摘要:

The characteristics of an insonified object are generally present in the signal carried by the returning scattered wave. Because an echo contains only a partial result of the wave interaction with a material structure, the inverse acoustical scattering process is an ill‐posed mathematical problem that may not have unique solutions. However, by decomposing the normalized echo signature (the form function), certain features that appear in the joint time and frequency domains can be exploited to relate the scatterer's acoustical signature to its structural response. The approach of the form function decomposition is based on the frequency domain Wigner distribution function formulation. A computer algorithm extended from image processing techniques is then implemented to depict the numerical results in a two.dimensional (tvs ω) display. From the interpretation and understanding of scattering phenomenon, the physical parameters of the scatterer can be extracted from the graphical representation. Examples from numerically simulated models and laboratory measurements are illustrated for estimating the general physical size and the corresponding elastic properties of the insonified object.

ISSN:0001-4966    

DOI:10.1121/1.2028214

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The problem of the scattering of acoustic waves by a large, sound soft (hard), corrugated surface in two dimensions is addressed. The surface undulates periodically up to a characteristic lengthLbeyond which it becomes planar. The height of the corrugation is measured by a characteristic length a and its period by Λ. The ordering of these scales is taken to be λ ∼ Λ ∼a⋘L, where λ is the wavelength of the incident plane acoustic wave. The method of matched asymptotic expansions is applied to analyze this problem in the limit as ε =a/L→0. This approach is both mathematically systematic and physically intuitive. The farfield results are identical to those obtained by using a finite beam approximation for a sound hard surface in two dimensions and almost the same for a sound soft case; the only difference being a sine factor that yields correct boundary behavior. Results are derived for the three‐dimensional scattering problems and these compare similarly. [Work supported by NSF and AFOSR.]

ISSN:0001-4966    

DOI:10.1121/1.2028217

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The random vibrations of a truss were minimized analytically by placing viscous dashpots where they would be most effective. It was assumed that there was a limited amount of damping available and the question was, where can the damping be used most effectively? Dashpots were placed parallel to the truss members, which deform only axially. The problem was developed as a nonlinear optimization problem. The response of the structure is represented in terms of its complex normal modes. The sensitivities of the individual modal natural frequencies, loss factors, and complex eigenvectors were obtained for nonproportional damping. The excitation was white noise applied as concentrated forces at nodal points. The mean‐square displacement was minimized using a modification of the method of steepest descent. An example is given for a 10‐bar truss. The problem was also formulated for the optimization programCONMINand the results are summarized for several constraint situations. The work is related to a previous paper dealing with the minimization of settling time of free vibrations by maximizing the energy dissipation rate [V. H. Neubert, Proc. DAMPING89, Int. Symp., W. Palm Beach. FL, February 1989].

ISSN:0001-4966    

DOI:10.1121/1.2028226

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The potential of the SAW devices as sensors has not been fully exploited. New SAW sensors are developed to sense the shear stress and the normal stress in turbulent flows. These devices are extremely sensitive to changes in the environment. The surface wave velocity is hence changed when forces are applied on the sensor. The direction of the surface wave can be changed and by suitable manipulation, the effect of each stress can be isolated. Experiments are conducted with a low‐frequency SAW oscillator sensor and the frequency shift is measured. The values obtained agree with the expected values for the experimental setup. As the frequency of operation is increased, the sensors become smaller and the values can be determined more accurately eliminating the effect of spatial variation of the stresses.

ISSN:0001-4966    

DOI:10.1121/1.2028229

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Two optical fibers, comprising the legs of a Michelson interferometer, are cast into a long rod (1.33‐cm diameter, 35. 1‐cm length) of epoxy at off‐axis coplanar distances of 5.3 _+ 0. I min. The free‐free bar is electodynamically excited in its gravest flexural, torsional, and longitudinal modes. The absolute displacement of the bar end for each mode was measured using the MTI fotonic vibration sensor and compared with the change in optical path length induced in the interferometer. At the fundamental flexural resonance of the bar (158.5 Hz), the correlation between measured transverse peak displacement (15.4 μm) and the measured number of strain‐induced optical fringes (14.5 at a wavelength of 817 nm) was within 7% of theoretical predictions. No fringes were observed for the torsional and longitudinal modes, as expected, since those modes produce equal strain in each leg of the interferometer. Experimental values and techniques used to obtain data for the complex Young's and shear modulus for the elastomeric casting material (Stycast 1266) will also be presented.

ISSN:0001-4966    

DOI:10.1121/1.2028236

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

For speakers of a tone language, a close functional association exists between segmental structure andF0contour (i.e., tone) in speech because both dimensions are needed to identify words. Using the speeded classification paradigm, which does not require lexical access, the hypothesis that segmental and tonal dimensions are perceptually more strongly integrated for speakers of a tone language (Mandarin Chinese) than for speakers of a nontone language (English) was examined. In four classification tasks, requiring attention to one dimension (either segmental or tonal) of CV syllables while ignoring the other,bothsubject groups showed strong interference from orthogonal variation in the unattended dimension. The Chinese subjects showed significantly more interference than the English subjects in only one of the four tasks (vowel classification with irrelevant tonal variation). These findings thus provide only weak evidence of differences between Chinese and English speakers in the perceptual integrality of segments and tones. [Work supported by NICHD.]

ISSN:0001-4966    

DOI:10.1121/1.2028239

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Recently short‐time waveform analysis has been used to analyze, manipulate, and synthesize speech [e.g., Lienard, ICASSP‐87]. Each waveform is described by six parameters: envelope attack and decay, reference instant, energy, internal frequency, and phase. In order to better understand the parameters, and to determine which carry pitch information in waveform‐analyzed speech, perceptual experiments were performed using synthetic stimuli of 300 ms (formed by repeating 10‐ms waveforms). The first experiments, with ten normal‐hearing subjects, verified that the parameters affecting the perceived pitch were internal frequency, offset (repetition interval), and change in phase in successive waveforms; the remaining parameters primarily affected timbre. Next, the parameters found to affect pitch were varied individually and jointly to explore their interaction. These experiments showed that both frequency and offset could dominate perceived pitch. In the region explored, 250–500 Hz (ABX test), when either frequency or offset was 500 Hz, variation of the other parameter did not change the response from 500 Hz. However, when either parameter was fixed at 250 Hz, varying the other changed the pitch percept. Varying the two parameters together, the response changed from 250–500 Hz at the offset frequency closest to the perceptual midpoint between the two references, even though the internal frequency was higher. Finally, the parameters were varied in opposition in order to determine whether a particular parameter dominates.

ISSN:0001-4966    

DOI:10.1121/1.2028242

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A previous presentation [P. T. Chen and J. H. Ginsberg, J. Acoust. Soc. Am. Suppl. 185, S139 (1989)] outlined a general procedure by which variational principles may be employed to analyze the surface pressure and structural response of a shell in the shape of a surface of revolution. The displacement in this treatment may be described directly as a set of admissible, linearly independent, but otherwise arbitrary, basis functions, in conjunction with a comparable description of the surface pressure. An alternative is to perform an intermediate analysis to determine thein‐vacuodisplacement modes, and then to truncate the modal series based on a comparison of the eigenfrequencies and the frequency of excitation. This paper assesses the ability of these alternative strategies to treat the response of a prolate spheroidal shell. A matter of specific concern is the situation where proximity of two natural frequencies produces high‐frequencyin‐vacuomodes whose localized displacement patterns suggest that they might be efficient radiators for low‐frequency excitation in the fluid‐loaded case. [Work supported by the Office of Naval Research, Code 1132‐SM.]

ISSN:0001-4966    

DOI:10.1121/1.2028244

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Cylindrical shells by the nature of their curvature exhibit coupling of extensional and flexural wave fields. This coupling produces unique characteristics when the shell is dynamically excited and subjected to acoustic radiation loading due to the interaction with a surrounding infinite fluid field. This paper outlines the governing equations of motion for the cylindrical structure and surrounding medium. The results of the coupled equations are presented in terms of acoustic power transmission. The acoustic power quantifies radiation loss, structural loss, and input power. Results are presented that describe the radiation efficiency of cylindrical shells under radial, axial, and circumferential point excitation. Parametric studies of structural loss, circumferential stiffening, and directional excitation are examined relative to the radiated acoustic power created in the farfield. A brief comparison of the theoretical calculation with an experimental result is presented as a validation benchmark for the theoretical calculations.

ISSN:0001-4966    

DOI:10.1121/1.2028247

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

From the classical theory of plane curves applied to two‐dimensional wave propagation in uniform media, it is known that the wave fronts are involutes of caustics or virtual caustics. (The involute of a curve C is constructed by the unwinding of a taut thread tangent to C.) in the present research, the involute construction is applied to wave fronts radiated into water by surface guided elastic waves (SEW) on scatterers having a uniform circular profile. Examples of SEW include circumferential leaky Rayleigh or Lamb waves on solid or hollow circular cylinders, respectively, with an outer radius =a. The outgoing wave appears to radiate from a virtual caustic that is a circle of radiussl=ac/cl, whereclis the phase velocity along the surface of the SEW andcis the sound speed in water. Consequently, the radiated wave fronts are the portion withr>aof the involute of a circle of radiussl. When the SEW are only weakly dispersive, radiated bursts imaged by schlieren photograph [W. G. Neubauer, J. Acoust. Soc. Am.45, 1134–1144 (1969)] approximate the location of wave fronts. In the present research such images are found to be involutes of circles of radiussl. Applications to scattering from spherical shells and to the localization principle were also investigated. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2028255

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP