摘要:

A compound lens exhibiting low spherical aberration over a wide field of view is described. The results of aberration analyses based on the wavefront deviation technique are presented and computed diffraction patterns are shown. The fabrication of a compound lens is described and experimental and theoretical results are compared at 700, 800, and 900 kHz.

ISSN:0001-4966    

DOI:10.1121/1.1912536

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The target‐strength characteristics of thin‐walled spheres filled with low‐sound‐velocity fluids have been investigated in the frequency range 100–225 kHz. Theoretical and experimental observations show that the target strength of fluid‐filled spheres is independent of orientation, can be optimized with proper choices of materials, and can be frequency independent. Comparisons of target strength are made with other well‐known passive reflectors.

ISSN:0001-4966    

DOI:10.1121/1.1912537

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The effect of surface irregularities upon the scattering performance of a composite rough surface that consists of scattering elements of structure sizes in quite different scale uniformly superimposed one on the other is described and analyzed. The theory is formulated on the basis of the principles of both geometrical optics and physical optics through the introduction of the surface‐slope distribution with a result clear in physical meaning and simple for analysis. Three factors determine the dominating role played by the component irregularities in wave scattering: surface slope, incident angle, and wavelength. An example showing the angular distribution of wave scattering by a composite rough surface at frequencies of 1 and 100 kHz about 7 oct apart is presented. At low frequency, the backscattering is confined to a narrow region of about 10° centered at the direction of normal incidence, while at high frequency, the angular distribution expands to about 150°. As for the bistatic scattering the angular distribution of the scattered acoustic wave increases from about 25° centered at specular reflection to 180°. This effect of the small‐scale surface irregularities is to broaden the angular distribution of the scattered acoustic wave at high frequency.

ISSN:0001-4966    

DOI:10.1121/1.1912538

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The purpose of this paper is to present development of a mathematical model for the Class V flextensional underwater acoustic transducer. The transducer is approximated through the consideration of three distinct problems. A thin piezoelectric disk with an arbitrary impedance on its edge is solved in terms of Bessel functions. The shell vibration problem is solved using a finite‐difference model to approximate the shell. The acoustic radiation problem is solved by obtaining the source density distribution for a system of quadrilaterals representing the transducer. With the source density of each quadrilateral, the near‐ and farfield pressures and velocities can be found. Utilizing these three components, a model is then constructed for the transducer. A comparison of the results from the mathematical model is made with those obtained from experiments in order to validate the model.

ISSN:0001-4966    

DOI:10.1121/1.1912539

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Propagation loss expressions were derived by means of perturbation theory corrections to the normal‐mode series solution to the acoustical wave equation with parabolic sound‐speed profile and sinusoidal point source. A fourth‐order power series was used to approximate nonparabolic speed profiles with coefficients determined by a least‐squares fit. Perturbation corrections were made to second order for the wave functions and to third order for the eigenvalues. Calculations were made for two realistic speed profiles and an idealized nonparabolic speed profile. Local focusing of sound in a parabolic speed profile had been reported in previous papers [H. Überall and N. C. Nicholas, J. Acoust. Soc. Amer.44, 1259–1261 (1968); and N. C. Nicholas and H. Überall, J. Acoust. Soc. Amer.48, 745–753 (1970)], but no local focusing was found for the nonparabolic cases computed. Numerical results showed good agreement with the few available measurements and ray‐acoustics estimates of propagation loss.

ISSN:0001-4966    

DOI:10.1121/1.1912540

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

An approach is presented to compute the near‐ and farfield transient radiation resulting from a specified velocity motion of a piston or array of pistons in a rigid infinite baffle. The approach, which is based on a Green's function development, utilizes a transformation of coordinates to simplify the evaluation of the resultant surface integrals. A simple expression is developed for an impulse response function, which is the time‐dependent velocity potential at a spatial point resulting from an impulse velocity of a piston of any shape. The time‐dependent velocity potential and pressure for any piston velocity motion may then be computed by a convolution of the piston velocity with the appropriate impulse response. The response of an array may be computed using superposition. Several examples illustrating the usefulness of the approach are presented. The farfield time‐dependent radiation from a rectangular piston is discussed for both continuous and pulsed velocity conditions. For a pulsed velocity of time durationTit is shown that the pressure at several of the field points can consist oftwoseparate pulses of the same duration, whenTis less than the travel time across the piston.

ISSN:0001-4966    

DOI:10.1121/1.1912541

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Extensive studies have been made on the mean or effective properties of waves propagating in a random medium. We present in this paper a set of correlation equations which are useful in the quantitative determination of the covariance properties of the waves. As an example, the problem of a scalar wave propagating in a random medium is considered.

ISSN:0001-4966    

DOI:10.1121/1.1912542

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The influence of turbulent flow on the sound attenuation in a narrow rectangular duct is calculated by a finite difference approximation of the differential equation for inviscid flow. The absorptive lining at one duct wall is characterized by a flow‐independent wall impedance. Results are found in good agreement with experimental data from a duct with 1‐in.2open area lined on one side with a resistive layer of very fine metal fibers in front of a partitioned air backing. For resistive layers of fibers with larger diameter or of perforated plate used in the same configuration, the wall impedance depends greatly upon the flow velocity. A correlation is found between the nonlinearity of resistive layers due to turbulent flow and due to high sound‐pressure levels in a duct. The influence of the nonlinearity due to high sound‐pressure levels up to 160 dB is studied in the absence of flow. Measured attenuation data agree with those from a theoretical model, which correlates the varying attenuation along the duct with the varying sound‐pressure difference across the resistive lining. By comparison of theoretical and experimental results, the influence of nonlinearity due to turbulent flow can be described by an equivalent level of sound‐pressure difference across the resistive lining. A biasing effect of flow on nonlinear duct lining material linearizes the attenuation along the duct, if that equivalent level due to flow exceeds the actual sound level.

ISSN:0001-4966    

DOI:10.1121/1.1912543

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

An investigation of the interactions between waves and steady flow fields was stimulated by the problem of combustion instability in rocket engines. A one‐dimensional analysis of traveling acoustic waves in flows of an inviscid nonconducting perfect gas, containing mass and heat sources, reveals the existence of a coupling between acoustic waves and nonhomogeneous steady flow fields. This coupling results in either amplification or attenuation of the waves, depending on the gradients of the steady flow variables, phase relationships between oscillations in mass/heat sources and flow field, and injection conditions. A portion of this amplification or attenuation is a result of energy transport between the waves and the flow field, while the remainder is a result of variations in group velocity and impedance. In addition higher‐order effects, such as the generation of backward‐moving acoustic waves and entropy waves by forward‐moving acoustic waves, can occur. Results of experiments with nonuniform velocity fields are in agreement with theoretical predictions. In particular, an air jet discharging into a straight organ pipe and a converging air flow through a tapered pipe are found to attenuate acoustic waves propagating through the pipes.

ISSN:0001-4966    

DOI:10.1121/1.1912544

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The angular dependence of the surface acoustic wave velocity and piezoelectric coupling coefficient have been measured forX‐cut quartz. The results in satisfactory agreement with the calculations of Coquin and Tiersten. Also reported is the first observation of pseudosurface waves onX‐cut quartz.

ISSN:0001-4966    

DOI:10.1121/1.1912545

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP