摘要:

A new ultrasonic imaging system that employs the orthogonal function wavefronts derived from theaprioriknowledge of the second‐order moment function about a class of objects is presented. First, it is shown theoretically that this new method has the ability to reconstruct images of sample objects of a given class with a higher signal‐to‐noise ratio (SNR) in a shorter time than the conventional beamforming methods. The assumptions are confirmed by numerical analyses and experiments carried out by using a prototype of an orthogonal function applied array imaging system.

ISSN:0001-4966    

DOI:10.1121/1.385206

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A new method of determination of the velocity of a moving noisy machine by the analysis of a signal detected at a fixed point is proposed. In this system, it is assumed that the trajectory of the object is known beforehand and the special characteristics of a machine noise in the bispectral region are used so that a measuring system which does not suffer from any surrounding Gaussian noise is realized. The nonstationary change of the bispectral characteristics, along with the movement of the object, is detected from a set of short‐time bispectra in segmented intervals and the method of minimum mean‐square‐error fitting is applied to get reliable velocity. The principle, concrete construction, and experimental results are shown. The results show the superiority of the new method over conventional methods.

ISSN:0001-4966    

DOI:10.1121/1.385207

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

An analysis concerning the response of a cavity‐backed panel when subjected to external excitation having arbitrary time and spatial variables is extended here to derive expressions for the pressure developed within the backing cavity. Two cases of practical interest are then considered as examples of application; a normally incident harmonic, and a normally incident ’N’ wave.

ISSN:0001-4966    

DOI:10.1121/1.385208

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Free vibration of annular plates has been studied using the method of initial functions. Numerical results of the natural frequencies are obtained for two typical support conditions. The present results depart from the classical plate theory for higher modes as well as for thicker plates, but is in good agreement with Mindlin’s improved plate theory.

ISSN:0001-4966    

DOI:10.1121/1.385209

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Acoustical signals at 270 Hz from SOFAR floats drifting in the region southeast of the Gulf Stream were recorded during most of 1975 from a near axis sound channel hydrophone near Bermuda. The amplitude levels received exhibit a large increase (12–18 dB) commencing about 24 July, following a long period (March to July) of relatively lower peak level amplitudes. A major part of the increase can be attributed to the influence of a large cyclonic eddy (Gulf Stream ring) that passed slowly between the SOFAR floats and Bermuda. Such an eddy produces a large sound speed anomaly that extends to depths below the axis of the sound channel. On 24 July, two SOFAR floats were known to have approximately the same sound transmission path through the edge of the large eddy. The sound transmission peaks occur when no ocean eddy is between the SOFAR floats and the receiver. Their spacing shows they occur at regular refraction caustics in the sound channel. When the sound transmission path passes through an eddy, these transmission focal distances are shifted to greater range and the signal level may be greatly enhanced. The decrease of caustic peak intensities with range is 5 dB per double distance, and this agrees with theory. Several different levels of peak acoustic intensity occur and these result from two float depths and oceanic thermocline oscillations.

ISSN:0001-4966    

DOI:10.1121/1.385220

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Scattering integrals are derived that represent the contribution of boundary scattering in a sound channel. These integrals are added to the normal mode sum which represents the coherent field in the channel. A comparison is made between experimental and calculated sound levels in a surface duct. For the case examined, it is clear that (except for short ranges) surface scattered energy is a significant source of sound at a below duct receiver.

ISSN:0001-4966    

DOI:10.1121/1.385141

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The frequency spectrum of log‐amplitude fluctuations is substantially modified (flattened) by a previously neglected effect: the vertical advection of short internal waves by the more energetic internal waves with large vertical wavelength. The log‐amplitude spectrum is sensitive to this vertical advection and to curvature of the equilibrium sound ray, whereas the phase spectrum is not. Noninternal‐wave ’’frozen’’ finestructure contributes to the log‐amplitude frequency spectrum through vertical advection, but not to the phase spectrum. The Cobb Seamount experiment is treated as an example; the spectrum of the modified theory agrees well with observations except at high frequency ( near the buoyancy frequency ).

ISSN:0001-4966    

DOI:10.1121/1.385221

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Experimental investigations of acoustically induced turbulence and shock waves in a traveling‐wave tube are performed with and without the introduction of an air flow through the system. Frequency (f) and intensity (I) effects of the acoustic field are studied using a hot‐film anemometer and FFT data processing unit. The effect of the acoustically induced turbulence on a laminar flow is also investigated. Sampled data are first conditioned, then processed to estimate the characteristics of turbulence. It is found that for sound pressure level ≳158 dB turbulence appears. Furthermore, at slightly higher level (∠160 dB) shock waves appear over the range of all frequency. Turbulence measurements were performed over a frequency range of 500–2200 Hz, with intensity over a range of 06.–4.0 W/cm2. BelowI=0.6 W/cm2no turbulent bursts are found. The turbulent spectrumFand the wavenumberkare found to safisfy a power lawF∝kαwith α?−1.7 to −2.1. The rms turbulent velocityu*is found experimentally to have aI1/2dependence, yet is relatively insensitive to variation off. Throughout the whole measuring range offandI, the rate of energy dissipation per unit mass ε is estimated to be 106–107 cm2/s3. It is also found that superimposing a laminar air stream through the tube will suppress the turbulence.

ISSN:0001-4966    

DOI:10.1121/1.385222

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Wave propagation is actually the transport of energy in a medium that also acts as a sink for the energy. In order to describe this process more accurately than is done in the classical approximation, it is necessary to modify the lossless equations of motion to account for losses in an inhomogeneous, arbitrary medium. This can be accomplished by the introduction of a generalized loss term (loss function) in the wave equation of motion. The inclusion of this term introduces into the force balance equation an effective dissipative force field that is the equivalent of the losses. The development of a general series solution in powers of the parameter (kx) for the complex wave vectorkand the displacementxyields a description of the wave field that incorporates the nonlinear effects of losses in its form. This approach gives insight into the higher order processes occurring in an arbitrary, inhomogeneous, lossy medium and shows how a frequency shift occurs in the wave as a result of travel through the medium.

ISSN:0001-4966    

DOI:10.1121/1.385169

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The effects of temporal variations of the sound velocity on acoustic propagation from a time periodic point source in a layer of uniform depthhare studied. It is assumed that the sound speed is a slowly varying function of time, and that the upper and lower surfaces of the layer are free and rigid, respectively. An asymptotic expansion of the acoustic pressure field is obtained by using the methods of normal modes, two times and stationary phase evaluations of multiple integrals. The small parameter ε that is used in the analysis is the ratio of the time scale of variation of the sound velocity to the frequency of the source. To lowest order in ε, the acoustic pressure is given by the sum of a primary wave and a secondary wave. The primary wave consists of a finite modal sum of propagating waves and an infinite modal sum of evanescent waves. If the dimensionless sound speed oscillates about the value 1/am, whereamis the propagation number of themth mode, then themth mode alternates between propagating, and evanescent, i.e., themth mode will alternately propagate and ’’fade.’’ In addition, it is shown that in the farfield the secondary wave is an infinite sum of evanescent waves which decay more rapidly than the evanescent components of the primary wave. Finally, it is shown that the long time, farfield limit of the primary wave is equivalent to the customary ’’approximation’’ that is obtained by first seeking time periodic solutions, where the initial conditions and the variations in the sound velocity are neglected. Then, the time dependence of the sound velocity is inserted into the resulting representation.

ISSN:0001-4966    

DOI:10.1121/1.385170

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP