Showing papers on "Acoustic wave published in 1981"

Elements of acoustics

Abstract: CHAPTER TWO Basic Properties of Acoustic Waves 2.1 Ideal Fluids 2.2 Linearization 2.3 Uniform Fluids 2.4 One-Dimensional Plane Waves Speed of Sound in a Perfect Gas Speed of Sound in Other Fluids Relationships between Acoustic Quantities 2.5 Monochromatic Waves Plane, One-Dimensional Monochromatic Waves Plane, Monochromatic Waves in Three Dimensions Relation between Variables in a Monochromatic Wave Time Averages 2.6 Fourier Analysis Periodic Waveforms —Fourier Series Nonperiodic Functions—Fourier Transform 2.7 Acoustic Energy Energy Density Acoustic Intensity Reference Levels

Acoustic radiation pressure produced by a beam of sound

Abstract: The second‐order force produced by a sound beam directed normally at a plane target is calculated. Previous theories on acoustic radiation pressures associated with plane acoustic waves are examined critically and erroneous results, where they exist, are noted and rectified. A number of general relations are established using a new approach which avoids the necessity of dealing with detailed solutions of the governing nonlinear equations. Some of the concepts inferred from known solutions obtained by previous authors require drastic revision in the light of the present study. Specifically, the notion that Rayleigh radiation pressure depends on the nonlinearity of the medium (while Langevin radiation pressure does not) is not true in the case where the medium is bound by a partially reflecting wall. Again, that the concept that Rayleigh radiation pressure depends on the acoustic field only through the energy density of the field is shown to be false. In one instance it is shown to depend also on how the field is maintained, while in another instance it does not appear to depend on the mean energy density of the field at all.

Reflection of acoustic waves at a water–sediment interface

Abstract: Reflection and refraction of plane acoustic waves are studied for the case where the sediment is modeled as a porous viscoelastic medium. The model is based on the classical work of Biot which predicts that three different kinds of attenuating body waves may propagate in the sediment. As a consequence when homogeneous plane waves in water are incident to a water‐sediment interface, three nonhomogeneous waves are generated in the sediment. In these waves the direction of phase propagation and the direction of maximum attenuation are not the same and particle motion follows an elliptic path. Moreover, the velocity and attenuation of the refracted waves become dependent on the angle of incidence and no “critical” angle occurs. Numerical examples show that the reflectivity of a porous viscoelastic model differs significantly from the case where the sediment is modeled as a viscoelastic solid with constant complex modulus. Finally, because of the frequency dependence of reflectivity in the porous model, it is ...

Plasma waves associated with energetic particles streaming into the solar wind from the Earth's bow shock

Abstract: Plasma wave and plasma data from ISEE 1 and 2 are examined. In the upstream solar wind, three dominant types of plasma waves are observed which are associated with energetic particle streams coming from the bow shock: ion acoustic waves, electron plasma oscillations, and whistler mode waves. The ion acoustic waves occur simultaneously with either ion beams or a dispersed ion population in the energy range from 0.5 to greater than 45 keV. The electron plasma oscillations are long-wavelength, nearly monochromatic electrostatic waves which are closely correlated with the flux of low-energy electrons, especially in the 0.2-1.5 keV range. Electromagnetic waves with frequencies below 200 Hz are observed when either ion beams or dispersed ion distributions are present; for these waves the refractive index determined from the wave B to E ratio is consistent with whistler mode radiation.

Nonlinear ion-acoustic waves and solitons in a magnetized plasma.

Abstract: A unified formulation is presented to study the nonlinear low‐frequency electrostatic waves in a magnetized low‐b plasma. It is found that there exist three types of nonlinear waves; (i) nonlinear ion‐cyclotron periodic waves with a wave speed Vp≳Cs (ion‐acoustic velocity); (ii) nonlinear ion‐acoustic periodic waves with Vp

Laser-induced phonon spectroscopy. Optical generation of ultrasonic waves and investigation of electronic excited-state interactions in solids

Abstract: Crossed laser pulse excitation generates high amplitude, counterpropagating, ultrasonic waves (acoustic phonons of selected wave vector) via direct coupling between the optical electromagnetic field and the material acoustic field. The technique allows optical generation of ultrasonic waves, conveniently tunable to at least 20 GHz. The coupling mechanism, which does not involve optical absorption, is discussed in detail in terms of electrostriction. The periodic density changes resulting from the acoustic waves cause spectral shifts whose magnitudes reflect the strengths of excited-state intermolecular interactions and excited-state phonon interactions. The first quantitative measurements of spectral shifts by laser-induced phonon spectroscopy (LIPS) are reported. In pentacene in $p$-terphenyl, spectral shifts on the order of 1 ${\mathrm{cm}}^{\ensuremath{-}1}$ are measured using laser-induced phonons propagating along the $b$ crystallographic axis. Orientation of the phonon wave vector along various crystalline directions allows investigation of the anisotropic excited-state intermolecular interactions.

Torque generated by orthogonal acoustic waves—Theory

Abstract: An analytical calculation of the torque generated by orthogonal waves is reported. This torque is a result of a viscous effect, rather than the Bernoulli effect as in Rayleigh's torque. The agreement between the reported experimental values and this calculation is excellent.

Precision acoustic measurements with a spherical resonator: Ar and C2H4

Abstract: The spherical acoustic resonator is a remarkably accurate and convenient tool for the measurement of thermophysical properties of gases at low and moderate densities. The speed of sound (c) in a gas of interest can be measured with an accuracy of 0.02% merely by measuring the frequencies of the radial resonances when the resonator is filled with the gas of interest and then repeating the frequency measurements with a reference gas such as argon. The resonance frequencies of the radial modes are easily measured because these modes have very high Q’s, typically 2000–10 000. In this work the precision and accuracy of speed of sound measurements have been substantially improved by including a detailed acoustic model of the resonator in the analysis. Many of the important parameters of the model can be determined from acoustic measurements: Painstaking mechanical measurements are not required. We have used a spherical resonator to measure the speed and attenuation of sound in C2H4 in the temperature range 0–100 °C and the pressure range 0.15–1.0 MPa. Our measured values of c in C2H4 have a precision of 0.003% and agree with those of Gammon within the scatter of Gammon’s data (±0.02%). This agreement is remarkable when one considers that our spherical resonator is operated in the frequency range 4–13 kHz while Gammon has used a more conventional, cylindrical, variable path, acoustic interferometer operating at 500–2500 kHz. To attain this agreement, we did not have to make any highly accurate measurements other than frequency. Our measured values of the bulk relaxation frequency of C2H4 are within the scatter of the more recent values of the literature. In the course of our ’’calibration’’ measurements with argon we have redetermined the leading acoustic virial coefficient of argon. Our values for the virial are in satisfactory agreement with those in the literature. We include several practical suggestions for increasing the accuracy and/or versatility of spherical resonators.

A Surface Acoustic Wave Gas Detector

Abstract: : Experimental results are presented on a new type of gas detector employing surface acoustic waves (SAW's). The SAW piezoelectric gas detector (SAWPG) consists of twin SAW delay lines fabricated on a single piezoelectric substrate each connected in an oscillator configuration. The propagation path of one delay line oscillator is coated with a selectively sorbent film, while the other is uncoated and used as a stable reference. Changes in phase delay resulting from mass loading or stress effects induced by gases sorbed on the delay line containing the film result in corresponding frequency shifts relative to the reference oscillator that are proportional to gas concentration. Since SAW energy is concentrated near the film, the detector is found to be highly sensitive. Further, the SAWPC offers the means to detect any gas given the corresponding selectively sorbent film.

Ultrasonic imaging apparatus

Abstract: Ultrasonic imaging apparatus including ultrasonic transducers for emitting acoustic wave beams of various apertures, apparatus for directing two acoustic wave beams of substantially different focusing distances on the same beam direction, circuitry for receiving the reflected waves corresponding to the beam directions, apparatus for modulating the scanning lines corresponding to the beam position directions of the reflected acoustic waves, and apparatus for composing the scanning lines corresponding to the two acoustic beams directed on the same beam direction into a single scanning line for display

Space–time dependence of acoustic waves in a borehole

Abstract: A combined theoretical and experimental study, using a scale laboratory model with a cylindrical borehole, has been performed to study the space–time dependence of modal propagation. Wave analysis has been expanded to include cased hole as well as the more usual situation of open hole propagation. A general formalism for cylindrically layered media is presented. Experimental results include 40 waveforms with transmitter–receiver spacings in increments of 12.7 mm which is sufficient to avoid significant spatial aliasing. Wavenumber‐frequency spectral estimation is used to resolve trapped modes in the data set. Good agreement between theory and experiment has been obtained for both modal dispersion and mode cutoff frequencies.

Linearly focused acoustic beams for acoustic microscopy

Abstract: A linearly focused acoustic beam is investigated to be introduced into acoustic microscopy for characterising materials in the nonscanning version. The new acoustic beam enables us to detect acoustic anisotropies of materials to be measured successfully. For forming the acoustic beam, an acoustic sapphire lens with a cylindrical concave surface of 1.0 mm in radius is made and the acoustic field distributions are investigated at 200 MHz.

Apparatus for producing ultrasonic waves in a workpiece

Abstract: Laser light for producing acoustic waves in a specimen to be examined is carried to the specimen from a single laser by a plurality of fiber optic cables. The fiber optic cables have different path lengths to cause a specific time delay between adjacent fiber optic cables to focus and steer the sonic waves in a desired direction.

Acoustic measurement of near surface property gradients

Abstract: Disclosed is a method for determining the dispersion of a surface acoustic wave in an object, including the steps of generating a broadband acoustic wave in a surface of the object, detecting the wave at first location on the surface, and detecting the wave at a second location on the surface. Fourier transforms of the first and second detected waves are calculated, then the change in phase Δφ(f) of the frequency component f of the detected wave, between the first and second locations, is computed from the phase components of the quotient of the two transforms. The dispersion of the wave in the surface is given by the formula v(f)=(2πf Δl/Δφ(f)) In a pulse-echo version of the method, the wave is generated and detected at the first location, and generated and detected at the second location, the dispersion then being according to the formula v(f)=4πf Δl/Δφ(f)).

Acoustic radiation force on a rigid sphere in a resonance chamber

Abstract: The acoustic radiation force on a rigid sphere has been measured in a resonance chamber for a range of pressures, positions, sizes, and for various gases. In the low to medium intensity region (<150 dB) the measured force is consistent with King’s theory [Proc. R. Soc. London, Ser. A 147, 212 (1934)] when analyzed in terms of the fundamental pressure. However, in the high intensity region (≳150 dB), the measured force starts to deviate systematically from King’s calculations.

Stimulated and spontaneous emission of acoustic waves from shock fronts

Abstract: In certain materials, shock waves of sufficient intensity spontaneously emit transverse rarefaction waves and are therefore unstable. A criterion is derived by calculating the relative amplitudes of reflected acoustic waves incident on the shock from behind. This criterion is found to delimit the conditions under which both acoustic amplification and acoustic emission can occur; it agrees with earlier results by Kontorovich.

Attenuation of acoustic waves in lithium niobate

Abstract: The attenuation of acoustic waves in the bulk of LiNbO3 at room temperature has been measured in the frequency range 500 MHz to 10 GHz. The measurement technique involved optical detection at the lower half of the frequency range and high resolution pulse echo method at 9.45 GHz. The attenuation is found to depend on the square of the frequency, so the attenuation coefficient may be given in terms of an effective viscosity value, according to a treatment based on the theory of perturbation. The attenuation coefficient has been determined for various modes and propagation directions allowing the complete determination of the viscosity tensor, from which the attenuation coefficient can be calculated for any desired type of acoustic wave in LiNbO3.

Damped acoustic transducers with piezoelectric drivers

Abstract: A tuned acoustic directional transducer for transmitting and receiving airborne sound, which provides enhanced efficiency and reduced cost without undue narrowing of bandwidth, makes use of an acoustic transducer element (2) coupled to a plate (10) having a higher order flexural mode resonance at approximately the desired frequency of operation, the plate being coupled to the air through low-hysteresis acoustic propagation material having an acoustic impedance much less than that of the plate and much greater than that of the air. The material is disposed so that in the desired direction of propagation there is no substantial reduction of sound intensity in the far field resulting from cancellation occasioned by interaction of sound radiated from adjacent antinodal zones. Preferably the thickness of the material is such that it acts as an efficient acoustic impedance matching transformer. Preferably, the transducer element is piezoelectric and coupled to the center of a circular plate to which the coupling material is applied in rings (16, 18, 20).

Method and device for the localization and analysis of sound emissions

Abstract: A method for localizing and analyzing sound emissions, wherein the position of the sound emission source is determined by incoming sound pulses at various measuring locations. The arrival time of a pulse or of a pulse characteristic is measured in three or more adjacent portions of each measuring location whose width is smaller than the expected sound field diameter. From this, the propagation time difference for the portion of each measuring location is determined and the sound arrival direction is determined from these propagation time difference values for each measuring location. Subsequently, the position of the sound emission source is determined from these direction values and the position coordinates of the measuring locations. The device for carrying out this method has three or more separate converter elements (1) for each receiver measurement head (E) whose total width is smaller than the expected sound field diameter.

Influence of cosmic rays on propagation of long magneto hydrodynamic waves

Abstract: The influence of relativistic particles on the dispersion properties of a cosmic plasma are considered. Use is made of the equation of magneto-hydrodynamics for the thermal plasma and the diffusion equation for cosmic rays. The mechanism of dissipation of the waves due to the diffusion of cosmic rays is shown to be predominant for magneto acoustic waves in the interstellar medium.

Nonlinear Properties of Surface Acoustic Waves: Applications to Oscillators and Sensors

Abstract: Abstrocr-The application of surface acoustic waves (SAW) to frequency generation is considered for stable oscillators and sensors. The nonlinear properties defining the coupling with external, thermal, and mechanical perturbations (temperature, forces, pressures, accelerations) are analyzed and compared with the experimental data.