LN/AO and LN/TO, respectively. Ultrasonic measurements up to 700 °C were performed and both transducers were able to confirm the reflected echoes and ultrasonic performance stability at 700 °C. Especially LN/TO showed the highest signal strength compared to previously developed high temperature lead-free sol-gel composite materials, Bi₄Ti₃O₁₂ (BiT)/BiT and CaBi₂Ta₂O₉ (CBTa)/BiT.

Improving spatial resolution is a crucial issue in medical ultrasound. One of the improving methods is the post-processing of the received ultrasound RF signal. In the present paper, we proposed a design method for a noise-robust broadband filter based on the singular value decomposition of the received RF signal. To design a noise-robust filter, we proposed a logical method to determine the optimal truncated order of singular values, which was validated by applying the filter to noise-contaminated signals. Furthermore, the proposed filter applied to the wire phantom resulted in a better axial resolution than that obtained without the filter and with our previously designed Wiener filter.

For the clinical application of high-intensity focused ultrasound (HIFU), improvement in monitoring and guidance methods is necessary to enhance the treatment accuracy. Among the ultrasonic imaging techniques, thermal strain imaging has a potential to estimate temperature change based on the linear relationship between the thermally induced strain and temperature change via a tissue-dependent coefficient. In this study, the coefficient was experimentally measured and the temperature rise induced by the HIFU irradiation was estimated based on the measured coefficient in a tissue mimicking material phantom. The temperature rise estimated using the measured coefficient and that simulated based on a bioheat transfer equation showed a good agreement when the spatial averaged effect in the elevational direction was considered. The exponential decay time constants also agreed well within the range of measurement.

A number of small ultrasonic transducers were placed on a flat surface to form a directional ultrasonic sound source. A standing wave field with a hexagonal distribution of sound pressure like a honeycomb was formed when ultrasonic waves were superimposed from three directions using three of these sources. Small objects could be trapped at the nodes of the sound pressure in the sound field. When the phase of the three sources was changed, the sound pressure distribution shifted in the direction of the sound axis of the sources, and the objects trapped at the pressure nodes also shifted. For more stable object trapping, the ultrasonic transducers were placed on the inner wall of a semicylinder and the ultrasonic waves were focused to form a thin two-dimensional planar standing wave field. Three of these sources were used in the experiment, and it was possible to manipulate the objects more stably.

In this paper, we introduce a method to measure the surface tension of a droplet on a solid substrate by observing the resonance oscillation excited by applying Maxwell stress using electric field tweezers in a noncontact manner. Additionally, we measured the frequency spectrum of the oscillation amplitude using a stroboscopic imaging technique. The resonance frequency of the droplet was inversely proportional to the 3/2th power of the droplet radius, with a contact angle of approximately π/2 rad. The acquired result is in good agreement with the theory derived by extending the established formula for a free-sphere droplet. Furthermore, the contact angle dependence of the resonance frequency can be qualitatively explained based on the behavior of waves on a confined liquid surface.

As a vibrator needs to be pressed onto the osseous parts of the head, bone conduction is often accompanied by pain and esthetic problems. To solve these problems, a "distant presentation" that involves presenting vibrators to the neck, trunk, or upper limb was proposed. Our previous studies focused on the perceptual and propagation characteristics of distantly presented bone-conducted sounds in the ultrasonic range. However, only a few studies have been conducted in the audible-frequency range. In this study, to examine the basic properties of distantly presented bone-conduction perception in the audible-frequency range, hearing thresholds, difference limens for frequency, and temporal modulation transfer functions were measured with insulated air-conducted sounds. The results indicate that the distance attenuation is much larger than that in the ultrasonic range, and the degradation of frequency and temporal information occurring in the propagation process of bone-conducted sounds is sufficiently small for the transmission of sound information.

We investigated the viability of vascular endothelial cells with the existence of lipid bubbles under ultrasound exposure. First, we estimated the various situations of bubbles on the cells including either adhesion, floating, or both of them using not only image analysis but also an experiment to retain the cells in flow. Then we examined the viability measurement of the cells using the ultrasound conditions with the frequency of 3 MHz, a maximum sound pressure of 400 kPa pp, and a maximum irradiation time of 60 s. We found that the floating bubbles caused more damage on the cells rather than the adhered bubbles. Because insufficient adhesion of bubbles might cause damage by floating bubbles, we consider that the adhered bubbles were protective of cells against floating bubbles. However, excessive bubbles with a higher concentration than the saturation also might cause damage by destructing both the floating and adhered bubbles.

A blood mimicking fluid (BMF) is imperative for the evaluation of Doppler ultrasound. Doppler ultrasound still causes errors due to some artifacts such as aliasing and presence of grating lobes. One of the other velocimeters is the optical particle image velocimeter (PIV). This study initially developed an in vitro measurement system for analyzing flowing BMF with ultrasonic and optical PIVs. The acoustic properties such as speed of sound, attenuation, and backscatter coefficient of BMF equivalent to the human blood, used for both ultrasonic and optical PIVs were analyzed in a frequency range of 4–12 MHz. The velocity profiles were estimated by ultrasonic and optical PIVs using a block matching method. A difference between velocities obtained by ultrasonic and optical data was within 4.0% using BMF with 20 μm polyamide particle at 0.2% concentration that realized the acoustic properties and speckle patterns similar to those in ultrafast ultrasound blood flow imaging.

In epidural anesthesia, it is difficult to specify the puncture position of the anesthesia needle. We have proposed an ultrasonic method to depict the thoracic spine using the different characteristics of reflection from bone and scattering from muscle tissue. In the present paper, we investigated the transmission aperture's width of the ultrasound probe to emphasize the differences in the reflection and scattering characteristics. First, we determined the optimum transmission aperture's width using a simulation experiment. Next, we measured reflection and scattering signals by changing the transmission aperture's width in a water tank experiment and confirmed that the results corresponded to the simulations. However, as the transmission aperture's width increased, the lateral resolution at the focal point improved. Therefore, better imaging of the human thoracic vertebrae can be achieved by selecting the transmission aperture's width, which considers the effect on lateral resolution.

One of the problems with ultrasound imaging during high-intensity focused ultrasound (HIFU) treatment is that the therapeutic ultrasound components interfere with the diagnostic ultrasound components, making it impossible to monitor the tissue changes during HIFU exposure. In this study, a convolutional neural network (CNN) framework was applied to the reconstructed ultrasound images with HIFU noise to remove the therapeutic ultrasound components while the diagnostic ultrasound components remain intact. In the experiments, the chicken breast was used as a tissue sample and exposed to HIFU in the water tank. The ultrasound images with and without noise were acquired during an intermission period of HIFU exposure and the noise-reduced images was predicted using the proposed multi-layer regression CNN model through the training process. As a result, ultrasound images with sufficient spatial resolution to detect the thermal lesion were acquired.

Scanning acoustic microscopy (SAM) is a useful observational tool in cellular study as non-invasive living observation is feasible, unlike in conventional optical microscopy. In a previous study, cell morphological changes were successfully visualized using acoustic impedance measurements. These acoustic impedance changes correspond to cell elasticity, mainly reflecting changes in the cytoskeleton. In this study, we evaluate the elastic changes in murine breast cancer cell C127I during mitosis. C127I cells were cultured to ∼75% confluency before measurement, using a transducer with a central frequency of 320 MHz. Dynamic changes during mitosis were successfully mapped using SAM and confirmed by laser confocal microscopy. Cells in prometaphase, anaphase, and telophase, which could previously only be confirmed through immunostaining, were successfully visualized using SAM. This suggests that SAM is capable of distinguishing cells in different mitotic phases based on the changes in acoustic impedance.

We confirmed that bubble-surrounded cells (BSCs) contained in flow were retained on the walls of an artificial blood vessel by forming an acoustic field with multiple focal points using tempo-spatial division emission. In order to realize the cell delivery system, we investigated the relationship between the concentration of T-cells and brightness in the microscopic images. Next, we defined the applied acoustic intensity, derived from the sound pressure distribution of every type of acoustic field. We studied the retention performance of BSCs versus various flow velocities, number and spatial intervals of the focal points, and maximum sound pressure. From the results, the optimal acoustic field to retain the cells depends on both acoustic intensity and flow velocity, where multiple focal points with an acoustic intensity of 50–120 mW cm⁻² were more effective than the single focal point with 180 mW cm⁻² in the range of a flow velocity of 10–20 mm s⁻¹.

The use of harmonics offers high resolution and low artifact imaging. However, the image intensity depends on the depth of field and is significantly weaker than the fundamental echo. The simultaneous use with the fundamental echo is therefore desirable. However, the frequency bands corresponding to the fundamental and harmonics are different, and the difference in value between their amplitudes is large. Imaging them simultaneously is difficult. Therefore, we propose a method that employs a single short-period transmission of a low-frequency pulse signal so that the fundamental and the harmonic meet. Then we use the subband compound method which treats the fundamental and harmonic bands as a single frequency band. Several regularized subbands of different frequencies are subsequently extracted from the entire frequency band. In effect, this method uses the phase information of the frequency band and improves the spatial resolution and signal-to-noise ratio through sub-band amplitude modulation while suppressing artifacts.

The agglomeration by acoustic waves is a phenomenon in which the vibration of fine particles is induced by irradiating the fine particles suspended in the atmosphere with ultrasonic waves, and the collided particles adhere to each other by causing frequent collisions between the fine particles, thereby increasing the particle size. Ultrasonic agglomeration of an aerosol flowing in a circular tube by using a conventional method with a sound field with a different diameter from the duct causes pressure loss. In this study, a cylindrical aerial ultrasonic source with the same diameter as the circular tube was used, and the aerosol was agglomerated without pressure loss. Several types of agglomeration chamber for the aerial ultrasonic source were fabricated, and the effect of the input power of the source on the agglomeration was measured.

In the ultrasonic vibration-assisted manufacturing method using complex vibration, the amplitude amplification factor of horn for each vibration is important. In this paper, we focus on longitudinal-torsional vibration as a complex vibration and propose a stepped horn with a hollow part as a horn that can design the amplification factors of these vibrations individually. The proposed equation for the amplification factor of the horn with a hollow part was derived from the equation of the conventional step horn without a hollow part, and the validity of the proposed equation was verified by the finite element method and experimental measurements. As a result, the validity of the equation was confirmed, and it was clarified that the proposed stepped horn with a hollow part can be individually designed for the amplitude amplification factors of longitudinal vibration and torsional vibration.

One of the challenges in underwater acoustic positioning is the occurrence of missing measurements and large errors in multipath environments, such as shallow water and harbor areas. In this paper, we propose a new underwater positioning method for multipath environments by using direct wave arrival time groups and database matching. The proposed method accurately measures baseline length from the impulse response of the underwater channel by calculating time window groups that cover the propagation time from the sound source to each hydrophone in advance and then extracting only the impulse response around the propagation time of the direct waves when the sound source exists in a certain region of the measurement space. The performance of the proposed method was experimentally evaluated in a static environment. The results showed that the proposed method achieved an accuracy of 0.03 m and a precision of 0.02 m in a test tank.

Tantalum pentoxide (Ta₂O₅) thin films were deposited on Pt(100)/Si(100) and SrRuO₃(SRO)/Pt(100)/Si(100) substrates using an RF magnetron sputtering system. From the evaluated orientation and piezoelectricity of the deposited thin films, it was clarified that the Ta₂O₅ thin films were crystallized to λ-Ta₂O₅ without piezoelectricity on the Pt/Si substrates and to β-Ta₂O₅ with piezoelectricity on the SRO/Pt/Si substrates. The electromechanical coupling factor k_t² of the deposited film containing β-Ta₂O₅ was measured to be 0.36% from the response of a high-overtone bulk acoustic resonator, whereas that of the deposited film containing λ-Ta₂O₅ was measured to be 0.03%. Furthermore, the enhancement of the electromechanical coupling factor of surface acoustic waves (SAWs) by adding a high-density Pt intermediate layer was clarified from the resonance property simulated by the finite element method. This enhancement was due to the distributed particle displacement of the SAWs throughout the Ta₂O₅ thin film.

In this study, the bulk and surface acoustic waves (BAW and SAW) propagation properties of (K,Na)NbO₃ (KNN) films deposited by hydrothermal synthesis or RF magnetron sputtering methods were evaluated to investigate the applicability of such films to high-frequency devices. For the {100}_c-oriented KNN epitaxial films deposited by the hydrothermal synthesis method, a BAW phase velocity of 6900 m s⁻¹ and an electromechanical coupling coefficient k_t² of 8.4% were obtained. From the measured Rayleigh-type SAW properties, a large electromechanical coupling coefficient K² of 4.0% in the 1st mode was obtained in the {110}_c-oriented KNN epitaxial films. On the other hand, for the preferentially {100}_c-oriented KNN film deposited on Pt(111) by RF magnetron sputtering, a BAW phase velocity of 7850 m s⁻¹ and k_t² of 7.4% were obtained. For the 0th mode of the Rayleigh-type SAW, a propagation loss of 0.13 dB/λ (λ: wavelength) at 440 MHz and a temperature coefficient of frequency of –42 ppm °C⁻¹ were obtained for the {100}_c-oriented KNN epitaxial film deposited on STO(100) by the RF magnetron sputtering method.

In sound wave propagation in the sea, it is important to evaluate the characteristics of sound reflection from the sea surface. The amplitude and phase of reflected sound waves fluctuate because of the changing sea surface with waves. In this study, using the finite difference time domain method and experiments in a water tank, we evaluated the variability characteristics of reflected sound waves on the sea surface generated by the Bretschneider-Mitsuyasu spectrum observed on the deep offshore coast of Japan. We introduced the concept of effective roughness of the sea surface and evaluated the variability characteristics of reflected sound waves on the sea surface. We confirmed the Rice distribution could express the amplitude variability, and the energy ratio γ is determined by the Rayleigh roughness parameter $2k{\sigma }_{z0},$ defined by the effective roughness ${\sigma }_{z0}$

We previously proposed a beamformer that adaptively compounds echoes for different subbands and transmission angles. This methodology requires the transmission and reception of multiple plane waves. Thus, in the present study, we examine a method that approximates the previous method with one transmission and reception. We assign different subbands to each transmission direction angle and simultaneously transmit one shot as a chirp signal; hence, echoes for all subbands can be received simultaneously. Then, through pulse compression, the received echo is separated into each subband, and we apply our previously proposed compound procedure to achieve imaging using one-shot beamforming. The evaluation of the method performance was conducted by finite element simulation. The results show that the obtained image is almost the same resolution as the original beamformer, but with a worse contrast. The cause and solution of the contrast deterioration are also reported in this paper.

This study explores the rigidity of Pickering-stabilised microbubbles subjected to low-amplitude ultrasound. Such microbubbles might be suitable ultrasound contrast agents. Using an adapted Rayleigh–Plesset equation, we modelled the dynamics of microbubbles with a 7.6-N m⁻¹ shell stiffness under 1-MHz, 0.2-MPa sonication. Such dynamics were observed experimentally, too, using high-speed photography. The maximum expansions were agreeing with those predicted for Pickering-stabilised microbubbles. Subjecting microbubbles to multiple time-delayed pulses yielded the same result. We conclude that Pickering-stabilised microbubbles remain very stable at low acoustic amplitudes.

Oil sand contains about 15 wt% bitumen which contains approximately 5 wt% sulfur. Bitumen must be extracted from sand and desulfurized before use as a fuel. Currently, bitumen is recovered from sand using hot water (80 °C) and sulfur is removed via hydrodesulfurization (200 °C–450 °C), which consumes large amounts of energy. Therefore, we investigated the separation of bitumen from sand and the oxidative desulfurization of bitumen using ultrasound and n-pentane at 20 °C. We successfully extracted 94 wt% bitumen from sand and removed 66.1% of the sulfur via oxidative desulfurization using 15 wt% H₂O₂ and 5 mol l⁻¹ NaOH.

We fabricated a non-contact identification system employing multiple-frequency air ultrasonic transducers and a microphone capable of broadband measurement. This study aims to perform non-contact identification of the state of cloth using broadband acoustic analysis and machine learning. We conducted experiments to obtain basic data on the relationship between the moisture content of cloth and the frequency–amplitude characteristics. Using the proposed system, which combines high-resolution acoustic measurement and machine learning, we succeeded in noncontact identification of the moisture content of fabric. In addition, we verified the feasibility of this system in identifying whether the fabric material is cotton or polyester.

Recently, remarkable developments in contact-less manipulation methods using ultrasonic transducer arrays have been reported. Ultrasonic non-contact manipulation enables precise control of small objects without contamination; therefore, it is expected to have a wide range of applications. In this paper, we present a novel approach for the stable non-contact pick-up of small particles from a rigid stage using a hemispherical ultrasonic transducer array. We achieved stable pick-up by employing adaptive phase and amplitude control against the distance from the reflection stage.