Kojiro Nishimiya

Visualization of Mode Conversion of Lamb-Type Waves in Glass Plates Using Pulsed Light Source

Presently, a number of visualization methods of sound waves are investigated in order to understand their behaviors in various mediums. In this study, we employ the visualization method using photoelasticity. In this method, the stress in elastic plates is visualized using the birefringence effect arising from ultrasound stress, through two orthogonal polarization plates. As visualization objects, we observed Lamb and Lamb-type waves under some conditions. Among them, the condition that the layer changes during the wave propagation is very interesting. Under this condition, not only Lamb and Lamb-type waves but also a propagating wave along the boundary appears at the same time. In this study, we can observe the mode conversion of elastic waves by changing the boundary condition during the wave propagation.

Negative group velocities of lamb-type waves in a glass/water/glass structure controlled by the thickness of water layer

In this paper, we described the characteristics of negative group velocities of Lamb-type waves in a glass/water/glass structure controlled by the thickness of the water layer. Lamb waves in an elastic plate have negative group velocities. It is known that the range of existence of negative group velocities depends on the Poissons ratio of the elastic plate. However, in the case of Lamb waves, their negative group velocities depend on the physical parameters of each material; therefore, we fabricated a structure such that the negative group velocities of Lamb waves are controllable without the limitation of the parameters of each material. The waves in such a structure are referred to as the Lamb-type waves. Using numerical calculations, we determined that the negative group velocities of the Lamb-type waves can be varied by changing the geometric parameters, and the negative group velocities are changed although the frequencies are fixed. Additionally, we found that the Lamb-type waves have various characteristics corresponding to the propagation modes.

Relationships between Existence of Negative Group Velocity and Physical Parameters of Materials for Lamb-Type Waves in Solid/Liquid/Solid Structure

We considered the negative group velocity of plate-mode waves. Lamb waves are a typical example of existence of negative group velocity. However, if we try to apply the negative group velocity of Lamb waves to some applications such as acoustical flat lenses, there is a problem about the existence of negative group velocity of Lamb waves. Its existence depends only on Poissons ratio. That is, the negative group velocities of Lamb waves depend only on physical parameters of materials. Consequently, we considered to control a negative group velocity. The negative group velocity of Lamb-type waves in a solid/liquid/solid structure can be controlled by changing the thickness of a liquid layer. In this research, we considered the relationships between the existence of negative group velocities and the parameters of each layer material with respect to Lamb-type waves in a solid/liquid/solid structure by numerical calculation. As a result, it was confirmed that the negative group velocity of Lamb-type waves depended not only on Poissons ratio but also on the density of each layer and the longitudinal wave velocity of the liquid layer. This result is useful when the negative group velocity of Lamb-type waves is applied to acoustical flat lenses, which require the negative group velocity.

Reconstruction of Three-Dimensional Sound Field from Two-Dimensional Sound Field Using Optical Computerized Tomography and Near-Field Acoustical Holography

We propose a method for reconstruction of a three-dimensional sound field using optical computerized tomography (O-CT) and near-field acoustical holography (NAH). The center of a transducer is the origin, and a sound wave is radiated along the z-axis. An ultrasonic wave affects the phase of light passing through the sound field, and the phase change is determined using a Michelson interferometer. Projections of the underwater sound field were obtained by mechanical scanning, and the sound field was reconstructed from 18 of these projections by O-CT. To determine sound velocity for reconstruction of the three-dimensional sound field, two-dimensional sound fields at z = 40 and 41 mm were reconstructed using O-CT in a region of 28 ×28 mm2. The sound velocity was determined to be 1508 m/s using inverse analysis of NAH. Using NAH and the determined sound velocity, the three-dimensional sound field of 28 ×28 ×5 mm3 was reconstructed from the two-dimensional sound field at z = 40 mm. The sound field reconstructed using NAH was in good agreement with the sound field reconstructed using O-CT.

Underwater ultrasonic ranging by digital signal multiplexing with hadamard matrix

In this paper, we propose a new signal processing method for underwater ranging systems, which is based on signal processing methods for wireless mobile communication, to improve SNR characteristics without an iterative measurement. We also evaluated the performance of the proposed method using a propagation simulation. The obtained results suggest that the proposed method can improve SNR characteristics from 1.0 to 3.5 dB, compared to conventional ranging methods.

Visualization of negative refraction in phononic crystal using pulsed light source

In recent years, the phononic crystal gets a lot attention because the size of phononic crystal is larger than that of photonic crystal for the relation between the wavelength of sound and that of light. Phononic crystal has many characteristics. Among them, we regard the negative refraction in phononic crystal. The negative refraction has a potential for the application to acoustical flat lens. We visualize the negative refraction by using optical method. In addition, the phononic crystals are made by self-produced. Therefore, they have some deviances of lattices in the crystals. We verified the influence depending on the deviances of lattices by using finite element method (FEM). As results, it is verified that the deviance of lattice have an influence on the behavior of wave propagation in phononic crystals. The behaviors are very different depending on the position of deviation of lattice. Also, we consider that it has possibilities of more effective focus method for the acoustical flat lens using the influence of deviances in a positive way.

Determination of Condition for Fastest Negative Group Velocities of Lamb‐Type Waves under each Density Ratio of Solid and Liquid Layers

Lamb‐type waves are coupling modes of leaky Lamb waves on a layer structure. The Lamb‐type waves have complicated propagation characteristics more than ordinary Lamb waves on a uniform elastic plate. In the characteristics, we examine the negative group velocities. Generally, the negative group velocities of Lamb waves are slower than positive group velocities under the same condition. If the negative group velocities are applied to fabricating some new application, it is desired that the speeds of negative group velocities are comparable to those of positive group velocities. Consequently, we aim to obtain the faster negative group velocities. Lamb‐type waves show more discriminative characteristics in negative group velocities than ordinary Lamb waves. In this research, we consider the Lamb‐type waves in a solid/liquid/solid structure. It is described the conditions for obtaining the fastest negative group velocities of Lamb‐type waves. The conditions, which are the acoustical impedance ratio, are expre...

An ultrasonic echo characterization approach based on particle swarm optimization

Presented is a hands-free approach for the extraction and characterization of ultrasonic echoes embedded in noise. By means of model-based nondestructive evaluation approaches, echoes can be represented parametrically by arrival time, amplitude, frequency, etc. Inverting for such parameters is a non-linear task, usually employing gradient-based, least-squared minimization such as Gauss-Newton (GN). To improve inversion stability, suitable initial echo parameter guesses are required which may not be possible under the presence of noise. To mitigate this requirement, particle swarm optimization (PSO) is employed in lieu of GN. PSO is a population-based optimization technique wherein a swarm of particles explores a multidimensional search space of candidate solutions. Particles seek out the global optimum by iteratively moving to improve their position by evaluating their individual performance as well as that of the collective. Since the inversion problem is non-linear, multiple suboptimal solutions exist, and in this regard PSO has a much lower propensity of becoming trapped in a local minima compared to gradient-based approaches. Due to this, it is possible to omit initial guesses and utilize a broad search range instead, which becomes far more trivial. Real pulse-echoes were used to evaluate the efficacy of the PSO approach under varying noise severity. In all cases, PSO characterized the echo correctly while GN required an initial guess within 30% of the true value to converge.

An automated framework for the extraction of ultrasonic echoes embedded in noise

Proposed is an automated framework for the extraction and characterization of the arriving echo in ultrasonic signals embedded in high noise. Commonly, in order to correctly characterize the first echo hidden within a noise-ridden signal, multiple traces are stacked in a gather to improve the SNR, hence facilitating easier extraction and characterization of the recorded echo. Such first order statistical methods require multiple traces and usually fall short in the accuracy of the echo estimate when the variance of the noise does not belong to a known distribution. To mitigate this problem, a framework has been developed comprised of a multi-step procedure, i.e., pre-processing, localization, gating and finally parameterization of the given echo. This automatic framework operates on single traces and does not require the setting of processing parameters. By means of this method, the true echo can be extracted in one-shot from other overlapping noise components. Furthermore, because the method operates on ...