摘要:

Imaging in a scanning photoacoustic microscope (SPAM) is analyzed using Fourier imaging concepts. In particular, the spatial frequency response and the resolution of the instrument are examined. It is shown that in the general case, the imaging is object dependent. However, for the special cases of negligible thermal diffusion or where the specimen is either optically uniform or acoustically and thermally uniform, explicit expressions for the spatial frequency response are derived. It is shown that a SPAM may exhibit superresolution provided an acoustic lens is used rather than a large area detector.

ISSN:0001-4966    

DOI:10.1121/1.391150

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The performance of elements in transducer arrays is evaluated, at various stages of construction, using an interferometric optical probe that is capable of measuring acoustic displacements of the order of 3 × 10−4Å. The vertical displacement is first measured on a single piezoelectric strip without matching layers or backing, and is compared with variational calculations of the surface displacement for both the open‐ and short‐circuit modes. Good agreement is obtained between experiment and theory. The measurement is repeated after a matching layer is added, then after a backing is added, and finally with the array water loaded after final completion. Our experimental results are in excellent agreement with theory, where theoretical predictions are available. Laser probe measurements are used to evaluate the cross‐coupling between array elements and predict the farfield illumination pattern of the array elements.

ISSN:0001-4966    

DOI:10.1121/1.391593

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A preliminary study using horizontal shear waves (SHwaves) in a bonded plate to estimate the bonding strength of adhesive is presented in this paper. The essence of the method is based on the observation that the characteristics ofSHwaves in a bonded plate are dependent upon the bonding stiffness of the adhesive; thus, an estimation of the adhering strength can be made by observing the behavior change of these wave motions. An adhesive can, in general, by modeled as a viscoelastic material. The mechanical properties determined by subjecting the material to a pure shear deformation are therefore the most pertinent properties that characterize the cohesive strength of the adhesive. AnSHwave produces a pure shear deformation in the adhesive layer in a direct manner. In this study, a long aluminum strip is bonded to the surface of a large aluminum block. AnSHwave is imparted to one end of the strip. The shear wave motions in the strip are periodically monitored at sequential times as the adhesive cures. The results indicate that (1) there are two measurable modes ofSHwaves in the strip during the early stage of adhesive curing and (2) the amplitude of the second modeSHwave decreases as the adhesive cures, finally disappearing after the adhesive curing process is completed. This study has successfully shown that the bonding stiffness of adhesives can be estimated by observing the amplitude change of the second modeSH‐wave motions in the adhered plate.

ISSN:0001-4966    

DOI:10.1121/1.391594

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A synoptic expendable bathythermograph survey of the warm‐core Gulf Stream ring 81‐D, conducted in September 1981 when the ring was located 550 km south of Halifax, Nova Scotia, provided the basis for a detailed study of the ring’s sound‐speed field. Salinities used for computing sound speed were derived from nonsynoptic conductivity–temperature–depth data collected during the survey. Analysis of the sound‐speed field, using vertical and horizontal sections and satellite imagery, revealed the ring’s typical high sound‐speed core, but also showed increased horizontal and vertical gradients along the ring’s western margin as compared to its eastern side. This asymmetry appears to be related to a surface streamer of warm, higher sound‐speed Gulf Stream water overlying colder, lower sound‐speed slope water, resulting in a subsurface sound duct centered 100 m below the surface along the ring’s western margin. The subsurface duct and increased vertical gradient resulted in 30–50 dB greater loss at depths of 100 and 200 m in the ring’s eastern margin as compared to its western side and center. Strong ray channeling and refraction of a 2000‐Hz source at a depth of 100 m was also noted. Based on these observations and predictions, warm‐core rings may possess regions of increased acoustic range dependence due to the interactions of waters along their margins.

ISSN:0001-4966    

DOI:10.1121/1.391595

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The vertical directionality of ambient low‐frequency (shipping) noise at midlatitudes has been measured by Wales and Diachok [S. C. Wales and O. I. Diachok, J. Acoust. Soc. Am.70, 577–582 (1981)] in the Atlantic and by Williams and Fisher (R. B. Williams, personal communication. The measurements were taken in October 1979) in the Pacific. At the sound axis the angular distribution in intensity is flat from 0° (horizontal) up to the inclination of the surface‐limited rays. The intensity falls off sharply for steeper rays (which are associated with lossy bottom reflections). This observed flat angular distribution is in contrast with the bimodal distribution (peaked near the inclinations of surface‐limited rays and zero for flat rays) which is expected from surface sources in a range‐independent environment. A flat angular distribution is equivalent to an equal partition of energy among modes; once such equipartition is attained, the distribution remains unaltered with propagation (or scattering) in a lossless channel. The simplest model consists of surface sources generating rays of steep axial inclination, which are then scattered into flat rays by internal waves throughout the ocean volume. The model fails because of the weakness of the scatterers, requiring 104km for equipartition. There are two regions for the noise energy to couple directly into flat (near‐axial) rays: (i) at high latitudes where the rising sound axis intersects the surface, and (ii) at the continental slope where the sound axis intersects the rising bottom. The second model consists of sources in the high latitude duct (where the sound axis is essentially at the surface), with energy broadly partitioned among modes from the very start (dipole sources in a linear sound speed gradient give precisely equipartition). Subsequently, the energy is ducted downward along the axis into midlatitudes, preserving equipartition. This model fails because of the low density of shipping at high latitudes. In the third model, the steeply descending rays from coastal shipping are bounced into flat near‐axial rays over the continental slope, as proposed by Wales and Diachok. Although there is some mystery as to how this will lead to the observed high degree of mode equipartition, we conclude (reluctantly) that reflections at the continental slope are probably the dominant mechanism for getting energy into the flat rays.

ISSN:0001-4966    

DOI:10.1121/1.391596

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

This paper described a theoretical investigation into the variations of the specific heats of airCpandCv, and their ratio γ, with relative humidity. The investigation is based on published equations forCp, and the predicted values are derived from theoretical and experimental thermodynamic data on the constituents of humid air. For acoustical applications, such as the determination of the variation of the speed of sound in humid air, the perturbation of the term γ/M̄ with relative humidity is also presented.

ISSN:0001-4966    

DOI:10.1121/1.391597

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A formulation is derived in Eulerian description governing finite amplitude compressible motion of uniaxial strain in an elastic‐plastic locking half‐space under an abrupt loading–unloading blast. For the material flow during the deformation phase, a wave characteristics formulation is presented valid for large strains but restricted to unloading stages which are moderate in comparison to linear elasticity. A method is also presented by which the permanent strain boundary is located in the space–time solution domain upon the termination of the unloading process. Finally, a scheme is given for the determination of the rigid body motion that takes place behind the permanent strain boundary.

ISSN:0001-4966    

DOI:10.1121/1.391598

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A computational method is developed for the wave characteristics formulation governing transient nonlinear deformations of locking solids where waves of a moderate unloading process overtake a shocked precursor wave of large uniaxial strain. The unique feature of the proposed all‐event integration scheme is that in order to evaluate the unknowns at a space–time point, not only past events are considered, but also present events surrounding the point in question are accounted for. Moreover, the solution by the proposed method propagates along all existing waves inside a variable space–time grid formed by the bicharacteristic curves associated with the chord‐shock speed of the leading wave (precursor). Based on these features, the method is essentially analytical where in its final stage of development a straightforward numerical integration is required which is free of iteration procedures, stability criteria, and of any other numerical analysis.

ISSN:0001-4966    

DOI:10.1121/1.391599

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Part III deals with the nonlinear transient response of an elastic‐plastic locking half‐space traversed by converging waves of uniaxial large strain generated as a result of a uniform loading–unloading blast suddenly applied with no rise time to the entire surface of the half‐space. For this type of boundary‐value problem where a shocked leading wave interacts continuously with faster overtaking unloading waves, a computational method was developed based on the theory of characteristics. This method, which employs an all‐events integration process propagating along the existing wavelets, is essentially analytical. In its final stage of the solution, a straightforward numerical integration is required free of iteration procedures, stability criteria, and any other numerical analysis. The resultant response of the half‐space is described and interpreted here.

ISSN:0001-4966    

DOI:10.1121/1.391600

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Scattering by a layered elastic obstacle submerged in a fluid is considered. The null field approach (T‐matrix method) is used, but the method is modified so that all surface fields can be expanded in spherical harmonics. The resulting method is then compared with the conventional null field approach (which uses expansions in regular wavefunctions), and it is concluded that the present method is the better one in general. It is, however, doubtful if the null field approach is applicable to cases where there is no sphere that circumscribes the core and inscribes the outer surface. Some numerical results are given for a layered spheroid and superspheroid.

ISSN:0001-4966    

DOI:10.1121/1.391154

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP