摘要:

System models consisting of linear partial differential equations which have been put into variational form, can be solved using the finite element method. The construction and use of a special class of one‐ dimensional finite elements, consisting of multiple convolutions of the unit pulse function, is described in this paper. As an example of how to use finite elements, the short circuit and open circuit normal modes of a radially polarized piezoelectric cylindrical shell are obtained. The computional procedures are shown, with special emphasis on how the system matrix equations can beconstructedas well assolvedby the computer. The finite element results are compared to exact values computed by classical methods. Convergence properties with respect to the order of finite elements used and the fineness of the grid structure are examined. Good results are observed for relatively coarse grids with second‐ or third‐order elements.

ISSN:0001-4966    

DOI:10.1121/1.381264

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

It is suggested that the damping affecting the forced radial oscillations of gas bubbles can be determined very accurately through the measurement of the subharmonic threshold for the oscillations. The advantage of the method is that, while the subharmonic oscillations have a large amplitude and can be easily detected, the theory can be accurately based on a perturbation approach. The theoretical basis for the subharmonic generation is also reviewed and its results illustrated for the case of an air bubble in water.

ISSN:0001-4966    

DOI:10.1121/1.381273

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A linearized theory of the forced radial oscillations of a gas bubble in a liquid is presented. Particular attention is devoted to the thermal effects. It is shown that both the effective polytropic exponent and the thermal damping constant are strongly dependent on the driving frequency. This dependence is illustrated with the aid of graphs and numerical tables which are applicable to any noncondensing gas–liquid combination. The particular case of an air bubble in water is also considered in detail.

ISSN:0001-4966    

DOI:10.1121/1.381252

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

We used realistic models of upper‐atmospheric winds in a three‐dimensional acoustic ray‐tracing program to calculate how much wind refraction alters acoustic transit speed and azimuth of arrival for long‐distance paths. Transit speed varied by 20% or more, and bearing deviations of up to ten degrees were found, depending on the season and on the direction the waves travel. On the average, the predicted seasonal trends corresponded to osbervations, but the calculated standard deviations exceed the mean seasonal values, making simple corrections useless in individual cases. Furthermore, it is doubtful that variations of horizontally uniform winds alone can account for the large observed variability in refractive effects, specifically the large differences in bearing error at adjacent observatories. We show that realistic horizontal gradients of either temperature or wind could cause as much azimuthal refraction as our height‐dependent wind models.

ISSN:0001-4966    

DOI:10.1121/1.381263

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

In acoustic measurements such as those conducted outdoors for static jet‐engine noise research, aircraft‐flyover noise certification, and motor‐vehicle regulation, ground reflections create multipath conditions characterized by different signal arrival times which cause signal reinforcements and cancellations at the microphone. This generates undesirable effects on the measured spectra. Cepstral techniques, developed in speech processing to identify echo delay times, have been applied to the ground‐reflection problem. Results indicate that cepstral techniques can produce a spectrum due to intensity addition only of the direct and reflected signals.

ISSN:0001-4966    

DOI:10.1121/1.381265

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Observations of sound attenuation in the out of doors have shown an attenuation factor approximately dependent on the five‐fourths power of frequency, rather than the square. Both power laws have been applied to calculations of yield‐scalable pressure signatures from explosions to allow comparison of results with measurements of explosion‐wave compression rise times. It appears that the five‐fourths law better explains the long rise times observed, but there are still serious underpredictions. Nevertheless, this model has been applied to the problem of determining the requirements for pressure‐gauge frequency response. At the low overpressures of concern in environmental monitoring, it appears that a 1‐kHz instrument response is more than adequate for recording explosion waves.

ISSN:0001-4966    

DOI:10.1121/1.381266

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

A two‐dimensional mathematical model for estimating the scattering of plane acoustic waves from an infinite, uniform, plane grating of compliant, elliptically, cylindric shells is developed. Calculations from the model are found to be in good agreement with insertion loss measurements of experimental arrays. The array is shown to be analogous to a multituned acoustic‐band rejection filter that has the shell’s fundamental flexural mode in the array for its center frequency. Low‐frequency approximations are also included. Conditions for an ultrawide resonance bandwidth having very high array resonance reflectivity are considered and an alternative application for compliant tube gratings, as an acoustic window that provides mechanical shading for a sonar receiving array, is discussed.

ISSN:0001-4966    

DOI:10.1121/1.381267

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

An outline showing the equivalence of several common expressions for array gain is developed and the relationship between the various correlation coefficients and other parameters used to describe array gain is shown. The signal constraints under which the relationships are valid are indicated. Based on the model developed by Chernov, two experimental methods used to determine patch size are described. The relationships between the array gain parameters and model parameters are indicated and three sets of data collected south of Bermuda are compared.

ISSN:0001-4966    

DOI:10.1121/1.381268

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Calculations were performed using the normal‐mode solutions for the parabolic profile to determine the behavior of the acoustic field when either the axial or maximum profile sound speed was altered slightly from an unperturbed value. It was found that incrementing either the axial or maximum profile speed by small amounts produced a change in the focal length of the parabolic profile, which in turn caused a range‐dependent compression or expansion of the unperturbed acoustic field with no noticeable distortion for frequencies less than 400 Hz and range less than 550 km. Approximating the effect of the first‐mode semidiurnal internal tide by appropriate variations in axial velocity, this fact was used to show that for cw propagation between fixed source and fixed receiver, the fluctuation in acoustic amplitude produced by the internal tide can be obtained by sampling the amplitude function for the unperturbed profile a distance Δreither side of the range in question, where Δris the distance the unperturbed acoustic field is shifted due to the change in axial speed produced by the internal tide. For a moving source, a range‐dependent critical source speed was determined such that for source speeds equal to or less than the critical speed, the effect of the internal tide predominates as a source of acoustic fluctuations, while for source speeds greater than this critical speed the effect of source motion is paramount.

ISSN:0001-4966    

DOI:10.1121/1.381269

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP

摘要:

Precision acousto‐optic transducer calibration requires an understanding of the influence of the nearfield of a transducer on the integrated optical effect. This influence has previously been studied theoretically for continuous waves from circular and rectangular plane piston transducers [F. Ingenito and B. Cook, J. Acoust. Soc. Am. 45, 572–577 (1969)]. For pulsed ultrasonic transducers, the nearfield can be considerably different than for continuous waves. The influence of the differences on the integrated acousto‐optic effect is analyzed by linear superposition. For a circular transducer with piston motion, it is possible to reduce the formulation with appropriate approximations and restrictions to a universal form. For long pulses, i.e., greater than ten wavelengths, the integrated acousto‐optic effect is the same as for continuous waves. For shorter pulses, the nearfield can influence the pulse shape, but not substantially. This influence is dependent on field position and can be minimized.

ISSN:0001-4966    

DOI:10.1121/1.381279

出版商:Acoustical Society of America    

年代:1977

数据来源: AIP