Showing papers on "Acoustic interferometer published in 2001"

Refractive acoustic devices for airborne sound.

Abstract: We show that a sonic crystal made of periodic distributions of rigid cylinders in air acts as a new material which allows the construction of refractive acoustic devices for airborne sound. It is demonstrated that, in the long-wave regime, the crystal has low impedance and the sound is transmitted at subsonic velocities. Here, the fabrication and characterization of a convergent lens are presented. Also, an example of a Fabry-Perot interferometer based on this crystal is analyzed. It is concluded that refractive devices based on sonic crystals behave in a manner similar to that of optical systems.

Investigation of acoustic waves in thin plates of lithium niobate and lithium tantalate

Abstract: The general properties of fundamental antisymmetric A/sub 0/, symmetric S/sub 0/, and shear horizontal SH/sub 0/ acoustic waves propagating in thin piezoelectric plates have been theoretically investigated on samples of lithium niobate (LiNbO/sub 3/) and lithium tantalate (LiTaO/sub 3/). The results obtained will be useful for a proper development of various physical, chemical, and biological sensors and devices for signal processing based on plate acoustic waves.

On the character of acoustic waves at the interface between hard and soft solids and liquids.

Abstract: Laser ultrasonics is used to optically excite and detect acoustic waves at the interface between a liquid and a solid or coated solid. Several case studies show that this technique is feasible to investigate experimentally the theoretically predicted fundamental properties of different aspects of interface waves at liquid–solid interfaces and to characterize the elastic properties of soft solids. The theoretical prediction that the leaky Rayleigh (LR)-type root of the characteristic determinant becomes forbidden when the shear velocity of the solid lies below the bulk velocity of the liquid was experimentally confirmed. The depth profiling and nondestructive testing potential of Scholte waves was experimentally illustrated and explained by the properties of the wave displacement profile.

Phase-shifting shearing interferometer.

Abstract: A single-element phase-shifting interferometer has been developed based on the lateral shearing interferometer. This new interferometer requires no precise alignment, and the phase is continuously varied by changes in the voltage across a commercially available liquid-crystal phase retarder.

Compressional acoustic wave generation in microdroplets of water in contact with quartz crystal resonators

Abstract: Resonating quartz crystals can be used for sensing liquid properties by completely immersing one side of the crystal in a bulk liquid. The in-plane shearing motion of the crystal generates shear waves which are damped by a viscous liquid. Thus only a thin layer of fluid characterized by the penetration depth of the acoustic wave is sensed by a thickness shear mode resonator. Previous studies have shown that the finite lateral extent of the crystal results in the generation of compressional waves, which may cause deviations from the theoretical behavior predicted by a one-dimensional model. In this work, we report on a simultaneous optical and acoustic wave investigation of the quartz crystal resonator response to sessile microdroplets of water, which only wet a localized portion of the surface. The relationship between initial change in frequency and distance from the center of the crystal has been measured for the compressional wave generation regions of the crystal using 2 and 5 μl droplets. For these volumes the initial heights do not represent integer multiples of a half of the acoustic wavelength and so are not expected to initially produce compressional wave resonance. A systematic study of the acoustic response to evaporating microdroplets of water has then been recorded for droplets deposited in the compressional wave generation regions of the crystals whilst simultaneously recording the top and side views by videomicroscopy. The data are compared to theoretically expected values of droplet height for constructive acoustic interference. Results are highly reproducible and there is good correlation between theory and experiment.

Electroacoustic waveguide transducing

Abstract: An acoustic waveguide system, having source of acoustic radiation and a source of opposing acoustic radiation. An acoustic waveguide has an open end and an interior. A first acoustic driver having a first radiating surface and a second radiating surface is arranged and constructed so that the first radiating surface radiates sound waves into free air and the second radiating surface radiates sound waves into the acoustic waveguide so that sound waves are radiated at the open end. A source of opposing sound waves in the acoustic waveguide opposes a predetermined spectral component of the sound waves radiated into the acoustic waveguide to reduce the acoustic radiation of the predetermined spectral component from the acoustic waveguide.

Multiplexed interferometer for ultrasonic imaging applications

Abstract: A multiplexed laser interferometer is presented that allows for the simultaneous detection of acoustic waves from a number of points on the surface of a specimen. Phase gratings are used to create an array of laser beams that are directed to the specimen. The beams are collected using a lens system, combined with a single reference beam in a photorefractive crystal, and imaged on separate photodetector elements. Detection of surface acoustic waves scattered from a defect in an aluminum specimen is demonstrated using a nine-element receiver. Focusing of the array interferometer, by applying appropriate phase shifts between the detected signals, is also shown. The wide bandwidth and noncontact nature of this receiver make it well suited for phased array detection and ultrasonic imaging applications.

Elastic properties of langasite-type crystals determined by bulk and surface acoustic waves

Abstract: We have determined the elastic constants of langasite-type crystals (La3Ga5SiO14, La3Ga5.5Nb0.5O14, and La3Ga5.5Ta0.5O14) from measurements of the sound velocity of acoustic waves. Starting with the elastic tensor determined from bulk acoustic waves, we optimized the data set by investigating the influence of the elastic tensor components on the angular dispersion of surface guided waves. This procedure is particularly useful for accurate determination of the nondiagonal elements of the elastic tensor. Phase velocity calculations based on the new set of constants show an increased agreement with experimental data compared to the data set derived from bulk waves and previously published material data.

High-resolution imaging of a single circular surface acoustic wave source: Effects of crystal anisotropy

Abstract: We present an experimental method for the high-resolution imaging of the excitation and propagation of surface acoustic waves (SAWs) on anisotropic piezoelectric substrates. By employing a scanning acoustic force microscope (SAFM), we are able to image acoustic waves that are excitable by a single circular electrode pair source through the mixing with well-defined reference plane waves. We show amplitude and phase images of the point-source wave field, containing the angular dependence of the phase velocity of these modes, as well as their electromechanical coupling strength. The SAFM allows easy access to acoustic material properties, which are important for the design of commercial SAW devices.

Acoustic effects caused by high-intensity internal waves in a shelf zone

Abstract: The sound field fluctuations caused by high-intensity, solitonlike, quasi-plane internal waves crossing a fixed acoustic path at different angles are numerically modeled for natural conditions of the shelf zone of the Sea of Japan The horizontal refraction of sound is considered for the case of an acoustic path parallel to the internal wave front

Laser velocimetry detection of underwater sound

Abstract: An apparatus for determining the velocity of sound waves which includes a liquid medium having a plurality of gas bubbles. In the liquid medium, a laser transmits a light pulse to interact with the bubbles excited by the sound wave. Backscattered light from the interaction of the light pulse is received. A processor is then responsive to the detector to provide detection of the acoustic wave through the fluid medium.

Acoustic anisotropy of piezoelectric PbB4O7 crystals studied by laser ultrasonics

Abstract: Acoustic anisotropic properties of PbB4O7 crystals are investigated and discussed. The slowness curves of three bulk waves as well as the surface waves for the plates with different cut directions are calculated by considering piezoelectric effects. The theoretical calculations are in agreement with experimental results determined by laser ultrasonic technique. On the other hand, the excitation efficiency of surface waves of PbB4O7 crystals is obtained by calculating the change of the velocities of acoustic surface waves get from the plates bare and deposited by a thin metal film. The results of this paper can be used for selecting PbB4O7 crystal orientations for designing the related devices.

Acoustic pulse propagation and localization in bubbly water.

Abstract: Acoustic pulse propagation in bubbly water is studied using a self-consistent method. The acoustic transmission and backscattering are evaluated numerically. Under proper conditions, the localization of acoustic waves is identified within a range of frequency. The results show that when a short pulse is transmitted the waves of frequencies within the localization regime will be trapped in the system and reveal a coherent behavior. A phase diagram approach is used to describe the localization behavior.

Enhanced laser generation of surface acoustic waves by discontinuities

Abstract: Laser generation of surface acoustic waves can be enhanced when the irradiated spot covers a surface crack. Experimental methods and simplified models are used to study how the amplitude, directivity, and spectrum of laser-generated surface waves depend on the size of the discontinuity. The results indicate that the described enhancement of laser-generated surface acoustic waves is very promising for detection of small cracks that are difficult to identify by conventional ultrasonic or laser-ultrasonic and other nondestructive evaluation methods.

Coupled guided acoustic modes in water-filled thin-walled tubes

Abstract: Recent results [J. D. N. Cheeke et al., J. Acoust. Soc. Am. 104, 3678 (1998)] reported a 20% decrease in group velocity for flexural mode circumferential waves propagating around water-filled thin-walled stainless-steel tubes. In the present work, the full theory of such modes is developed to explain the structure modes which originates from the coupling between the water filler and stainless tube. Calculated values of the group velocity for the first two coupled modes are in excellent agreement with experiment. The results are of interest for the physics of acoustic waves in fluid-loaded structures and have potential application for liquid level detection.

Acoustic-field distribution within a fan-shaped filter

Abstract: We have investigated the surface-acoustic wave field variation in a fan-shaped filter on a 112/spl deg/ LiTaO/sub 3/ substrate as a function of position and frequency. The acoustic field has been measured with the help of a scanning laser interferometer and the SAW-field distribution in the complete filter has also been numerically simulated in detail. Our modelling scheme utilizes a discretization of the filter into elementary cells. The simulations serve to render features of the acoustic-field distribution in astonishing quantitative agreement with the laser-interferometric detections.

Effect of selective absorption on nonlinear interactions in high intensity acoustic beams

Abstract: The effect of second harmonic selective absorption on nonlinear interactions in high intensity acoustic beams is studied. Diffraction and focusing phenomena are taken into account in addition to nonlinearity and selective absorption. The results are compared with those obtained for one-dimensional plane waves in a selective absorption medium and for the focused beams in a medium without absorption. It is shown that selective absorption of the second harmonic does not prevent shock formation, but results in specific waveform distortion, less asymmetry of the waveform in case of the beam, less amplitude of the shock and corresponding nonlinear attenuation.

Characteristics of the Diffraction of Acoustic Waves in Stratified Sound Channels

Abstract: The results of calculations performed in the framework of the approximate approach developed by the authors are presented for the diffraction of sound waves by a stiff spheroid in an acoustic waveguide. The scattered sound field is analyzed as a function of the following parameters of the problem: the spheroid dimensions, its position relative to the sound source and the receiver, the vertical profile of sound velocity in the waveguide, and the acoustic parameters of the waveguide bottom.

Conditions for anomalous acousto-optical diffraction by backward propagating acoustic waves

Abstract: Experimental evidence is given of acoustic plate waves, whose group velocity is contradirected with respect to the phase velocity, through a determination of the acoustic wavelength dependence on frequency, in a limited range of frequencies. The dependence dLambda/dOmega>0 between wavelength and frequency is experimentally verified, as the required condition for acousto-optical diffraction, where higher frequency components would scatter light into smaller diffraction angles.

Propagation of Longitudinal Acoustic Waves in a Thin Cylindrical Pipe Filled with a Liquid

Abstract: In [2, 3], the effect of reducing the wave velocity was found for elastic rods made in the form of a thin strip or fiber and immersed in a liquid. A reduction of the velocity is observed only in thin samples with a thickness on the order of a hundred microns. The effect found is caused by the fact that the so-called boundary layer of a liquid vibrates together with the rod. The thickness of this layer depends on the vibration frequency and the density and viscosity of the liquid. When the sound velocity depends on the viscosity, the effect discovered is employed for the determination of the viscosity of liquids. An advantage of this method is its ability to provide high-rate measurements, which makes it possible to control the chemical reactions with a time-dependent viscosity that proceed in liquids. As a sensor, we used metallic strips and fibers with a diameter of approximately 0.1 mm [2, 3], which were immersed in the liquid under investigation. The use, for this purpose, of a thin capillary filled with the liquid presents a number of advantages, among them the possibility of operating with a very small quantity (specified by the capillary volume) of liquid. The problem of the effect of liquid on the propagation of sound in a thin pipe (capillary) has not yet been solved. The goal of this study is to theoretically seek a solution to this problem.

Propagation of acoustic waves scattered in a lossy three-dimensional channel

Abstract: In previous papers we discussed the scattering of acoustic waves by random sound-speed fluctuations in lossless two- and three-dimensional channels. Here we include the effect of bottom loss in a three-dimensional channel. The bottom loss is included by replacing the rigid bottom condition by one allowing for loss. We find an asymptotic solution for the angular distribution of the scattered acoustic energy and the characteristic propagation distance at which the asymptotic solution is valid. Beyond this characteristic distance the angular distribution saturates and is no longer dependent on the propagation distance. Before the mathematical development is given a simple physical argument is presented which shows why a saturation region is expected.

Amplitude multiplexing of Mach-Zehnder interferometer array and its application to locating acoustic emission

Abstract: We report an amplitude division multiplexed interferometric sensor array for locating acoustic emission. The interferometer is composed of 3 arms, the two sensing arms have different interference amplitude with the reference arm, so we can judge whether a sensor receives sound or not from the composite interference amplitude. For single sound source, the sound-receiving delay of two sensors can be obtained from the time when the interference amplitude changes, and the sound source location relative to the sensors can also be achieved in a 1-D system. Preliminary experiments were carried out and demonstrated a one-dimensional (1-D) location accuracy of better than 1cm. The system can be extended for 2-D and 3-D location of acoustic emissions by adding one or two more sensing arms, respectively, in the interferometer.

Broadband measurements of ultrasonic waves using optical beam deflection

Abstract: A technique of optical beam deflection by ultrasonic waves was developed to measure acoustic nonlinearity parameters B/A and ultrasonic absorption in liquids. The laser beam is deflected by ultrasonic waves under the condition that the sound wavelength is larger than the laser beam waist. The deflection angle was detected with a high-speed photodiode having double elements, yielding a time derivative of pressure amplitude of the ultrasonic waves. Two kinds of experiments were performed using this technique. The distorted waveforms of finite-amplitude ultrasonic waves of 6.65 MHz were observed. Nonlinearity parameters were obtained from the fundamental and second-harmonic wave amplitudes in water and ethanol. Second, a broadband ultrasonic pulse was observed as function of propagation distance. Spectral analysis of the pulsed waveform enabled absorption measurements in the 5–35 MHz range in gelatin sols and gels.