Tomoki Yokoyama

3-D FDTD Analysis of Sound Field Focused by Biconcave Acoustic Lens for Normal Incidence

Recently, the finite difference time domain (FDTD) method has been frequently used for the analysis of underwater sound propagation. There are demonstrated advantages of this FDTD method in terms of obtaining data regarding snapshots of sound pressure distribution and a series of waveforms at any point. In addition, the method facilitates the modeling of factors, such as the sound source and media into the analysis domain. In this study, a three-dimensional FDTD analysis was carried out in order to obtain the sound field focused by a biconcave acoustic lens specialized to measure the normal incidence of the spherical wave. Additionally, the results of the analysis were compared with experimental results obtained in a water tank. When the frequency of the sound source was 500 kHz, the range between the acoustic lens and the sound source was 1.78 m, and the attenuation constant was 0.5–1.0 dB/λ, the experimental results regarding the position of the focal point, the on-axis characteristics and the beam pattern were all found to agree well with the simulation results obtained by FDTD method.

Finite Difference Time Domain Analysis of Underwater Acoustic Lens System for Ambient Noise Imaging

Much attention has been paid to the new idea of detecting objects using ocean ambient noise. This concept is called ambient noise imaging (ANI). In this study, sound fields focused by an acoustic lens system constructed with a single biconcave lens were analyzed using the finite difference time domain (FDTD) method for realizing an ANI system. The size of the lens aperture that would have sufficient resolution–for example, the beam width is 1° at 60 kHz–was roughly determined by comparing the image points and -3 dB areas of sound pressure fields generated by lenses with various apertures. Then, in another FDTD analysis, we successfully used a lens with a determined aperture to detect rigid target objects in an acoustic noise field generated by a large number of point sources.

Temperature Distribution in Circular Space Reconstructed from Sampling Data at Unequal Intervals in Small Numbers Using Acoustic Computerized Tomography (A-CT)

The propagation velocity of sound is often used to measure temperature. The sound probe has the advantage of non-contact sensing and quick response. By combining a sound probe with computerized tomography (CT) and data interpolation, an image of temperature distribution through space can be reconstructed. We have proposed a method for measuring the temperature distribution in two-dimensional space using an acoustic CT (A-CT) method with a very small number of acoustic transducers. The measured object is a circular space with a radius of 1,480 mm. Sixteen transducers are installed on the circular stage. Without a mechanical motion, projection data for the reconstruction is acquired by electronic scanning. We reconstruct the temperature distribution by interpolation from a small number of data set. Electrical heaters create a temperature gradient in space. The temperature profile is measured by 19 thermocouples and used for a computer simulation. Experimentally reconstructed images are in agreement with the simulated images. The measurement system proposed in this paper has advantages for use in atmospheric monitoring, air conditioning and heat management.

Detection and Selective Focusing on Scatterers Using Decomposition of Time Reversal Operator Method in Pekeris Waveguide Model

The decomposition of the time reversal operator (DORT) method is a selective detection and focusing technique for the pulse-echo mode that uses an array of transmit-receive transducers. Because it is an application of the acoustic time reversal mirror, the irregular shape of the array and the aberration of the propagation medium are compensated and the selective focusing on targets is made possible. In this paper, we examined the application of this method to the underwater acoustics and the application to active sonar and communication are made possible. To clarify the relationship between the focusing effect of the DORT method and the sound propagation property in shallow water, we examined the selective focusing effect with respect to the propagation distance by using the cylindrical spread model without the sea surface and bottom and the Pekeris model, which is typical of shallow water models. There are few influences of the propagation distance on the focusing effect in shallow water model as a result, and it is shown that the focusing effect improves with the increase of the sound velocity of the bottom.

Generation of Bessel Beam from Equiamplitude-Driven Annular Transducer Array Consisting of a Few Elements

We present a method for generating a nondiffraction beam using an annular transducer array. In this method, each element is driven with equiamplitude and with an antiphase from its neighboring elements. Theoretical and experimental analyses of an array of this type have been carried out, and the feasibility of this method is confirmed. The beam from a continuous wave is shown using radiated pressure magnitude distributions and it is shown that there exist most suitable values of the width and number of elements. When the array is driven by a burst signal, the beam propagates as a plane wave which has an amplitude corresponding to the zeroth-order Bessel function of the first kind, J0. Since this beam is realized by an annular array consisting of a few elements, it suggests the possibility of a transducer of this form developing into a source which generates nondiffraction beams.

Effects of Mutual Interactions on a Phased Transducer Array

In the conventional design method of the SONAR array system, variations of the transducers velocity occur because the acoustic interactions between transducers are not considered. This research was conducted first, to analyze the acoustic interaction effects on the vibration velocity of transducers using an equivalent circuit. Then, we studied the acoustic interaction effects on the amplitude and phase of the velocity, the radiation power outputs and the far-field directivity pattern in the case of a phased array. It is obvious that there have been drastic effects on the radiated field from the phased array operated at the resonance frequency.

Consideration of radiation impedance calculation methods for cylindrical array sound source

In a phased array, the interval between the centers of the sound source elements which adjoin each other is set below half the wavelength in order to suppress the gratinglobe. At this interval, the array sound source can gain strong radiation power. The estimation of radiation impedance is important in the analysis of the radiation power. The Greenspons method which is a radiation impedance calculation method of rectangular pistons on a cylinder is explained, and some defects of this method are clarified. For the evaluation of Greenspons method, we introduce the combined Helmholtz integral equation formulation (CHIEF) method. The radiation impedance is calculated using both Greenspons method and the CHIEF method, and the results are compared. On the basis of comparison of the radiation power measured while driving the cylindrical phased array with the theoretical value of radiation power determined using the caluculated value of radiation impedance, it is considered that Greenspons method and the CHIEF method give accurate estimations.

Nondiffraction beam generated from an annular array driven by uniform velocity amplitude

This paper presents a method for generating J/sub 0/ beam approximately using an annular array. In this method, each element is driven by an anti-phase from its neighboring element and at the same amplitude. The validity of this method is verified with simulations and experiments and is shown to the generation of nondiffraction beam from the array consist of few elements. Using an annular array consisting of 5 elements driven by a continuous wave, simulation results and experimental results using piezoelectric rubber annular array transducer show good agreement. Driving by RF burst signal, the pulse from the annular array arrives slightly earlier than that of an ideal J/sub 0/ source.

Doppler Measurement of Moving Object Across the Ultrasonic Field Radiated from Circular Planar Transducer

We improve the procedure for measuring the velocity component of a moving object perpendicular to the direction of ultrasonic wave propagation. The conventional procedure uses a concave transducer. We adopt a circular planar transducer. The use of the transducer offers advantages. The both ranges of the distances between the path of the moving object and the transducer and between the path and the central axis of the transducer could be extended by using not the converging beam but the radiating one. However, this degrades the accuracy of the measurement. We derive the equation approximately representing the relationship between the velocity across the field radiated from the planar transducer and the bandwidth of the reflected wave. Then the field is exactly calculated for a concrete example to estimate the accuracy of the approximation. Finally an experiment is performed to confirm the validity of the discussion.

Effect of received beam width on passive detection of silent target using ambient noise

Much attention has been paid to the new idea of detecting objects by using ocean ambient noise affirmatively. This concept is called acoustic daylight imaging. The authors have attempted to detect a silent target object using ambient noise by employing a wide-band beamformer consisting with an array of receivers. In the experimental results obtained in air, we applied the delay-sum array method to successfully detect the silent target object in the acoustic noise field generated by many transducers with a line array of receivers. In this paper, some experimental results of the effect of the beam width formed by the wide-band beamformer on target detection are reported. The ocean ambient noise is simulated by transducers decentralized to many points in air. The spherical target object is detected by changing the number of receivers. The beam width is compared with the angle subtended by the spherical target at the center of the receiver array.