共查询到20条相似文献,搜索用时 31 毫秒
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Traer J Gerstoft P Hodgkiss WS 《The Journal of the Acoustical Society of America》2011,129(4):1825-1836
A model is presented for the complete passive fathometer response to ocean surface noise, interfering discrete noise sources, and locally uncorrelated noise in an ideal waveguide. The leading order term of the ocean surface noise contribution produces the cross-correlation of vertical multipaths and yields the depth of sub-bottom reflectors. Discrete noise incident on the array via multipaths give multiple peaks in the fathometer response. These peaks may obscure the sub-bottom reflections but can be attenuated with use of minimum variance distortionless response (MVDR) steering vectors. The seabed critical angle introduces discontinuities in the spatial distribution of distant surface noise and may introduce spurious peaks in the passive fathometer response. These peaks can be attenuated by beamforming within a bandwidth limited by the array geometry and critical angle. 相似文献
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We demonstrate that a strong longitudinal, nonpropagating field is generated at the focus of a radially polarized beam mode. This field is localized in space and its energy density exceeds the energy density of the transverse field by more than a factor of 2. Single molecules with fixed absorption dipole moments are used to probe the longitudinal field. Vice versa, it is demonstrated that orientations of single molecules are efficiently mapped out in three dimensions by using a radially polarized beam as the excitation source. We also show that there is no momentum or energy transport associated with the longitudinal field. 相似文献
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A possible design of the mode tomography of the ocean with the use of a scheme requiring no expensive low-frequency radiators is considered. The design is based on the widely discussed method of estimating the Green’s function from the cross-coherence function of noise field received in a great number of observation points. The relationship between the Green’s function and the noise coherence function is derived from the Helmholtz-Kirchhoff integral. The use of the vertical multielement arrays composed of vector receivers is suggested to decrease the duration of noise signal accumulation required for a reliable determination of the Green’s function. The solution of the tomographic problem is based on the determination of the mode structure of acoustic field from the eigenvectors and eigenvalues of the cross-coherence matrix of the received noise field. 相似文献
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Legislation in several countries restricting noise emitted by individual road vehicles recommends the use of international standard ISO R362, or something very similar, as a standard. This is a pass-by test with strict conditions laid down regarding the test site, weather conditions, and so on. As such it is not well suited for simple routine and quick roadside enforcement of vehicle noise. This paper describes some initial work aimed at clarifying and resolving a number of the difficulties commonly encountered with stationary tests and describes a procedure which gives good correlation with the ISO pass-by test and may lead to a test which could be used for routine service check and roadside enforcement. 相似文献
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During multi-bubble cavitation the bubbles tend to organize themselves into clusters and thus the understanding of properties and dynamics of clustering is essential for controlling technical applications of cavitation. Sound field measurements are a potential technique to provide valuable experimental information about the status of cavitation clouds. Using purpose-made, rugged, wide band, and small-sized needle hydrophones, sound field measurements in bubble clusters were performed and time-dependent sound pressure waveforms were acquired and analyzed in the frequency domain up to 20 MHz. The cavitation clusters were synchronously observed by an electron multiplying charge-coupled device (EMCCD) camera and the relation between the sound field measurements and cluster behaviour was investigated. Depending on the driving power, three ranges could be identified and characteristic properties were assigned. At low power settings no transient and no or very low stable cavitation activity can be observed. The medium range is characterized by strong pressure peaks and various bubble cluster forms. At high power a stable double layer was observed which grew with further increasing power and became quite dynamic. The sound field was irregular and the fundamental at driving frequency decreased. Between the bubble clouds completely different sound field properties were found in comparison to those in the cloud where the cavitation activity is high. In between the sound field pressure amplitude was quite small and no collapses were detected. 相似文献
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《Ultrasonics》1987,25(2):114-118
A comparison of the characteristics of various designs of hydrophones by means of laser interferometry is performed in a field radiated by a pulse-echo diagnostic device. The results are presented in the time and frequency domain in terms of temporal waveforms, frequency spectra and transfer functions. With regard to the measurement of characteristic ultrasonic field data, distinct differences between the various designs and units were found. 相似文献
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We examine a method for estimating the transient impedance of an emitter and a receiver in a free field based on sliding complex
averaging of the frequency dependence of the transient impedance of the emitter and the receiver obtained under continuous
emission in an undamped pool. The method makes it possible to weaken the influence of reflected signals and to obtain frequency
characteristics almost coinciding with measured ones in free-field conditions. We present results from experimental studies
of the method and its application for calibration of hydrophones by field with the reciprocity method in a reverberating field. 相似文献
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O.?é.?Gulin 《Acoustical Physics》2010,56(5):684-692
A further development of a previously proposed approach to calculating the sound field in an arbitrarily irregular ocean is presented. The approach is based on solving the first-order causal mode equations, which are equivalent to the boundary-value problem for acoustic wave equations in terms of the cross-section method. For the mode functions depending on the horizontal coordinate, additional terms are introduced in the cross-section equations to allow for the multilayer structure of the medium. A numerical solution to the causal equations is sought using the fundamental matrix equation. For the modes of the discrete spectrum and two fixed low frequencies, calculations are performed for an irregular two-layer waveguide model with fluid sediments, which is close to the actual conditions of low-frequency sound propagation in the coastal zone of the oceanic shelf. The calculated propagation loss curves are used as an example for comparison with results that can be obtained for the given waveguide model with the use of adiabatic and one-way propagation approximations. 相似文献
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Modes of the waveguide are selected from signals of individual receivers by time reversing them with the use of the frequency
response of a plane wave. The experimental distribution of the modes on the “arrival time — mode number” plane corresponds
to the model of an ideal waveguide, differing in that higher modes advance lower ones in the experiment. A modification of
the frequency response, which eliminates that contradiction, is proposed, the modification accounting for the dependence of
the effective thickness of the waveguide on the frequency. As a result, a method of determining the distance between the sound
source and receiver is proposed and tested, and the interpretation of the noise immunity of signal reception on the basis
of the time reversal is presented. The experimental data are obtained in the Barents Sea, at depths of about 100 m and distances
of 7, 10.5, and 12 km, with the signals in a band of 100–300 Hz. 相似文献
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Many problems of current interest in underwater acoustics involve low-frequency broadband sonar interrogation of objects near the sea surface or sea floor of a shallow-water environment. When the target is situated near the upper or lower boundary of the water column the acoustic interactions with the target objects are complicated by interactions with the nearby free surface or fluid-sediment interface, respectively. A practical numerical method to address such situations is presented. The model provides high levels of accuracy with the flexibility to handle complex, three-dimensional targets in range-independent environments. The model is demonstrated using several bottom target scenarios, with and without locally undulating seabeds. The impact of interface and boundary interactions is considered with an eye toward using the sonar return signal as the basis for acoustic imaging or spectral classification. 相似文献
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We construct exact solutions describing trapped water waves over an underwater ridge of small height in the shallow water
approximation and in the complete formulation. Resonances (antibound states) in the case of an underwater trench are also
constructed. 相似文献
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