摘要:

The deformed cylinder method (DCM) is a very versatile model that can be used on a wide range of acoustic scattering problems. To date, it has been applied to uncoated axisymmetric bodies of circular cross section, whether they be rigid, elastic solids, or shells. In this article, the method is applied to three bodies that are covered to varying degrees by a viscoelastic absorbing layer. These bodies include a coated spheroidal shell, a spheroidal shell partially covered with an axisymmetric coated band, and a cylindrical shell with a lengthwise coated strip. For the coated spheroidal shell, the effects of varying coating thickness and coating loss on the scattering are examined. Results indicate that the coating has little effect below 2 kHz, but above this level, the target strength decreases with increase in either thickness or loss. At the higher frequencies, the analytical solution used in the DCM becomes unstable. To overcome this, a high‐frequency solution is obtained by combining the DCM together with the physical optics method and layered plate theory. This high‐ frequency solution can also be used in situations where the shape of a partial coating is of a form that prohibits the analytical solution from being used. This was the case with the cylindrical shell with the lengthwise strip.

ISSN:0001-4966    

DOI:10.1121/1.414491

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The matched asymptotic expansions method is applied to investigate the problem of diffraction by the edge of a three‐dimensional body at high frequencies. The present approach is an extension of a theory which was originally elaborated for plane screens [J. Acoust. Soc. Am.97, 796–806 (1995)]. The theoretical analysis provides an asymptotic solution which is a refinement of physical optics by the addition of two edge components as in the physical theory of diffraction. Explicit and simple approximate solutions are derived to calculate the edge line integrals leading to a practical integrative procedure that is well suited to complex curved edges and that can overcome some disadvantages of ray theories. Last, the problem of diffraction by a finite cylinder is analyzed and a comparison with other asymptotic methods is shown.

ISSN:0001-4966    

DOI:10.1121/1.414492

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Scattering of leaky Rayleigh waves from a flat fluid–solid interface is studied. The fluid half‐space is taken to be ideal and homogeneous while the solid half‐space has randomly inhomogeneous anisotropic elastic constants due to the microstructure of the material. For plane waves incident from the fluid onto the interface at the critical Rayleigh angle the singly scattered incoherent field is obtained by utilizing a first Born approximation. When the solid is a crystal aggregate and when the correlation function is of exponential form, the mean‐square scattered signal level is found to be inversely proportional to the dimensionless frequency in the geometrical (high‐frequency) limit but proportional to the third power of frequency in the Rayleigh (low‐frequency) limit. Numerical results are given for water–aluminum (cubic polycrystals) interface scattering.

ISSN:0001-4966    

DOI:10.1121/1.414493

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Near‐field acoustical holography (NAH) is used in the present work to measure the reflection coefficient of porous layers from normal to grazing incidence and for inhomogeneous waves. Measurements are compared with predictions obtained from a recent model. A lateral propagating mode inside the porous layer is evident.

ISSN:0001-4966    

DOI:10.1121/1.415222

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The finite element method is used to calculate the impulse response of an irregular room. Although only a finite number of degrees of freedom are used, there is sufficient information in the model to predict the early reflections from the walls to a source at any location within the room’s interior. The results of calculations, performed for a room with a curved roof, show how absorption material placed at strategic points on the walls can affect the temporal response.

ISSN:0001-4966    

DOI:10.1121/1.414494

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

This paper discusses a hybrid ray–mode (wavefront–resonance) approach to analyze wave radiation and scattering by fluid loaded targets with internal structures. The approach consists of the following three methods: (1) coupling of plate and shell modes at joints and junctions; (2) spectral approaches (such as ray asymptotics, collective rays, guided modes, resonances, ray modes, etc.) for separable and weakly nonseparable structures; (3) a combination of methods (1) and (2) for nonseparable structures. The general theory is applied to a prototype structure of revolution consisting of a cylindrical pipe, hemispherical endcaps, a bulkhead, and a rib. First a conventional surface ray–normal mode approach is applied to the normal coordinate of each shell element, reducing the submerged structure into an equivalent multilayer–multiwave medium in the lateral domain. A matrix Green’s‐function formulation is then employed to systematically synthesize the acoustic radiation or scattering returns in terms of angular spectra of surface modes of the structural elements. This allows efficient bookkeeping of various spectral objects such as ray, collective ray, modes, ray–mode, resonance, etc., to be maintained.

ISSN:0001-4966    

DOI:10.1121/1.414495

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

An acoustic system, defined as a liquid‐filled piezoceramic shell with an asymmetric internal insertion, is considered. The mathematical model is formulated. The problem of sound radiation by such a system is solved using the partial domains method. The acoustic characteristics of the system for a number of insertions with different configurations and acoustic characteristics of surfaces are studied extensively. The analysis of the obtained quantitative data allows the following conclusions. When exciting the piezoceramic shell in the zero (pulse) mode, redistribution of energy among various circumferential modes of vibration of the shell occurs through the asymmetric disposition of the insertions. This fact sets up the conditions for the occurrence of specific low‐frequency resonances of the system, absent in the eigenfrequency spectrum of the shell in vacuum.

ISSN:0001-4966    

DOI:10.1121/1.414496

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Data from an underwater continuous wave signal are analyzed using methods derived from the study of chaotic systems. Distortions by the environment effectively add three degrees of freedom (dimensions) to the signal. The dimension required to reconstruct and analyze the received data is five in contrast to a pure tonal or sine wave that only requires two dimensions. The data are examined with a chaotic data analysis toolkit that includes determination of degrees of freedom using local and global false‐nearest‐neighbor statistics, average mutual information, correlation dimension, and local Lyapunov exponents. These results are important for the development of methods for correction of propagation distortions and nonlinear noise reduction algorithms. Application of this knowledge should lead to improvements in the detection and classification of underwater signals.

ISSN:0001-4966    

DOI:10.1121/1.414497

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Previous impedance models for grounds whose porosities vary exponentially with depth have been based on the microstructural theory of sound propagation in rigid porous materials. These models were derived by incorporating the spatial dependence of the complex wave number on the porosity into the wave equation. The predicted impedances do not behave as expected and produce nonphysical results for porosities which decrease with depth. In this paper the change in porosity is incorporated into the equivalent complex medium Euler equation and continuity equation and a new wave equation is derived. The solution to this wave equation produces physical meaningful results for exponentially decreasing porosity (the case for most naturally occurring ground surfaces) and produces well‐behaved results for exponentially increasing porosity.

ISSN:0001-4966    

DOI:10.1121/1.414498

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The present investigation is concerned with the sound‐production mechanism of primary small plate tones (S1 tones) generated by the impingement of a choked jet on normal flat plates with a diameter equal to the nozzle exit diameter. The power spectra associated with S1 tones typically contain a fundamental tone and many harmonics. These tones occur when the plate is located in a particular region of the jet. Photographic evidence indicates that S1 tones occur when a Mach disk cuts off a portion of the first conical shock wave leaving a frustum and a standoff shock wave forms in front of the plate. Timed schlieren photographs indicate that sound is generated when the jet boundary near the standoff shock wave contracts suddenly. This occurs as the first shock wave moves downstream and begins to interact with the standoff shock wave. The resulting sound appears to be somewhat impulsive. Reflector tests indicate that the tones may not be part of a feedback loop to the nozzle. Simplified calculations show that once the first shock wave is set into motion, its continued movement downstream is due to an increase in the compression of the upstream fluid.

ISSN:0001-4966    

DOI:10.1121/1.414499

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP