摘要:

The coupled equations of motion for linearly interacting elastic foam and saturating fluid are formulated and solved for excitation at fixed frequency and wave number. As known from previous work on porous media, the longitudinal waves in the foam and fluid are coupled, and the shear wave in the foam is modified while imparting rotational motion to the fluid. Resulting fields are expressed suitably for use in situations of axisymmetry. The characteristic equation for free waves and relations between field coefficients are given explicitly for propagation within a rigid tube. Acoustically, a characteristic feature of the application to liquid‐saturated foam is the high mass porosity and compressibility of the empty foam, commending a corresponding limiting approximation. Applied to propagation in a rigid tube, this yields a modification of of the acoustic wave speed and impedance due to foam that is measured by (sQ)1/2(ωa/ct)−1, whereQ=iΔ/(1+iΔ), Δ is a drag‐interaction coefficient,sthe ratio of the mass of foam to that of fluid,athe tube radius, andctthe shear‐wave speed of empty foam. The interaction effect thus exhibits saturation, sinceQbecomes independent of Δ for sufficiently strong interaction. In a contrary limit where the foam is nearly stationary, the effect of foam is measured rather by Δ. Poiseuille flow in the presence of foam is also treated. The hydrodynamic determination of Δ is carried through for idealized foam geometries. In the viscous domain where the viscous depth exceeds the filament diameter, Δ is real and varies as ν/ωD2, whereDis pore size, with coefficient weakly dependent on porosity. At high frequencies Δ becomes negative imaginary on account of the induced‐mass effect.

ISSN:0001-4966    

DOI:10.1121/1.382236

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Sound transmission and reflection properties of a lossless fluid layer imbedded in another fluid are analyzed in terms of a resonance formalism similar to one that has been used in nuclear scattering theory, and more recently in acoustic scattering theory. Expressions for the transmission and reflection coefficients have been obtained which manifestly exhibit the resonance character of these functions. The formalism provides an explicit determination of the position, width, and strength of each individual resonance contribution, and links these resonance quantities to the material constants of the layer and its surroundings, thus indicating a novel way to determine these constants by a measurement of the acoustic transmission and/or reflection coefficients. Within the limits of the resonance approximation, the resonance positions may be obtained by solving a purely real characteristic equation, which constitutes a distinct computational advantage over the solution of the complex eigenvalue equation thought to be necessary for a loaded layer.

ISSN:0001-4966    

DOI:10.1121/1.382275

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

We have studied the scattering of plane waves from cylinders and spheres with complex impedance boundary conditions using the Kirchhoff method and the Luneburg–Kline method. A stationary phase approximation is made in the Kirchhoff derivation to eliminate spurious contributions from the boundary of the insonified region. Terms of order up to (ka)−2are retained. Dependence of the cross section on the complex impedance, the incident wave frequency, as well as on the azimuthal angle is illustrated. The curves are found to obey several near symmetries. Further, the Luneburg–Kline method is found to lead to predictions which are often qualitatively different from those of the Kirchhoff method, and the former is observed to be more reliable. Convergence of the Luneburg–Kline terms is studied and is found to be quite satisfactory.

ISSN:0001-4966    

DOI:10.1121/1.382258

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

The problem of sound propagation in a duct is investigated. Several results which have been deduced for isentropic duct flow by previous authors are extended to the case where entropy convection is included. For this more general case, a concept is developed to guide the solution of wave‐propagation problems in an optimal manner. Furthermore, it is shown that some new aspects of isentropic wave propagation emerge. To illustrate the methods, the stability of waves behind a shock wave is considered.

ISSN:0001-4966    

DOI:10.1121/1.382247

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

The acoustic radiation force on a solid sphere in a wave field in an inviscid fluid has been investigated theoretically and numerically taking into account the effects of elasticity of the sphere material for the following three cases: (I) the wave field is stationary, (II) the wave field is quasistationary, (III) the sound absorption in the sphere material is taken into account in each case above. The results show several features quite different from the rigid sphere solution.

ISSN:0001-4966    

DOI:10.1121/1.382263

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

The acoustic radiation force on a sphere in quasistationary waves has been investigated experimentally. The radiation forces measured with fused silica spheres have been compared with a previous theory by the present author which takes into account the elasticity of the sphere material. Good agreement has been found. It has been demonstrated that the acoustic radiation forces on fused silica spheres show much deviation from the conventional theory for rigid spheres forkalarger than approximately four.

ISSN:0001-4966    

DOI:10.1121/1.382264

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

The interference spectrum between direct and ground‐reflected sound waves is strongly influenced by fluctuations in their phase and amplitude induced by propagation through turbulence. Measurements of the interference spectrum at distances up to 45 m over a flat asphalt surface were shown previously to be in satisfactory agreement with a spherical wave theory that allowed for partial covariance between the phases of the interfering waves. The amount of covariance was claculated from simultaneous measurements of wind and temperature fluctuations in the atmosphere. This theory has now been extended to include propagation over ground of finite acoustic impedance and provides an explanation for measurements of jet noise over grass to a distance of 1100 m. Above 300 Hz the sound levels are up to 10 dB higher than predicted by coherent acoustic theory. The meteorological parameters were used as constants adjusted for best fit and agreement is obtained by assuming standard deviations in phase velocity of about 1 in 1000, and a correlation length of about 1 m—typical of values measured in Ottawa. Covariance of sound amplitude, being relatively more important at long distances, is now also included—at short distances its effect is negligible compared with that of phase convariance. Phase‐amplitude convariance is negligible at all distances.

ISSN:0001-4966    

DOI:10.1121/1.382265

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Acoustic velocimeters for fluids traditionally assume a straight ray between transmitter and receiver, and use acoustic phase velocity for calculating fluid speed or velocity from upstream and downstream transit times. Corrected ray theory which uses group or signal velocity yields a relation between fluid velocity and differential transit time which differs from the traditional expressions. Fluid velocities calculated from differential transit times in the range 0.1–50 μs over a short path (5 cm) in air differ by as much as 1.4% using the traditional and corrected ray methods.

ISSN:0001-4966    

DOI:10.1121/1.382266

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Horizontal bending of long range underwater sound rays can be caused by repeated reflection from a sloping sea bed, and in a previous paper the author used ray invariants to calculate the horizontal projection of ray paths analytically. At low frequencies ray‐mode equivalence imposes restrictions on the initial ray elevation angles, which in turn cause shadowing in the horizontal plane. Analytical ray paths and shadow zone boundaries are derived here in a simple way for some bottom topographies of interest including a wedge, a ridge, and a seamount. Horizontal shadow zones have important implications for hydrophones near the edge of the continental shelf because the shadows are well developed in shallow water and may easily extend into deep water.

ISSN:0001-4966    

DOI:10.1121/1.382267

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Measurements of the stability of an underwater acoustic channel were made at long range (500 km) and relatively low acoustic frequency (206 Hz). The source depth and depth of the two receiving hydrophones were carefully chosen to limit the number of totally refracted acoustic paths. In addition, a discrete spectrum coded waveform was transmitted to allow time domain resolution of refracted and reflected arrivals. The results show considerably less channel time spreading than is predicted by recent theoretical models, and indicate that coherence bandwidths of 100 Hz are obtainable with proper source and receiver geometry. Received pulses wander systematically with changes in acoustic path length strongly influenced by energy at the frequency of the semidiurnal tide. The spatial coherence of signals propagated to two receivers separated by 250 m indicates that acoustic paths are similarly modulated by ocean processes with frequencies below the Brunt–Väisälä internal wave cut‐off. Above the Brunt–Väisälä frequency, coherence is small indicating that processes with space scales much less than 250 m are responsible for path length fluctuations.

ISSN:0001-4966    

DOI:10.1121/1.382268

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP