Kenji Saijyou

Data extrapolation method for boundary element method-based near-field acoustical holography

A new data extrapolation method for boundary element method (BEM)-based near-field acoustical holography (NAH) is proposed to reduce an error of the reconstructed result obtained from the pressure measured on an aperture small compared with the structure. The finiteness of the measurement aperture is a serious impediment to actual large-scale implementation of NAH, because NAH requires the measurement of the pressure field over a complete surface of structure. To eliminate the requirement, the wave number-space data extrapolation method for fast Fourier transform (FFT)-based NAH has been proposed. In this paper, the extension of this data extrapolation method to BEM-based NAH is presented. The effectiveness of this method is demonstrated by experiments. The experiment results confirm that the reconstruction error is sufficiently suppressed by the proposed method.

Regularization method for the application of K-space data extrapolation to near-field acoustical holography

A regularization method for applying the K-space data extrapolation method to near-field acoustical holography (NAH) is proposed. The influence of the finiteness of measurement aperture is a serious impediment to the NAH reconstruction of structural vibration when the size of the measurement aperture is smaller than that of the structure. The K-space data extrapolation method has been proposed to reduce the influence of this finiteness. An appropriate regularization method is necessary to apply the K-space data extrapolation method if the measured pressure is contaminated by noise. Some regularization methods for NAH have been proposed. However, it is difficult to determine an appropriate K-space filter if the finiteness of the measurement aperture becomes an issue. An estimation method of appropriate regularization filter for applying the K-space data extrapolation method is proposed to solve this difficulty. The effectiveness of this method is demonstrated by simulations. The simulation results confirm ...

Measurement of structural and acoustic intensities using near-field acoustical holography

A non contacting method for measuring structural and acoustic intensities is proposed by developing the signal processing in near-field acoustical holography (NAH). The effectiveness of this method is illustrated by applying it to the measurement of radiation from ribbed and unribbed submerged plates. The spatial derivatives for determining the structural intensity are evaluated by a K-space filtration method newly developed by the authors. The experimental data obtained at 928 Hz and 1464 Hz are selected for specific and detailed investigation. The attached ribs have a small influence on the structural intensity flow at 928 Hz, so the vibrational mode of the ribbed plate is the same as that of the unribbed plate. However, since the attached ribs control the direction of vibrational energy flow at 1464 Hz, the vibrational mode of the unribbed plate is markedly changed.

Application of Generalized Near-Field Acoustical Holography to Scattering Problems

Generalized near-field acoustical holography (GENAH) is applied to scattering problems, making use of its wavelength-limitation-free characteristic in the reconstruction process. GENAH signal processing is directly adapted for the reconstruction of the scattered field, while a method to extract the scattered-pressure hologram is independently proposed to create scattering GENAH. The measurement of the near field of a scatterer yields the superimposed incident and scattered fields. Their decomposition is carried out by subtracting the incident-pressure hologram from the superimposed-pressure hologram in K space. The validity of this decomposition method was demonstrated by comparing the experimental and theoretical results of finite submerged shells. Reconstruction of shell-surface vibration induced by scattering displayed a reasonable result.

Regularization method for measurement of structural intensity using nearfield acoustical holography

The regularization method for measurement of structural intensity using nearfield acoustical holography is proposed. Spatial derivatives of normal displacement are necessary to obtain the structural intensity. The derivative operations amplify high-wave-number components of measurement noise. Therefore, the estimation of an appropriate wave-number filter is crucial for implementation of the measurement of structural intensity. In conventional methods, this wave-number filter is determined from the flexural wavelength. And the same wave-number filter is applied to obtain all spatial derivatives. As a result, structural intensity obtained from the pressure hologram, whose signal-to-noise ratio is low, is seriously contaminated by the noise. To overcome this difficulty, regularization theory is applied to determine the appropriate wave-number filter for each order of derivatives. The effectiveness of the proposed method is demonstrated by experiments.

Design optimization of wide-band Tonpilz piezoelectric transducer with a bending piezoelectric disk on the radiation surface

Wide-band Tonpilz piezoelectric transducer with a bending piezoelectric disk on the radiation surface has been proposed to improve sonar detection performance in shallow water. This transducer is driven by utilizing two vibration modes, i.e., longitudinal and bending. Consequently, to achieve a wide-band signal transmission by this transducer, the phase difference between signals, which drive the ring-stack and the bending-disk piezoelectric resonators has to be optimized. In this paper, optimization approach of this phase difference in the design process is proposed. The effectiveness of this approach was confirmed by water-pool experiments.

Measurement of structural intensity using boundary element method-based nearfield acoustical holography

The regularization method for measurement of structural intensity (SI) using boundary element method (BEM)-based nearfield acoustical holography (NAH) is proposed. Spatial derivatives of normal displacement are necessary to obtain the structural intensity. The derivative operations amplify high-wavenumber component of measurement noise and contaminate the measurement result of SI. To overcome this difficulty, regularization method for measurement of SI using fast Fourier transform-based NAH has been introduced. In this paper, this regularization method is modified for the BEM-based NAH. The BEM-based NAH avoids the aperture replication problem; therefore, measurement aperture for BEM-based NAH can be set smaller than that for FFT-based NAH. The effectiveness of the proposed method is demonstrated by experiments.

ANALYSIS OF SCATTERING ENERGY FLOW BY NEAR-FIELD ACOUSTIC HOLOGRAPHY

Acoustic scattering from a submerged cylindrical shell is experimentally investigated from the viewpoint of energy flow. Acoustic and structural intensities, which respectively indicate acoustic and vibrational energy flows, are obtained by processing data from near-field acoustical holography. Both acoustic and structural intensities of the scattering wave are treated by using a radiation propagator. An incident wave propagator is also introduced to measure both acoustic and structural intensities of the incident wave from an external source. The interrelationship between structural and acoustic energy flows of the present scattering problem is examined by comparing experimental and simulated results.

Accuracy Improvement of Reconstruction in Near-Field Acoustical Holography by Applying Data Extrapolation Method

Near-field acoustical holography (NAH) is a non-contacting measurement method to visualize structural vibrations by means of backward reconstruction. A relevant data-smoothing process is necessary for the NAH experiments. We had previously proposed a new data-smoothing method with the aid of data extrapolation. In this paper, we report the effectiveness of this proposed method based on the experimental results. Furthermore, our data-smoothing method is modified for applications to the generalized NAH (GENAH). The experimental results show that the reconstruction error of the surface velocity by the proposed method is less than one-fifth of that by the conventional real-space window method for S/N=33 dB. These results indicate that the prospect of improving the accuracy of reconstruction in NAH for the quantitative analysis of structural vibrations is obtained by the proposed method.

Fiber optic interferometric hydrophone based on fiber Bragg grating with polarization diversity receiver

The objective of this work is to overcome polarization‐induced signal fading (PSF) that occurs in fiber optic interferometric hydrophones based on fiber Bragg grating (FBG). The use of FBGs as reflectors in fiber optic interferometric hydrophones would greatly simplify the construction of the devices’ wet‐end portion by eliminating couplers, joints, and other connections that are required for other types of fiber optic interferometric hydrophones. Unfortunately, the visibility of the interference signal is reduced by PSF, and the demodulated signal is contaminated greatly by the phase noise. A polarization diversity receiver (PDR) is therefore applied to reduce the PSF influence. We developed a prototype of fiber optic interferometric hydrophone based on FBG with the PDR and demodulate an acoustic signal to confirm the effectiveness of the PDR application. Experimental results described here show that the phase noise is suppressed by selecting an optimum port that has high visibility. Consequently, the ef...