Nobuyuki Endoh

Analysis of Characteristics of Underwater Sound Propagation in the Ocean by a Finite Difference Time Domain Method

Many methods of calculation that are useful in deep water like the Pacific Ocean have recently been reported for obtaining sound propagation characteristics. The development of new method is expected for shallow water. In this paper, the finite difference time domain (FDTD) method, which takes account of attenuation, is proposed to calculate the propagation of sound in shallow water with a lossy seabed. The benchmark problem proposed by the Acoustical Society of America was solved by the FDTD method and its results were compared with other methods. The contour pattern of the transmission loss in water and in sediment agreed well with the results of an accurate coupled-mode calculation. A series of propagated pulse waveforms in the same model is also calculated. It is useful for us to recognize the sound propagation not only in water but also in sediment. The FDTD method shows the pulse waveforms with interference between direct wave and reflected waves. These results show the validity of the FDTD method.

Numerical Analysis of Underwater Acoustic Lens Using Wide-Angle Parabolic Equation Method

The autonomous underwater vehicle mounted on the forward obstacle avoidance sonar is developed to investigate ocean environments such as that of the Arctic Ocean. In order to obtain real time, high efficiency and clear acoustic images, the acoustic lens sonar system has been adopted. Usually, the design of the acoustic lens as well as that of the optical lens is based on geometrical optics theory. In this paper, the acoustic characteristics of the acoustic lens are studied by using the two- and three-dimensional parabolic equation methods.

Estimation of Receiving Waveform of Ultrasonic Aerial Back Sonar Calculated by Finite Difference Time Domain Method

In order to develop a high-performance aerial back sonar for a car, we predicted the waveforms of the received pulse using the finite difference time domain (FDTD) method. The echo pulses reflected from the target were calculated as a function of the targets height in air at an inhomogeneous temperature. The maximum amplitudes of the pulse train changed with the targets height. The amplitude and propagation time of the reflected pulse markedly differed with shape of targets at a constant temperature. The result shows that an aerial sensor should be able to detect a square target at an inhomogeneous temperature such as that in summer. However, it is very difficult to detect a slanted target.

Numerical Analysis of Temperature Rise in Tissue Using Electronically Focused Ultrasound

Recently, the use of higher power ultrasonic equipment has been extended to not only therapy but also diagnosis because the new diagnostic imaging techniques, such as Doppler color flow imaging and harmonic imaging, require a higher ultrasound power than conventional imaging techniques. It is very important to ensure the safety of temperature rise caused by the absorption of ultrasound energy in new ultrasonic imaging systems. In this two-dimensional finite difference time domain–heat conduction equation study, the temperature rise in tissue has been simulated at a focal point radiated by a phased array focused transducer, such as like a common B-mode imagine system. The center frequency of radiated wave pulses is 2.5 MHz and ISPTA=0.72 W/cm2. When the sound pulse repetition frequency (PRF) is changed from 100 to 400 kHz, the temperature rise in tissue at a focal point is proportional to the PRF. The maximum temperature rise in tissue has been simulated only at 0.0004 °C at a focal point of a transducer when PRF is 400 kHz.

Development of ultrasonic propagation analysis method for estimation of inner state of bone phantom

The preliminary study on the ultrasonic pulse propagation in the bone is described in this paper. The proposed method concerns about not only longitudinal waves but also shear waves in the bone. A simple phantom composed of three materials is also proposed, because the cancellous bone has the trabecular bone and the bone marrow. Calculated results show that the characteristics of received pulse waveforms are affected by the contents of the trabecular bone. The experimental results agree with the calculated results

Design and Convergence Performance Analysis of Aspherical Acoustic Lens Applied to Ambient Noise Imaging in Actual Ocean Experiment

In this study, an aspherical lens with the aperture diameter of 1.0 m was designed for utilization in an actual ocean experiment of ambient noise imaging (ANI). It was expected that this ANI system would realize directional resolution, which is a beam width of 1° at the center frequency of 120 kHz. We analyzed the sound pressure distribution focused by the designed lens using the 3D finite difference time domain method. The frequency dependence of a -3 dB area was then compared between 120 kHz and the higher or lower frequency. The analysis results suggested that the designed lens has fine directional resolution over the center frequency of 120 kHz. We had measured the directivity of the designed lens in an actual ocean experiment in Uchiura Bay in November of 2010. It was verified that the ANI system with this lens realizes a beam width of 1° at 120 kHz.

Evaluating Directional Resolution of Aplanatic Acoustic Lens for Designing Ambient Noise Imaging System

In our previous studies, it was verified that a spherical biconcave lens with an aperture diameter of 2.0 m has a sufficient directional resolution (e.g., a beam width of 1° at 60 kHz) for realizing an ambient noise imaging (ANI) system. In this study, an aplanatic lens that corrects both spherical and coma aberrations with the same aperture was designed for an ANI system, and its directional resolution was evaluated. First, in order to predict the resolution, we performed a numerical analysis using the finite difference time domain (FDTD) method. Second, the numerical analysis results were verified by a small-scale trial of one-fifth of full size in a water tank. The shapes of the -3 dB areas were similar between the numerical analysis and experimental results at small incidence angles, and the -3 dB areas do not overlap at 1° increments of incidence angle. The resolution of the aplanatic lens was closer to that of an ideal lens than to that of the spherical lens. Finally, it was satisfied that the present lens has sufficient directional resolution for use in an ANI system.

Numerical Simulation of Target Range Estimation Using Ambient Noise Imaging with Acoustic Lens

In ambient noise imaging (ANI), each pixel of a target image is mapped by either monochrome or pseudo color to represent its acoustic intensity in each direction. This intensity is obtained by measuring the target objects reflecting or scattering wave, with ocean background noise serving as the sound source. In the case of using an acoustic lens, the ANI system creates a C-mode-like image, where receivers are arranged on a focal plane and each pixels color corresponds to the intensity of each receiver output. There is no consideration for estimating a target range by this method, because it is impossible to measure the traveling time between a transducer and a target by a method like an active imaging sonar. In this study, we tried to estimate a target range using the ANI system with an acoustic lens. Here, we conducted a numerical simulation of sound propagation based on the principle of the time reversal mirror. First, instead of actual ocean measurements in the forward propagation, we calculated the scattering wave from a rigid target object in an acoustic noise field generated by a large number of point sources using the two-dimensional (2D) finite difference time domain (FDTD) method. The time series of the scattering wave converged by the lens was then recorded on each receiver. The sound pressure distribution assuming that the time-reversed wave of the scattering wave was reradiated from each receiver position was also calculated using the 2D FDTD method in the backward propagation. It was possible to estimate a target range using the ANI system with an acoustic lens, because the maximum position of the reradiated sound pressure field was close to the target position.

Comparison of Sound Pressure Distribution Determined by Numerical Analysis and Scaled-Up Experiment for Small Ultrasonic Probe with Lens

Ultrasonic medical equipment is used in not only diagnosis but also therapy such as that for treating hyperthermia. Many researchers are also studying high-frequency ultrasonic imaging systems with interluminal or catheter transducers. In both applications, an acoustic lens might improve the characteristics of ultrasonic medical probes. In this paper, a small acoustic lens for an ultrasonic catheter-type probe is described. The conventional three-dimensional finite-difference time-domain (3D FDTD) with orthogonal coordinates requires a large memory and a long calculation time to estimate the characteristics of the lens. To overcome these disadvantages, a simple two-dimensional (2D) FDTD calculation based on symmetry is proposed in this paper. A virtual spherical sound source whose amplitude distribution is equal to that of the sound propagation field of an actual sound source is also used to simplify the calculation. A numerical model of the lens with a lens holder is constructed. The experimental results agree well with the calculated results with the lens holder.

Numerical Analysis of Temperature Rise in Tissue Using Ultrasound

In order to avoid burns from ultrasonic radiation, it is important to accurately predict the temperature rise caused by the absorption of ultrasonic radiation in tissue. We describe a preliminary study on the simulation of the temperature rise distribution when the thermal diffusivity is changed. The sound pressure distribution is calculated by the finite difference time domain (FDTD) method, and the thermal conduction distribution in tissue is calculated by the heat conduction equation (HCE) method. Because the boundary condition affects the calculation results, the measured experimental temperature is used for calculations.