Showing papers on "Acoustic wave published in 1978"

Generalized theory of the photoacoustic effect

Abstract: The theory of the photoacoustic effect is extended to include the contribution of mechanical vibration of the sample. Coupled equations for thermal and acoustic waves are solved in both sample and gas. It is shown that the pressure signal in the gas may be significantly affected by acoustic coupling in the sample, and experimental confirmation of this extended theory is given. The results of the fully coupled treatment are shown to be accurately reproduced by an extension of the Rosencwaig piston model: the pistonlike motion of the gas boundary layer adjoining the sample is superimposed on the mechanical vibration of the sample surface to give a composite piston displacement which then produces the pressure signal in the gas. The composite‐piston model provides relatively simple algebraic results applicable to many cases of physical interest.

The Equations of Motion for Thermally Driven, Buoyant Flows.

Abstract: In this paper a set of approximate equations is derived which is applicable to very nonadiabatic, nondissipative, buoyant flows of a perfect gas. The flows are assumed to be generated by a heat source in which the heat is added slowly. The study is motivated by the occurrence of such flows in fires. There, the time scale associated with the fire growth and resultant fluid motion is usually long compared with the transit time of an acoustic signal (based on the temperature derived from the heat added) across the spatial extent of the fire. The approximate equations are characterized by a spatially uniform mean pressure appearing in both the energy equation and the equation of state with the spatially nonuniform portion of the pressure only appearing in the momentum equation. Therefore, the pressure remains almost constant in space while significant density and temperature variations, such as might occur in a fire, are allowed. The approximate equations are shown to reduce to the Boussinesq equations when the heat addition is mild. These equations are also shown in general to admit internal-wave motions while "filtering out" high-frequency, acoustic waves. In addition, they are shown to be expressible in conservation form, the pressure satisfying an elliptic equation whose homogeneous terms are derivable from the wave equation by letting the sound speed become infinite. An equation for the mean pressure is also obtained. For the special case of a room heated at a uniform rate with a small leak to the outside, an approximate solution for the mean pressure is determined explicitly.

Acoustic harmonic generation at unbonded interfaces and fatigue cracks

Abstract: The generation of a second‐harmonic signal at a flat unbonded interface between two solids has been observed. This signal is caused by passage of a longitudinal acoustic wave across the interface. The harmonic amplitude depends upon the pressure applied normal to the interface and is largest close to zero pressure, as expected theoretically. The effect has also been used to detect the presence of microcracks on the surface of Al 2024, developed during fatigue.

An angular‐spectrum approach to contrast in reflection acoustic microscopy

Abstract: The scanning acoustic microscope in the reflection mode has proved to be a rather simple and direct means for monitoring the elastic properties of a solid surface. When smooth surfaces of crystalline material are examined in a liquid with a highly convergent sound beam they exhibit a distinct response. This characteristic response, which can be treated as a ’’signature’’, is obtained by recording the output of the microscope as the spacing between the acoustic lens and the object is varied. An angular‐spectrum approach is used to derive an expression for this output in terms of the reflectance function. This function has an angular dependence determined by the bulk constants of the material itself. The expression resulting from this treatment can be used to explain the source of contrast in acoustic images.

Laser‐generated stress waves in liquids

Abstract: The generation of laser‐induced stress waves in liquids by the vaporization process and the thermoelastic effect was studied experimentally. A high‐speed camera and special high‐sensitivity stress transducers with a response time of a few nanoseconds have been used for these investigations. The experimental results obtained for water, n‐heptane, and carbon tetrachloride are discussed. For the first time, the individual contributions of vaporization and the thermoelastic effect on stress generation are separated. In addition, tunable high‐frequency acoustic waves, with frequencies up to 60 MHz, have been generated in water by the impact of a laser pulse exhibiting longitudinal mode beating. Since existing theories on the thermoelastic generation of acoustic waves do not yield satisfactory agreement with our experimental data, a new spherical model is proposed, where the transient heating caused by the laser impact, is represented by the three‐dimensional heat pole. This solution of the equation of heat conduction corresponds to a Gaussian distribution of the excessive temperature in space, and thus to the TEM00 mode of the incident laser beam. An analytical solution of the thermoelastic pressure wave is derived for this case of temperature distribution. Its good agreement with the experiment is discussed for various liquids and for two different laser characteristics.

The application of reciprocity theory to scattering of acoustic waves by flaws

Abstract: The reciprocity theorem and the scattering matrix formalism of electromagnetic theory have been adapted to obtain formulas for scattering of acoustic waves from flaws. The technique has been applied, in the Born approximation, to determine scattering of plane waves by small flaws, and with an elastostatic approximation to find the scattering from a flat elliptical crack. A third example is a treatment of the optical approximation to scattering from a curved interface between two media. In all cases results are obtained which are applicable for flaws placed either in the Fresnel region or Fraunofer region of unfocused transducers, or near the focal point of a focused transducer. A fourth example, given in the Appendix, uses the reciprocity theory to derive the axial field of a piston transducer in contact with the surface of a solid.

Ion-Acoustic Waves in the Solar Wind.

Abstract: Plasma wave measurements on the Helios 1 and 2 spacecraft have revealed the occurrence of electric field turbulence in the solar wind at frequencies between the electron and ion plasma frequencies. Wavelength measurements with the Imp 6 spacecraft now provide strong evidence that these waves are shortwavelength ion acoustic waves which are Doppler-shifted upward in frequency by the motion of the solar wind. Comparison of the Helios results with measurements from the earth-orbiting Imp 6 and 8 spacecraft shows that the ion acoustic wave turbulence detected in interplanetary space has characteristics essentially identical to those of bursts of electrostatic turbulence generated by protons streaming into the solar wind from the earth's bow shock. In a few cases, enhanced ion acoustic wave intensities have been observed in direct association with abrupt increases in the anisotropy of the solar wind electron distribution. This relationship strongly suggests that the ion acoustic waves detected by Helios far from the earth are produced by an electron heat flux instability, as was suggested by Forslund. Possible related mechanisms which could explain the generation of ion acoustic waves by protons streaming into the solar wind from the earth's bow shock are also considered.

Exact solitary ion acoustic waves in a magnetoplasma

Abstract: It is shown that finite amplitude ion acoustic solitary waves propagating obliquely to an external magnetic field can occur in a plasma.

A perturbation analysis of the attenuation and dispersion of surface waves

Abstract: A perturbation formulation of the equations of linear piezoelectricity is obtained using a Green’s function approach. Although the resulting equation for the first perturbation of the eigenvalue strictly holds for real perturbations of real eigenvalues only, it is formally extended to the case of purely imaginary perturbations of real eigenvalues. The extended equation is applied in the calculation of the attenuation of surface waves due to the finite electrical conductivity of thin metal films plated on the surface and air loading. The influence of the viscosity of the air is included in the air‐loading analysis, and the calculated attenuation increases accordingly. Since the metal films are thin compared with a wavelength, an approximate thin‐plate conductivity equation is employed in the determination of the attenuation due to the electrical conductivity of the films. The resulting attenuation is obtained over a very large range of values of sheet conductivity. This is accomplished by using the equatio...

Sound production in a moving stream

Abstract: Jet noise is modeled by particle-attached quadrupoles convected with the velocity of the actual fluid but positioned near a hypothetical instability-free vortex sheet. The strength of each quadrupole is Lighthill's stress tensor per unit mass. The work of Mani (1976) has shown that this type of model agrees well with experiment, and the present work establishes some of the equivalent sources needed for an exact analogy. The instability waves of the shear layer, as they grow into turbulence, are heard as sound that builds up as a precursor of the main turbulence-driven field. The circular compact jet is examined in some detail, and it is found that when the jet is very light it can provide a waveguide in which the effects of source activity persist for some time but eventually leak out as sound. This interaction greatly distorts the free field characteristics of the turbulent sources, so that Reynolds-stress-induced waves have an intensity that scales with the fourth power of jet velocity.

On the origin of periodic surface structure of laser‐annealed semiconductors

Abstract: Observations on laser‐irradiated GaAs are reported in which the phenomenon of induced surface periodicity is related to a nonlinear interaction between simultaneously oscillating axial modes of the laser. It is suggested that during surface melting and regrowth material is distributed along nodal lines of a standing acoustic wave pattern corresponding to the axial mode beat frequencies. The several patterns observed are not consistent with an interference effect at optical frequencies.

Excitation of instability waves in a two-dimensional shear layer by sound

Abstract: The excitation of instability waves in a plane compressible shear layer by sound waves is studied. The problem is formulated mathematically as an inhomogeneous boundary-value problem. A general solution for abitrary incident sound wave is found by first constructing the Green's function of the problem. Numerical values of the coupling constants between incident sound waves and excited instability waves for a range of flow Mach number are calculated. The effect of the angle of incidence in the case of a beam of acoustic waves is analyzed. It is found that for moderate subsonic Mach numbers a narrow beam aiming at an angle between 50 to 80 deg to the flow direction is most effective in exciting instability waves.

Acoustic microscope and method

Abstract: An acoustic apparatus and method for microscopic imaging and spectroscopy. The apparatus includes a plurality of devices for exciting an object of interest so that acoustic waves are propagated from the object. These devices include lasers, x-ray sources, microwave generators, ultraviolet sources, and electric current generators. The acoustic waves propagated from the object of interest are detected and the object of interest and the acoustic wave detector are moved with respect to each other in a raster scanning pattern. The magnitude of the detected acoustic waves and the corresponding raster pattern of the object are recorded so that a visual image of the object can be obtained. In addition, the frequency of the exciting electromagnetic radiation that excites the object can be varied so that both the absorption spectra and the Raman frequency mode of the object can be determined.

Acoustic radiation and scattering from elastic structures

Abstract: This paper presents a numerical technique for the linear dynamic analysis of a finite elastic structure immersed in an infinite homogeneous acoustic medium. It is required to determine the vibrational motion of the structure and also the associated acoustic field in the fluid, when the structure is either subjected to internal applied forces or is acting as a scatterer of an incident acoustic wave. A finite element analysis of the structure is matched at the structure-fluid interface with an integral equation representation of the exterior acoustic field, leading to a coupled system of equations which may be cast in either acoustic or structural from. The former approach is preferred here for which numerical results are presented when the method is applied to plane wave scattering by thick and thin elastic spherical shells.

Effects of atmospheric turbulence on the interference of sound waves near a hard boundary

Abstract: The mean sound levels resulting from the interference between direct waves and those reflected from the ground are strongly influenced, especially at frequencies near interference minima, by fluctuations in phase and amplitude of the sound waves induced by propagation through atmospheric turbulence. Since it was found experimentally that the correlation length (∠1.1 m) of the meteorological fluctuations is comparable to the separation between the interfering sound paths, previous theoretical work by Ingard and Maling [J. Acoust. Soc. Am. 35, 1056–1058 (1963)] has been extended to allow for partial covariance between the two waves. The theory has been further extended to use the calculations of fluctuations in phase and amplitude of spherical waves, and to include the explicit calculation of the fluctuating acoustical index of refraction from the fluctuating values of temperature and wind velocity. Measurements (1–6 kHz) have been made of the interference spectrum at 15, 30, and 45 m from a point source 1....

Active control of sound waves

Abstract: A sound wave propagated along a duct through a fluid contained in the duct is attenuated by generating sound waves from an array of sound sources spaced along the duct. Each source generates two waves travelling in opposite directions, the array being operated so that the resultant of those travelling in the same direction as the unwanted wave interferes destructively with the unwanted wave while the resultant of those travelling in the opposite direction is negligible. The array is operated in response to detection of the unwanted wave, the sound detector(s) being so positioned as to introduce a degree of acoustic coupling between the source array and the detection system.

Nonlinear self‐modulation of ion‐acoustic waves

Abstract: The nonlinear evolution of an ion‐acoustic wave packet is studied. Experimentally, it is found that (i) nonlinear phase modulation develops in the wave packet; (ii) the phase modulation, together with the dispersion effect, causes expansion and breaking of the wave packet; (iii) the ions trapped in the troughs of the wave potential introduce self‐phase modulation; and (iv) the ion‐acoustic wave is stable with respect to the modulational instability. Computer simulations have reproduced the experimental results. The physical picture and the model equation describing the wave evolution are discussed.

Mud gas ratio and mud flow velocity sensor

Abstract: A remote sensor for the measurement of the amount of gas present in the drilling fluid or "mud" in an offshore well-drilling operation, for the purpose of giving prompt warning of geologic conditions conducive to the occurrence of a blow-out. An acoustic projector projects an acoustic wave of fixed wavelength through the mud returning upwardly from the borehole. The wave is received by a first array of acoustic receivers locked in a known phase relationship with the projector. Changes in the speed of sound due to changing relative amounts of mud and gas produce a shift in frequency of the acoustic wave which is proportional to the speed of sound in the mud/gas mixture. The change in frequency is electronically detected and scaled to yield the value of the speed of sound, from which the mud to gas mass ratio is calculated by means of a computer. A second array of acoustic receivers is arranged with the first array and the projector to form a phase-shift velocimeter which uses the value of the speed of sound obtained in conjunction with the mud/gas mass ratio measurement to calculate the velocity of the mud flow, which is used as a further indicator of potential blowout conditions.

Effects of particle shape and orientation on propagation of sound in suspensions

Abstract: Particles are assumed to have oblate and prolate spheroidal, spherical, disklike, and needlelike shapes, with symmetry axes for oblate and prolate spheroids assumed to be either parallel or perpendicular to the acoustic field. The disk‐shaped and needle‐shaped particles are assumed to be either broadside or edgewise (or end on) to the acoustic field. A wave equation is developed for dilute suspensions of the particles which yields a formula for sound velocity and viscous absorption coefficient. For large differences in densities between suspension components, computations indicate substantial effects of particle shape, orientation, and ultrasound frequency on the velocity of sound through the suspension. A particle moving edgewise or end‐on to the acoustic field causes a greater loss of sound energy than a particle moving broadside to the acoustic field.

Theory of nonlinear electroacoustics of dielectric, piezoelectric, and pyroelectric crystals

Abstract: Based on a fully electrodynamic Lagrangian theory of elastic dielectrics, a completely deductive derivation of the dynamical equations and constitutive relations for elastic, electric, and electroelastic phenomena is presented. The equations apply to crystals of arbitrary symmetry, structural complexity, and nonlinearity. A key technique of this work is the use of material frame electromagnetic fields. We show that the electrostriction tensor differs from the low‐frequency limit of the elastooptic tensor by the Maxwell stress tensor of a polarizable medium. A general expression containing three new terms for the effective third‐order elastic constant measured by mixing of two input acoustic waves is obtained. Our expression is more general in applying to all crystals and to acoustic waves of all types propagating collinearly or noncollinearly in any direction. We derive the coupled mode equations governing the Thompson‐Quate experiment, that is, the parametric interaction of two counterpropagating bulk ac...

Resonance scattering of acoustic waves from cylindrical shells

Abstract: The theory of resonance scattering, recently developed for acoustic‐wave scattering from elastic objects and elastic‐wave scattering from cavities, is applied here to the problem of sound scattering from fluid‐filled elastic cylindrical shells in a fluid, in particular air‐filled aluminum shells in water. The eigenvibrations of the shell appear as resonances in the scattering amplitude of each normal mode; these are superimposed on a smooth background which if the shell is made thinner, converts from the amplitude of a rigid to that of a soft cylinder. The resonances themselves are exhibited by subtracting the background from the total amplitude and are seen to become less numerous as the shell gets thinner.

Small amplitude gasdynamic disturbances in an exploding atmosphere

Abstract: The propagation of disturbances through an atmosphere that is, in its undisturbed condition, undergoing a spatially uniform chemical explosion is analysed on the assumption that the disturbances are of small amplitude. When the latter are arbitrarily small, and therefore classifiable as acoustic, the progress of the ambient explosion is undisturbed to first order and a rather complete history of the acoustic waves (including weak shock waves) can be constructed. The generally amplifying effect of the explosion on the disturbances, which has previously been identified at wave heads, is found to occur throughout the disturbed domains.When the disturbance amplitude becomes comparable to the ratio of the thermal energy of the gas to the combustion-reaction activation energy, the ambient explosion becomes involved in the disturbance to first order. For large activation energies a small disturbance theory can be constructed to account for the disturbance behaviour; at present it is limited to time intervals from initiation that are shorter than the ‘no-depletion’ homogeneous ignition time.

Transducer arrays suitable for acoustic imaging

Abstract: : The design and development of piezoelectric transducer arrays suitable for use in electronically scanned and focused acoustic imaging systems is described. These arrays are designed to operate in the frequency range 2-5 MHz within 45% to 80% fractional bandwidths. Linear arrays of up to 180 elements have been built and used extensively in acoustic imaging systems. Particular attention is placed on achieving high transduction efficiency and angular beam- widths of at least + or - 15 deg. Design techniques based on the transmission line model of the transducer of Krimholtz, Leedom, and Matthaei are formulated for achieving efficient, broadband operation. These techniques involve the use of quarter-wave matching layers between a high impedance ceramic and a low impedance load. Broadband matching criteria are generated, and a novel technique for selecting the quarter-wave matching layer impedances is described. The theoretical transient response of the transducer is obtained by taking the Fourier transform of the transfer function of the transducer. An experimental transducer built using the formulated design techniques is described and its characteristics compared to theory. Slotted transducer arrays employing tall, narrow elements are described. A one-dimensional model of this kind of element is formulated, and the effects of coupling between element resonant modes are examined.

On the emission of sound by an ionized inhomogeneity

Abstract: The emission (including generation, propagation, scattering, convection and radiation) of sound by an ionized inhomogeneity in a non-uniform mean flow is studied as an extension of the hydrodynamic problem (Howe 1975 a, b ) to include a variable electromagnetic field. The stagnation enthalpy is the thermodynamic function of state taken as the acoustic variable, and satisfies an inhomogeneous wave equation, which describes the propagation and generation of sound in a compressible mean flow, and which assumes a simplified form at low Mach numbers. The effective acoustic source consists of two dipoles, associated with the Laplace-Lorentz (or electromagnetic) force and the hydrodynamic force. The latter consists of displacement and vortical terms that are shown to be analogous to the classical electric and magnetic forces, respectively. The level of radiation depends on the changes of the activity (work per unit time) of these forces experienced by the ionized inhomogeneity, e.g. as it is convected past electrified bodies, which are responsible for the variable hydrodynamic and electromagnetic fields, and also scatter the wave field. By taking as an example a model problem featuring biaxial symmetry, the amplitude and shape of sound pulses are discussed and illustrated in a number of generation conditions, hydrodynamic and electromagnetic, and observation positions in the far-field. In principle there can be destructive interference between hydrodynamic and electromagnetically generated sound, although in practice this is not expected to occur. In aerodynamic and atmospheric conditions the hydrodynamic sound predominates, whereas in plasma and stellar situations, electromagnetic sound may be dominant.

Dynamics of the solar transition zone.

Abstract: Time-resolved profiles of the 1548-A C IV line arising from the solar transition region are analyzed in order to determine whether the 300-sec oscillations characteristic of the photosphere and chromosphere penetrate into the transition zone and to measure the rms amplitude of transition-zone disturbances as well as their dependence on solar activity. The rms velocity amplitude is used to set limits on the mechanical energy flux available for solar heating. A power-spectrum analysis indicates that acoustic waves appear to have been detected in the transition zone, that at least one case of a strong 200- to 300-sec oscillation was observed, but that strong periodicities are not found on the average in either the intensity or the velocity field. It is suggested that the rms velocity that may be attributed to directly observable wave motion is between 3 and 7 km/s, depending on whether the individual emission elements seen in rocket spectra are coherent or independent in phase.

Piezoelectric acoustic boundary waves propagating along the interface between SiO 2 and LiTaO 3

Abstract: The propagation characteristics of the layered structure of thick Si02 film on 126° rotated Y-cut X-propagation LiTa03 are studied. The existence of the new piezoelectric acoustic boundary waves (Stoneley type waves) has been found. The waves are tightly bound to the thick Si02 film-LiTa03 substrate interface. Piezoelectric acoustic boundary waves appear beyond H/λ = 0.5, where H is the film thickness and λ is the acoustic boundary wave length. At H/λ = 1.0, the energy concentration coefficient, η = |Us|2 / l |Ui|2 is 0.08, where Us and Ui, are displacement at the surface and interface, respectively, and piezoelectric coupling coefficient K2 is 0.02. The experimental results agree with the calculated results.

Saw IF Filter on LITaO3 for Color TV Receivers

Abstract: More than eight years have passed since DeVries et al. reported the first attempt at installing SAW (Surface Acoustic Wave) filters in a TV set(1). In Japan, SAW IF filter development has been continued by several companies and TV receivers incorporating SAW filter have been placed on the market since 1977.