摘要:

Resolution in the scanning acoustic microscope is determined by wavelength which is in turn limited by attenuation in the acoustic medium. In order to make use of the low attenuation found in cryogenic liquids we have developed an acoustic microscope suited for use at low temperatures. In this paper we present images taken in liquid argon held at 85°K and in superfluid helium at 1.95°K. In liquid argon, wavelengths as short as 0.43 μm were achieved while in preliminary work with superfluid helium a wavelength of 0.36 μm was used. In order to operate in liquid helium, it was necessary to improve the power transfer from the acoustic lens to liquid helium by using double quarter‐wave matching layers. Techniques of fabricating and testing these layers are described. Finally, prospects for operating at still shorter wavelengths in superfluid helium held at temperatures below 0.5°K are discussed.

ISSN:0001-4966    

DOI:10.1121/1.384285

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A new type of algorithm based on a modified least‐square method has been developed to analyze ultrasonic absorption for a single relaxation. Compared with the usual heuristic methods, this technique is a more direct one, which enables us to determine with high accuracy the relaxation parameters from a single initial estimated value of the relaxation frequencyfc. Computer simulation of the α/f2vsfcurves are presented for various conditions. This method is satisfactorily applied to the ultrasonic absorption of propionic acid over a frequency range from 2.8 to 95 MHz.

ISSN:0001-4966    

DOI:10.1121/1.384286

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Theoretical calculations of the backscattering of a plane sound wave by a rib stiffened Timoshenko–Mindlin plate are presented. An exact solution is compared with two saddle point integral approximations. One saddle point integral approximation takes into account the near coalescence of the leaky wave pole and the saddle point. Mass loading of the rib and different rib constraints are discussed. The range of validity of the model is also given.

ISSN:0001-4966    

DOI:10.1121/1.384287

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Theoretical calculations of the backscattering of a plane sound wave by a rib stiffened Timoshenko–Mindlin plate are presented. Equations are derived for the calculation of the influence ofNarbitrarily situated, arbitrarily loaded ribs on the amplitude of the reflected wave. Calculated results for two, nine, ten, and an infinite number of equally spaced ribs are given.

ISSN:0001-4966    

DOI:10.1121/1.384288

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A deterministic bandpass signalx(t) bandlimited to ‖ ω ‖ ε I1≡(ω0−σ/2, ω0+σ/2) can be represented in the formx(t)=p1(t) cosω0′t−q1(t) sinω0′t, wherep1,q1are low‐pass (the ’’in phase’’ and ’’quadrature’’) components bandlimited to ‖ ω ‖?σ/2+‖ ω0−ω0′ ‖ and ω0′ is an arbitrary frequency within the bandI1. Quadrature sampling has as its aim the recovery of the low‐pass componentsp1,q1directly from samples of both the bandpass signalx(t) and its quarter‐wavelength translationx(t−π/2ω0′), the samples being taken at a low‐pass rate. Grace and Pitt [J. Acoust. Soc. Am. 44, 1453 (1968)] obtained a result for the case ω0′=ω0?σ requiring an overall oraveragesampling rate of (σ/π)(γ/[γ]) samples/s, where γ=ω0/σ and [⋅] denotes the greatest integer function. By reducing the problem to an application of the classical Shannon sampling theorem in a special case (2ω0/σ =integer) and then proceeding to the general case via an embedding technique, we arrive here at an improved average sampling rate of (2γ+1/[2γ+1])(σ/π) samples/s. The method, assuming only ω0?σ/2 and finite energy signals, provides a simpler and more accessible treatment of the quadrature sampling problem than the earlier techniques of Grace and Pitt, Kohlenberg [J. Appl. Phys. 24, 1432 (1953)], and the author [IEEE Trans. Aerosp. Electron. Syst. AES‐15, 366 (1979)].

ISSN:0001-4966    

DOI:10.1121/1.384289

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The dynamic history of narrow band ’’line’’ components in spectrograms has customarily been characterized by bandwidth variations and changes in the center frequency of the ’’line.’’ Analysis techniques are described that enable ’’instantaneous’’ amplitude and frequency estimates to be obtained. Characterization by the new techniques describes significant spectral features in terms more closely related intuitively to the mechanisms causing them and helps visual identification of correlated effects in a frequency versus time spectrogram display. Spectrograms obtained with the new techniques are shown to illustrate the improvements obtainable. The term ’’microanalysis’’ is suggested here to describe the new techniques, since they allow feature resolution to be improved in the time and frequency domains simultaneously for sufficiently strong signals.

ISSN:0001-4966    

DOI:10.1121/1.384290

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A seismic processing technique has been developed to remotely determine the effective layer velocity and layer depth of a multilayered ocean floor. The computer algorithm making up the seismic signal processor is based on the Kirchhoff diffraction integral and is referred to as the inverse diffraction processor (IDP). An iterative solution for the effective interval velocity and layer depth is obtained with the use of the Fourier transform. The required input data to the processor are the measured magnitude of the reflected acoustic signal recorded by an array of hydrophones located on the ocean surface. The performance of the seismic processor has been evaluated using synthetic seismic data. This insures that the actual parameters being solved for are known, thus the accuracy and resolution of the processor can be ascertained.

ISSN:0001-4966    

DOI:10.1121/1.384291

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The theory and experimental results for a new type of surface acoustic wave phase shifter are described. The device consists of three interdigital transducers, including one with sine‐shaped apodization. The phase shifter produces an arbitrary constant phase difference, and has two operation frequency ranges with 180° out of phase, separated at the center frequency of the device.

ISSN:0001-4966    

DOI:10.1121/1.384292

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A review of Békésy’s experiments on postmortem preparations shows the fundamental features of cochlear macromechanics. Early mathematical theory was able to explain Békésy’s observations in terms of the hydrodynamics of surface waves and to account for some experimental artifacts. More recent experimentsinvivohave shown that the cochlear frequency analysis is sharper than observed by Békésy and may occur in two interacting stages. It is suggested that the second stage may result in part from a resonance of hair‐cell stereocilia loaded by the mass of the tectorial membrane, and it is calculated that the damping is small enough to allow such a resonance. The resonance would make the basilar membrane with the organ of Corti appear as a fourth‐ rather than a second‐order system.

ISSN:0001-4966    

DOI:10.1121/1.384294

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The sensory cilia of the mammalian cochlea show an orderly gradation in height along the length of the cochlea. The cilia are taller in the apex and shorter in the base. Differences in ciliary gradation also exist between the different rows of outer hair cell cilia along the length of the cochlea. There are gradations in width and thickness in the moveable portion of the tectorial membrane paralleling those in the basilar membrane. There are also gradations in the relationship between the tectorial membrane and the slope of the reticular lamina along the length of the cochlea. This suggests that there may be additional mechanical fine‐tuning capability built into the organ of Corti besides the basilar membrane. The tectorial membrane is firmly attached to the outer hair cell cilia along the entire length of the cochlea in all species examined. The inner hair cell cilia do not have the same firm attachment to the tectorial membrane as outer hair cell cilia. This suggests that the modes of mechanical coupling between the tectorial membrane and the inner and outer hair cell cilia are different.

ISSN:0001-4966    

DOI:10.1121/1.384295

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP