摘要:

Impedance characteristics of various sintered stainless‐steel resistance elements were measured. These elements are essentially pure resistances, exhibiting a linear relationship between sound‐flow velocity and normalized‐flow resistance. Processing parameters that permit consistent attainment of desired stable acoustical characteristics were discovered.

ISSN:0001-4966    

DOI:10.1121/1.1919087

出版商:Acoustical Society of America    

年代:1964

数据来源: AIP

摘要:

An account is given of the construction of a double form of Helmholtz resonator, which is compared with a resonator used by Tucker in earlier work. The variation of its resonance response and resonance frequency with pressure reduction have been investigated and the effects of pressure, temperature, and source‐current variation on its damping at resonance are described. The neck corrections of the resonators have been calculated from their calibration constants and are compared with values derived from simple formulas.

ISSN:0001-4966    

DOI:10.1121/1.1919089

出版商:Acoustical Society of America    

年代:1964

数据来源: AIP

摘要:

In this paper, the author solves the partial‐differential equations governing the motion of plates, which were derived by Mindlin, Reissner, and Uflyand, by using product solutions and finds, by taking an infinite series of product solutions, that the boundary conditions are satisfied. The elimination of the arbitrary constants occurring in each boundary problem leads to the frequency equation for the normal modes of vibration. The frequency equation for each problem is an infinite determinant and each element in it is an infinite series of Mathieu functions containing as unknown the frequency. A method is described permitting one to find the roots of the infinite determinant. These roots represent the normal modes of vibration for the elliptical plate. Since the classical (Lagrange) theory of plate is good only for plate when the wavelength is large in comparison with the thickness of the plate, it is restricted to low‐frequency vibrations. The present theory gives good results for high‐frequency vibrations, essentially because it includes coupling between flexural and shear motions.

ISSN:0001-4966    

DOI:10.1121/1.1919091

出版商:Acoustical Society of America    

年代:1964

数据来源: AIP

摘要:

The sound radiation from a slightly compressible flow can be calculated by estimating the pressure that would exist at an enclosing surface just as if the fluid were incompressible, this pressure then being inserted in the appropriate point‐multipole expression for the sound radiation in a compressible fluid. The source strength is specified in terms of the vorticity, a simple result being given for the pressure in the incompressible flow; the final result for the farfield sound pressure is then the same as that of the theory of vortex sound.

ISSN:0001-4966    

DOI:10.1121/1.1919093

出版商:Acoustical Society of America    

年代:1964

数据来源: AIP

摘要:

The normal‐mode solution of the problem of the radiation field of a simple harmonic source in a layered waveguide is extended to the case of small irregularities in the surface. The effect of the irregularities is to decrease the coherence of acoustical propagation and increase the attenuation. The theory is used to calculate the radiation field of a 147.8 cps simple harmonic source in about 20 in of shallow water over a thick layer of unconsolidated sediment. The calculations indicate that the effects of small surface roughness, i.e., rms waveheight about12 m, are to increase the attenuation as a function of range and to decrease the mode‐interference maxima and minima as evidence of the decrease in coherence. The experimental dependence of the acoustical pressure upon source distance similar to that calculated with the theory.

ISSN:0001-4966    

DOI:10.1121/1.1919095

出版商:Acoustical Society of America    

年代:1964

数据来源: AIP

摘要:

The effects of shading on number of minor lobes, level of minor lobes with respect to the principal lobe, and beamwidth are analyzed for symmetrically shaded, uniformly spaced 6‐ and 4‐element arrays. For a 6‐element array shaded 1:a:b:b:a:1, there are no minor lobes above the parabola125−30 a+a2−20b=0. Between this parabola and5−2 a+a2−4b=0, there is one minor lobe, while between the second parabola and the straight lineb−3a+5=0there are two minor lobes, which may or may not be equal in amplitude. Below the straight line, there is again only one minor lobe. Curves in thea‐bplane for constant first and second minor‐lobe levels are presented, as well as curves of constant beamwidth for the half‐wavelength spacing. For the 4‐element array shaded 1:c:c:1, there are no minor lobes whencis greater than 3. Curves for minor‐lobe level and beamwidths for the half‐wavelength spacing as a function ofcare presented.

ISSN:0001-4966    

DOI:10.1121/1.1919097

出版商:Acoustical Society of America    

年代:1964

数据来源: AIP

摘要:

In the past, underwater acoustic data have been obtained with pressure‐sensitive hydrophones, which neglect the particle displacement in an acoustic wave. Displacement reception possesses certain advantages, among which are the possibilities of direction finding and also of acoustic observations at the sea surface. The theory of operation of a displacement hydrophone is discussed and the technique is given for constructing instruments sensitive to a displacement of 0.1 Å at 1000 cps. The propagation of displacement observed at sea is compared with theoretical predictions. The vertical component of displacement is found to be maximum on the sea surface as predicted; the horizontal component vanishes at the sea surface, but at greater depths can be used to observe the wave‐propagation direction. At short ranges, the acoustic‐displacement field is closely approximated by a dipole source; at longer ranges, the displacement field diminishes less rapidly (1/rdependence) than is predicted by the dipole theory.

ISSN:0001-4966    

DOI:10.1121/1.1919099

出版商:Acoustical Society of America    

年代:1964

数据来源: AIP

摘要:

The fluctuation of the response‐signal intensity as a result of correlated phase and amplitude fluctuations is considered in connection with the performance of various common types of linear arrays. The results of Chernov's treatment of sound transmission through a random medium are used extensively for the formulation and solution of the problem of signal degradation by phase and amplitude fluctuations. This method of solution indicates that amplitude and phase fluctuations must be considered simultaneously. A generalization of Chernov's method shows that the bearing angle of the array, relative to the direction of propagation, is of importance in connection with the autocorrelation of the phase and amplitude fluctuations. The simultaneous inclusion of phase and amplitude fluctuations as well as bearing angle effects leads to quite general results for the computation of mean signal intensities, along with higher‐order moments. Explicit expressions are given for the case of large values of the parameterD=4R/ka2, whereRis the range,kis the wavenumber, andais a characteristic correlation distance for the transmission medium.

ISSN:0001-4966    

DOI:10.1121/1.1919101

出版商:Acoustical Society of America    

年代:1964

数据来源: AIP

摘要:

Underwater ambient‐noise spectra and amplitude distributions are described for data acquired during field experiments made in the spring, late summer, and winter within the Canadian Arctic Archipelago. Measurements in each case were made using a bottom‐mounted hydrophone. The ice cover was fast to the shore during the spring and winter experiments. In the summer experiment, the ice cover, although 10/10, was free to move. The ambient noise was generally impulsive and at times highly non‐Gaussian. The significant noise was the result of mechanical activity associated with the ice cover. For shore‐fast spring and winter ice, surface cracks as a result of thermal stresses were important; for late summer ice, relative motion of the floes was important.

ISSN:0001-4966    

DOI:10.1121/1.1919103

出版商:Acoustical Society of America    

年代:1964

数据来源: AIP

摘要:

Measurements of ocean‐surface backscattering were made off Nassau in 2500‐f water using 2‐lb TNT charges as acoustic sources. Analysis in octave frequency bands between 0.8 and 12.8 kc/sec shows surface backscattering decreasing down to grazing angles of 2° and confirms that scattering strength is proportional to frequency and wind velocity, with wind velocity becoming increasingly significant as grazing angle is decreased. Volume backscattering predominates over surface backscattering at low wind velocities and small grazing angles; the volume backscattering is presumed to be due to the presence of a layer of large numbers of microscopic marine life. General agreement was obtained on backscattering coefficients at grazing angles where previously published data are available for comparison.

ISSN:0001-4966    

DOI:10.1121/1.1919105

出版商:Acoustical Society of America    

年代:1964

数据来源: AIP