Showing papers on "Acoustic radiation published in 1969"

Calculation of the acoustic radiation field at the surface of a finite cylinder by the method of weighted residuals

Abstract: In the design of sonar systems it is desirable to compute the acoustic radiation field at the transducer surface, upon which all the significant radiation properties (radiation impedance, beam patterns, etc.) depend. Like other practical array geometries of interest, the finite cylinder does not belong to the class of separable coordinate surfaces of the Helmholtz equation, and consequently, the acoustic field for this geometry cannot be determined analytically. In this paper the surface field is computed numerically from the interior Helmholtz integral equation by the method of weighted residuals. Since the pressure fields over the three surfaces of the finite cylinder must coincide along the locii of intersection between the cylindrical surface and the end caps, the interior Helmholtz integral equation must he constrained to meet this requirement. The matrix representation of this equation which is not self-adjoint is solved by the method of least squares. This enables the constraints to be introduced via Lagrange multipliers. The procedure is used to calculate the surface pressure and radiation impedance of the finite cylinder for a range of axis ratios (diameter/length) and frequencies of interest in sonar applications. Calculations of the radiation resistance and the directivity index determined in this manner are shown to differ from those previously evaluated from the far-field solution. The weighted-residual methods considered are shown to have excellent convergence properties which make them more versatile than alternative numerical methods for solving the problem.

Flextensional Underwater Acoustics Transducer

Abstract: The basic principles of the flextensional underwater acoustic transducer are investigated by developing a mathematical model representation for this type of transducer system, and then establishing the effects that the various geometrical parameters have on the systems performance. The acoustic radiation problem is solved by numerically evaluating the Helmholtz integral for the farfield and nearfield pressure distributions. The mechanical shell vibration problem is simplified by replacing the continuous shell with an analogous framework consisting of a series of bars and joints and having a finite number of degrees of freedom. Finally, the coupled stack wave equation is solved in terms of an arbitrary terminal impedance that simulates the combined impedance of the shell and acoustic radiation impedances. Values of transducer impedance, farfield radiation pressures, and effective electromechanical coupling coefficients are calculated and compared with published experimental values for the University of Miami flextensional underwater acoustic transducer.

Acoustic radiation from fluid loaded rectangular plates

Abstract: The acoustic radiation into a fluid‐filled infinite half‐space from a randomly excited, thin rectangular plate inserted in an infinite baffle is discussed. The analysis is based on the in vacuo modes of the plate. The modal coupling coefficients are evaluated approximately at both low and high (but below acoustic critical) frequencies. An approximate solution of the resulting infinite set of linear simultaneous equations for the plate modal velocity amplitudes is obtained in terms of modal admittances of the plate‐fluid system. These admittances describe the important modal coupling due to both fluid inertia and radiation damping effects. The effective amount of coupling, and hence the effective radiation damping acting on a mode, depends on the relative magnitudes of the structural damping—i.e., on the widths of the modal resonance peaks and on the frequency spacing of the resonances. Expressions are obtained for the spectral density of the radiated acoustic power for the particular case of excitation by a turbulent boundary layer. [Work supported by the Office of Naval Research.]

Acoustic and Internal Damping in Uniform Beams

Abstract: Experimental results on the overall damping of freely supported uniform beams of circular and rectangular section are reported. These results show that the loss factor is, beyond a certain stage, highly frequency dependent but that this dependency largely disappears when the experiment is carried out in vacuo. The frequency dependency is shown to be due to acoustic radiation from the beam. Theoretical results using an approximate theory for rectangular sections agree well with the observed values. Over a certain frequency (wavelength) region the acoustic damping may be many times the internal damping of the beam for the material tested (mild steel).

Development of a Mathematical Model for the Class V Flextensional Underwater Acoustic Transducer

Abstract: 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.

Acoustic radiation efficiency of trucated conical shells

Abstract: Acoustic radiation efficiency and structural vibration characteristics of truncated conical shells

Acoustic imaging by holography

Abstract: : A method for visualizing objects immersed in water is formulated analytically and demonstrated experimentally. The technique, called 'acoustic holography,' is an adaptation of Gabor's two-step imaging process known as wavefront reconstruction or holography. The hologram is first formed from coherent acoustic radiation and then the image is reconstructed optically using coherent light source. Acoustic holography has advantage over other schemes for imaging in optically opaque media in that lenses or other focusing devices are not required, and a complete amplitude and phase reconstruction of the scattered field may be obtained. Since instantaneous amplitude is an acoustic observable, the reference field may be simulated electronically. Moreover, by resorting to heterodyne or phase detection the cross product term between object and reference signals may be generated without the undesired extraneous terms which occur in conventional holography. A scanning technique for generating acoustic holograms of underwater objects in the laboratory is described in detail. Using this system acoustic holograms have been recorded which show angular resolution of 3.6 milliradians, approximately 1.5 times the Rayleigh limit. A variable contrast television display was used to view the acoustic holograms.

Acoustic Field Produced by an Arbitrary Body in Good Vibration. I. Theory and Three‐Dimensional Synthesis

Abstract: Arbitrary‐body acoustic radiation is simulated by an inside source distribution whose field satisfies the boundary vibration. Together with the expansion of Kirchoff's integrand in the ratio of source points and observation point locations, a proposed perturbation process is shown to converge for ratios that are less than unity. Within the frame of perturbation, the class of vibration (good vibration) was shown to bypass the eigenvalue difficulties by analytical inversion. The results are consistent with spherical pulsation, spherical oscillation, and Morse's solution for spherical‐cap vibration. Application of the method to transient analysis is also indicated.