Showing papers on "Acoustic wave published in 1975"

Acoustics of Aircraft Engine-Duct Systems

Abstract: Noise generated in aircraft engines is usually suppressed by acoustically treating the engine ducts. The optimization of this treatment requires an understanding of the transmission and attenuation of the acoustic waves. A critical review is presented of the state of the art regarding methods of determining the transmission and attenuation parameters and the effect on these parameters of (1) acoustic properties of liners, (2) the mean velocity, including uniform and shear profiles and nonparallel flow, (3) axial and transverse temperature gradients, (4) slowly and abruptly varying cross sections, and (5) finite-amplitude waves and nonlinear duct liners.

Propagation of Ion-Acoustic Waves in a Two-Electron-Temperature Plasma

Abstract: The propagation of ion-acoustic waves (IAW) in a double-electron- temperature plasma is investigated both experimentally and theoretically. It is found that the presence of even a small fraction of the lower-electron- temperature component can dominate the behavior of the waves. The results have important implications both for the use of IAW as a diagnostic tool for measuring electron temperature and for the interpretation of turbulent IAW spectra. (AIP)

Nonlinear behavior of acoustic waves in combustion chambers

Abstract: This paper is concerned with the general problem of the nonlinear growth and limiting amplitude of acoustic waves in a combustion chamber. The analysis is intended to provide a formal framework within which practical problems can be treated with a minimum of effort and expense. There are broadly three parts. First, the general conservation equations are expanded in two small parameters, one characterizing the mean flow field and one measuring the amplitude of oscillations, and then combined to yield a nonlinear inhomogeneous wave equation. Second, the unsteady pressure and velocity fields are expressed as syntheses of the normal modes of the chamber, but with unknown time-varying amplitudes. This procedure yields a representation of a general unsteady field as a system of coupled nonlinear oscillators. Finally, the system of nonlinear equations is treated by the method of averaging to produce a set of coupled nonlinear first order differential equations for the amplitudes and phases of the modes. These must be solved numerically, but results can be obtained quite inexpensively. Subject to the approximations used, the analysis is applicable to any combustion chamber. The most interesting applications are probably to solid rockets, liquid rockets, or thrust augmentors on jet engines. The discussion of this report is oriented towards solid propellant rockets.

Propagation of ion-acoustic waves in a multi-component plasma

Abstract: The authors derive the Korteweg-de Vries equation in a multicomponent plasma that includes any number of positive and negative ions. The solitary wave solutions are also found explicitly for the case of isothermal and non-isothermal electrons.

Analytic BGK modes and their modulational instability

Abstract: Physically acceptable solutions of the time-independent Vlasov–Poisson system are found by applying a method which is analogous to the soliton method in Korteweg–de Vries theory. Cnoidal wave solutions for small-amplitude Langmuir and ion acoustic waves are derived, in which the nonlinearity is determined by trapped electrons. The class of weakly distorted waves is shown to be modulational unstable.

Surface Acoustic Wave Multielectrode Transducers

Abstract: In the basic SAW transducer, the fingers are connected alternately to the two bus-bars. Interesting responses can be ob- tained if there are more than two fingers per fundamental acoustic wavelength, and the fingers are connected to the bus-bars in various ways. Several of these structures are discussed qualitatively. The transmission spectra obtained from these various schemes are con- firmed experimentally. Quantitative calculations of admittance are also performed for several transducer structures. The measured data agree well with calculations.

Multiple scattering by two cylinders

Abstract: Theoretical calculations and experimental measurements of the backscattering of a plane acoustic wave by two parallel, rigid cylinders are presented. The effects of multiple scattering between cylinders are included in the calculations. Two methods of solution of the equations are discussed. Good agreement is found between theory and experiment. Multiple‐scattering calculations show the influence of the shadow of one cylinder upon the other. Variations in level of the interference peaks are correctly predicted.Subject Classification: 30.30, 30.40; 20.30, 20.15.

II. Sonar System Technology

Abstract: T of thispaper is to present the fundamentals of sonarsystem technology. Specific sonar applications require many space-time processing procedures and associated tradeoffs. To optimize acoustic performance,system analysis and design should reflect the dynamics of the medium, sonar, and target. The organization and emphasis of thematerial was influenced by the fact that in the past twenty-five years, the most significant advancements made in sonar technology have been in: 1) improving understanding of shallow and deep water propagationmodes andevaluatingtheirconstraints on system design, 2) establishing the correlation between signal design andsonarparameters,and assessing the impact’ on the receiver structure, 3) adapting the receiver beam pattern to the changing structure of the masking background, and 4) utilizing dynamic range compression and normalizationtechniques to best “match” the received signal to the display and/or decision device. Particular attention is given here to a system model for active and passive sonar operatmion, the signal waveforms, transmission and reception modes commonly used in echo ranging, the propagation effects and reverberation mechanisms of the acoustic channel, the multidimensional aspects of conventional beamforming and the feasibility of adaptive control in an operational environment, the effectiveness of gain control receivers and hard clipping for dynamic range compression and normalization of acoustic data, the techniques presently employed for signal detect,ion and parameter estimation-for video and aural presentation,andthedynamicalsystem measures required for system performance evaluation. The material is presented in a somewhat “hit-and-run” fashion since it’ is meant t,o be tutorial. However, it is hoped that there is enough continuity to provide the conceptual link between the operational constraints and engineering design criteria of a sonar system for any given application.

On the Dynamics of Sonic-Langmuir Solitons

Abstract: The nonstationary evolution of nonlinear Langmuir waves is considered. In particular the process of soliton formation is studied in the case where the perturbation propagates with a speed close to that of sound. The influence of electron collisions on the motion of the soliton is also described.

Calculation of the effect of internal waves on oceanic sound transmission

Abstract: The signal received by a hydrophone in the ocean many kilometers from a steady sound source fluctuates dramatically due to variations of the speed of sound in sea water. By inserting an empirical model of internal‐wave‐generated sound‐speed variations into an acoustic‐transmission computer code, we have shown that internal waves cause significant variations in sound transmission at 100 Hz, comparable in size and frequency to the variations observed in field experiments. We have also studied the usefulness of vertical hydrophone arrays.Subject Classification: 30.25, 30.82; 28.60.

Acoustic telemetry system for oil wells utilizing self generated noise

Abstract: An acoustic communication method and system are disclosed for transmitting information through a well-bore drill string by using the acoustic noise inherently generated in the drill string by drilling operations as a "carrier" propagated therealong to be modulated by the information to be transmitted. In the drilling of wells, the action of the drill tool on the bottom of the bore hole creates acoustic noise within the drill string. Other suitable sources of "noise" are often present during "drilling operations", as broadly defined, even while bore drilling, as such, is suspended. This noise usually consists both of wide-band continuous acoustic energy and of narrow spectral lines of acoustic energy and is usually present in a variety of acoustic modes such as longitudinal and torsional acoustic waves. The detailed characteristic of this acoustic noise depends on the type of drill tool used, the drilling speed and the type of formation being drilled. When this inherently generated noise is modulated by an information containing signal at a signal-transmitting station, acoustic communication of information is possible while drilling is in progress, or, in some cases, while drilling as such is suspended. The communication occurs between the transmitting station and a signal-receiving station spaced along the drill string, using the intervening length of drill string between the signal-transmitting and signal-receiving stations as an acoustic transmission channel.

Thermoelastic waves generated by laser beams of low power

Abstract: A modulated or chopped beam from an argon ion laser, with CW power at the 1−W level, is used to generate thermoelastic waves in several liquids. The ceramic cylinder surrounding the liquids is the transducer for the acoustic waves. Output signal is measured as a function of frequency, with different laser powers and different amounts of absorption in the liquids. The analysis makes use of thermoelastic potentials in the liquid and in the ceramic cylinder, with appropriate continuity conditions between the two regions, and predicts results in good agreement with those measured. Conversion efficiency from laser power to acoustic power is extremely low at this power level, but increases directly with the laser power. Thus it is concluded that the acoustic wave generated by a modulated laser beam at the 1−W level propagating through sea water would only be detectable near the beam, but for a pulsed beam at the megawatt level, acoustic power might be detectable for several kilometers.Subject Classification: 35.65.

Passive transponders using acoustic surface wave devices

Abstract: A passive transponder sensitive to electromagnetic pulses received in the form of surface acoustic waves. The device utilizes a plurality of interdigital transponders each consisting of a metallic deposit on a piezoelectric substrate and serving as an electrode. Transponder deactivation and energy interruption means provide programability and coding.

Acoustic scattering by penetrable homogeneous objects

Abstract: The scattered field is found when a time harmonic acoustic wave is incident upon a finite object characterized by a density and wavenumber different from that of the surrounding medium. The interface is assumed to be either Lyapunoff or piecewise Lyapunoff. The problem is cast as a pair of coupled surface integral equations for the total field and its normal derivative on the interface. The Neumann series obtained by straightforward iteration is proven to be convergent for ranges of density and wavenumber, and specific bounds on these ranges are given. For small enough wavenumbers, the series converges for all values of the interior density. The iteration appears simpler than the usual Born approximation, which involves volume as well as surface integrals. The method is illustrated in the case of a spherical interface.

Ion-Acoustic Solitary Waves in Multicomponent Plasmas with Negative Ions

Abstract: By using the perturbation technique, a Kortewege-de-Vries (K-dV) equation for a multicomponent plasma with negative ions and isothermal electrons has been derived. We have discussed the stationary solutions of K-dV equation and it has shown that in the presece of multiple ions, the amplitude of solitons exhibits interesting behaviour, especiallY when the negative ions are present.

Obtaining subsurface profiles from surface−acoustic−wave velocity dispersion

Abstract: Surface acoustic waves can be used to probe nondestructively subsurface gradients (caused by physical processes) by changing their penetration depth with frequency. A perturbation−theory integral equation that describes the influence of a known gradient on the velocity dispersion is reviewed. The experimental situation requires solution of the ’’inverse problem’’: obtaining the profile from measured velocity data. The inverse problem is solved exactly by the use of Laplace transforms for gradients of the general form F (z). The nature of the solution allows observations about the gradient−dispersion relationship, the physical interpretation of dispersion curves, and the representation and measurement of gradients to be made. Six commonly occurring gradient functions and their dispersion curves are compared by use of an equivalent−area parameter formulation. The solution agrees well with published experimental results for residual stress in an aluminum block and for damaged layers on YZ LiNbO3 substrates. ...

Large, quiescent, magnetized plasma for wave studies

Abstract: The development of a large magnetized plasma source for wave propagation experiments is described. The method employs a dc discharge in a weak magnetic field region and subsequent particle diffusion into a strong, uniform B‐field region. The resultant steady‐state plasma is uniform (8 cm⊥B0, 120 cm∥B0), quiescent (δn/n≲1%), has high density (ne≳1011 cm−3), a B0=1 kG, argon (3×10−4 Torr), and low collision rates (νen≃105 sec−1). The source can be easily scaled to larger dimensions. Plasma diagnostics with probes, microwaves, and spectroscopy for steady state and afterglow are described. Examples of wave propagation for ion acoustic waves, electron Bernstein waves, and lower hybrid waves indicate the versatility of the device.

Sound Propagation in Fogs.

Abstract: The propagation of plane, infinitesimal acoustic waves through fog media is studied theoretically using a continuum formulation. It is shown that the correction of theories previously published decreases the effects of mass transfer between vapor and droplets and produces lower attenuation and dispersion predictions. General features of attenuation and dispersion phenomena as functions of fog and sound frequency parameters are otherwise unchanged. Correction of the theory improves the agreement between attenuation predictions and experimental measurements.

Thermal instability in high−power laser discharges

Abstract: The thermal and acoustic instabilities that could possibly cause arcing in high−power pulsed and cw gas lasers have been investigated A cubic dispersion relation is obtained from linearization of the coupled hydrodynamic and Ohmic equations in which the electrical conductivity is dependent on the neutral density We find that when the thermal wave is unstable the acoustic waves are damped, and vice versa The growth rate of the instability is proportional to the power density for low input powers while it goes as the cube root of the power density for high input powers

Finite−element approach to acoustic scattering from elastic structures

Abstract: An approximate method is presented for solving the equations of motion that describe the scattering of an acoustic wave by an elastic structure immersed in an infinite fluid medium. The mathematical model that is developed uses the finite−element method to calculate the vibrational response of the elastic body and to calculate the acoustic pressure field of a finite volume of the fluid medium which closely surrounds the elastic body. Analytical methods are used to obtain the boundary conditions for this mathematical model. Results are presented for the total scattering cross section, axial pressure, and scattering patterns for an acoustic plane wave at normal incidence on an aluminum disk.Subject Classification: 30.30, 30.40.

The propagation of solitary waves in piezoelectric semiconductors

Abstract: A non-linear theory of the propagation of acoustic waves in piezoelectric semiconductors is presented, based on an asymptotic expansion in multiple time and space scales. The envelope of the wave amplitude is found to satisfy a non-linear Schrodinger equation with complex coefficients. The solution of this equation is obtained using a perturbation technique. This result, a solitary wave, is used to study the saturation of the instability and it is found that the saturation mechanism is a local change of d.c. electric field and carrier concentration.

Characteristics of Wedge Transducers for Acoustic Surface Waves

Abstract: Wedge transducers have long been used to excite and detect elastic surface waves, but there has beenno complete theoretical description of the phenomena that underly their operation. In this paper we show that the phenomena, when the wedge operates as a detector, are: 1) the excitation by the incident surface wave of a leaky wave propagating beneath the wedge; 2) the radiation of the leaky wave fields into the wedge; and 3) the detection of the nonuniform leaky-wave fields by the bulk wave transducer on the slant face of the wedge. By studying these phenomena, we have found the factors influencing the efficiency of the wedge transducer and have determined the conditons under which it may be made as high as 80% of the radiation e5ciency of the bulk-wave transducer. Because of reciprocity, the same efficiency is obtained when the wedge is used to excite surface waves. To obtain high efficiencies, it is necessary to control the perturbation of the surface wave due to the presence of the wedge; methods for achieving such control are described in a companion paper.

Uniform Ray Theory of Surface, Internal and Acoustic Wave Propagation in a Rotating Ocean or Atmosphere

Abstract: First a short wavelength asymptotic expansion, employing rays, is presented for waves propagating in a rotating compressible fluid layer of nonuniform depth. This theory applies to surface, internal and acoustic waves in an ocean or atmosphere. It yields infinite amplitudes at caustics and shorelines. Therefore two different asymptotic expansions, uniform in a region containing a caustic and a shoreline respectively, are constructed. They yield the correct finite amplitude at the caustic and at the shoreline. In addition, expansions uniform in a region containing a caustic and a shoreline, or two or more caustics and shorelines, are constructed. Both general time-dependent waves and time-harmonic waves are considered. Linear theory is employed.

Frequency and damping of ion acoustic waves

Abstract: The frequency and damping of ion‐acoustic waves in a singly ionized plasma have been determined for wave lengths at which collisions are important but not dominant and for electron‐ion temperature ratios of 0.5, 1, 2, and 4, by numerically solving the linearized Fokker–Planck equation. The collisional and collisionless limits are in good agreement with existing theory. As a by‐product the ion viscosity and thermal conductivity were evaluated and agreed with those of Braginskii.

Acoustic method for detecting leaks from submerged pipelines

Abstract: A method for locating leaks in liquid-transporting submerged pipelines. A sonar device directs acoustic waves along the path of a submerged pipeline and the reflected waves are detected and recorded. The location of escaping liquid is then determined from the recorded reflected acoustic waves, the differences in the reflected acoustic waves resulting from the difference in acoustic impedances between the escaping liquid and seawater. Gas and/or a liquid having a different acoustic impedance from the seawater in or under which the pipeline is located may be introduced into the pipeline to enhance the detection of the liquid being transported through and leaking from the pipeline. Also, sediments stirred up by fluids leaking from a submerged pipeline and, particularly, a buried or partially buried pipeline will cause determinable differences in the recorded reflected acoustic waves. Further, the crater excavated by the escaping fluids will be revealed by the reflected acoustic waves. In addition, a fluid-transporting pipeline may be surveyed along the path of the pipeline with a sonar device and the reflected acoustic waves detected and recorded during normal no-leak conditions. Then the path of the pipeline is again surveyed by a sonar device and the reflected acoustic waves detected and recorded during abnormal or leak conditions. The "no-leak" and "leak" surveys are then compared. Variations in the surveys will indicate escaping fluids and/or a cavity or anomoly in the ocean floor caused by the flow of fluids from the leak and/or the stirred up ocean floor sediments that are dispersed by the flow of fluids from the leak.