摘要:

An acoustic backscattering technique for detecting transient cavitation produced by 10‐μs‐long pulses of 757‐kHz ultrasound is described. The system employs 10‐μs‐long, 30‐MHz center frequency tone bursts that scatter from cavitation microbubbles. Experiments were performed with suspensions of hydrophobic polystyrene spheres in ultraclean water. Transient cavitation threshold pressures measured with the active cavitation detector (ACD) were always less than or equal to those measured using a passive acoustic detection scheme. The measured cavitation thresholds decreased with increasing dissolved gas content and increasing suspended particle concentration. Results also show that ultrasonic irradiation of the polystyrene sphere suspensions by the ACD lowered the threshold pressure measured with the passive detector. A possible mechanism through which suspensions of hydrophobic particles might nucleate bubbles is presented.

ISSN:0001-4966    

DOI:10.1121/1.399091

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Air‐filled bubbles and bubblelike scatterers (balloons, very thin shells, and cavities in rubberlike layers) of radiusa, in a liquid of sound velocityc, satisfy the compactness conditionka<1 at frequencies ω=kcin the neighborhood of their resonant ‘‘bubble’’ frequency ω0. One can therefore describe the scatter from a monolayer of such scatterers at an interface between two media using the generalized smoothed‐boundary theory developed by this writer elsewhere [I. Tolstoy, J. Acoust. Soc. Am.75, 1–22 (1984);79, 666–672 (1986)] in which the scatterers are replaced by distributions of monopoles and dipoles. For bubblelike scatterers, and for ω≂ω0, the monopole contributions outweigh the dipole contributions by many orders of magnitude; there is then a layer of monopoles whose effective scattering cross sections near resonance are between 1012(bubbles in water) and 106(cavities in rubber) times that of a small hard sphere of the same radius. Such monolayers lead to major changes of reflectivity—which can be calculated rather simply, using plane distributions of scatterers, e.g., a square lattice of basislon a hard wall, with or without taking into account monopole interaction through multiple scatter.Ineithercase, the models predict the existence ofklvalues at which the wall/fluid interface becomes anechoic at or close to ω=ω0. Numerical examples are given for air‐filled cavities in rubberlike materials next to a perfectly hard wall. In practice, anechoic conditions are approximated for a narrow band of frequencies ωA±Δω, ωA≂ω0, and Δω/ωA≂10−2for soft rubber (FJ95) and ≂5×10−2for harder rubbers [FJ65: using constants given by Gaunaurdetal., J. Acoust. Soc. Am.65, 573–594 (1979)]. For given ωA, the effect is, in practice, less sensitive to spacingkl(departures of ±25% from the exact value are tolerable). Simple theoretical considerations suggest that similar anechoic conditions will be achieved for elastic plates with the same cavity distributions in an interfacial rubber layer. A constant ambient pressure is implicitly assumed throughout (horizontal interfaces and monolayers).

ISSN:0001-4966    

DOI:10.1121/1.399092

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

When phase information is absent (incoherent detection), the basic statistical entity relevant to data processing is the bichromatic four‐point coherence. Using the strength and size parameters Φ 2and Λ, an integral expression for this quantity is formulated and evaluated for a high‐frequency signal scattered by a phase screen that represents the randomly reflecting ocean cover (or, alternatively, a randomly refracting thin layer). The evaluation is illustrated by a treatment of the monochromatic mean‐square intensity 〈I 2〉, then applied to the bichromatic intensity correlation 〈I1I2〉. Viewed as a function of the frequency‐independent parameter ΛΦ, 〈I2〉 rises from 1 to 2, passing through a maximum which scales as log k(the wavenumber) at some intermediate value of ΛΦ that depends only on the form of the screen statistics. By constrast, 〈I1I2〉 saturates at 1. Moreover, even when ΛΦ is large, the second and fourth coherence moments are found to be consistent with the assumption that the bichromatic signal possesses joint normal statistics only within a narrow range of frequency separations(∼O(1/Φ)), which nonetheless encompasses the coherent bandwidth(∼O(1/ΛΦ 2)).

ISSN:0001-4966    

DOI:10.1121/1.399093

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The effect of hydrodynamic nonlinearities on ocean surface scattering is considered. A recently developed expression for the bispectrum of the surface elevation is used to calculate the lowest‐order correction due to this effect. The result is comparable in magnitude to the tilt correction, which has been recently calculated.

ISSN:0001-4966    

DOI:10.1121/1.399094

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This paper extends the study on reflection and refraction of waves at the interface of water and porous sea ice by Yew and Weng [J. Acoust. Soc. Am.82, 342–353 (1987)]. The behavior of leaky Rayleigh waves and Scholte waves generated by a line cylindrical wave source at a distance below the interface is analyzed. It is demonstrated that a wave receiver in the water would pick up a direct wave from the source, a specularly reflected wave from the interface, a leaky Rayleigh wave, and, if the source and receiver are close to the interface and to each other, a Scholte wave. The effects of porosity and frequency on the behavior of these surface and interface waves are also studied. PACS numbers: 43.30.Ma

ISSN:0001-4966    

DOI:10.1121/1.399095

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Dependence of the noise spectrum of acoustically induced cavitation on the vibrational level of a radiation surface was investigated in a range of vibrational displacement from 0.1 to 24 μm; a heterodyne fiber‐optic hydrophone was used to detect cavitation noise. As vibrational level is increased beyond cavitation threshold, the fundamental component and the second and third harmonics cease to increase andn/2‐ andn/4‐order subharmonics (n: integer) start to increase. At a vibrational level wheren/2‐order subharmonics become maximum,n/4‐order subharmonics make a dip in their spectral level. At high vibrational levels, subharmonics ofn/4 order become dominant.

ISSN:0001-4966    

DOI:10.1121/1.399096

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Capon’s maximum likelihood (ML) method has been used with some success in matched field processing for source range and depth estimation. One reason for the interest in the ML is that it is data adaptive; that is, it adapts to the actual noise field present rather than requiring anaprioriestimate of the noise component for prewhitening. When modal noise is present the ML can become sensitive to any deviations from the unperturbed case (i.e., from the model) as would be introduced by phase errors or model parameter errors. Using a dimensionality‐reduction procedure a more stable data adaptive method, the ‘‘reduced’’ ML (RML), is obtained. The RML is compared here with the ML on simulated data from a 21‐sensor array in a Pekeris waveguide supporting eight normal modes. Under modal noise conditions the RML provides a significant improvement over ML when phase errors occur. Although the deviation from the model considered here is that caused by phase errors, the nature of the perturbation is not important since the sensitivity of ML is not to any special type of perturbation.

ISSN:0001-4966    

DOI:10.1121/1.399097

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Given well‐known environmental conditions, matched field processing has been shown to be a promising signal processing technique for the localization of acoustic sources. However, when environmental data are incomplete or inaccurate, a ‘‘mismatch’’ occurs between the measured and model fields that can lead to a severe degradation of the localization estimator. The possible mismatch effects of surface and internal waves on matched field processing in a shallow water waveguide were investigated. A modified ray theory was utilized, based on the work of Tindle [J. Acoust. Soc. Am.70, 813–819 (1981);73, 1581–1586 (1983)], to calculate the acoustic pressure field. Thus it was possible to simply incorporate range‐dependent environmental conditions as well as to generalize this work to deeper waveguides. In general, the conventional (Bartlett) matched field beamformer does not provide sufficient resolution to unambiguously locate a source, even in a perfectly matched environment. The maximum likelihood method matched field beamformer has much better resolution, but is extremely susceptible to mismatch. The mismatch due to surface roughness can result in a large reduction of the estimator peak. Part, but not all, of the peak can be regained by (1) using a model that includes incomplete reflection at the surface based on actual sea surface statistics and (2) short‐time averaging of the measured signal, with times on the order of the period of the surface waves. Mismatch due to internal waves can also result in a large degradation of the estimator. Averaging over the same time period as surface waves provides little improvement and leads one to surmise that internal waves may be a limiting constraint on matched field processing. Finally, the surface and internal wave fields were combined with a slowly moving source. This example highlights the necessity for the development of a beamformer that has a broader mainlobe while maintaining adequate sidelobe suppression. This issue is addressed by looking at two such beamformers.

ISSN:0001-4966    

DOI:10.1121/1.399098

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The performance of line arrays is significantly affected by the irregular spatial variations that develop in acoustic waves when they propagate in the ocean. A numerical technique that has been successful in simulating the spatial variations observed in ocean acoustic experiments has been used to study their effect on array performance. Analytical expressions, helpful for describing the output of an array in such a spatially varying sound field, have been derived on the basis of the parabolic second‐ and fourth‐moment equations. The present paper is confined to treating vertically deployed arrays.

ISSN:0001-4966    

DOI:10.1121/1.399045

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Wavenumber‐frequency calibration of underwater, planar, receiving arrays requires the ability to generate single‐wavenumber pressure fields over the surface of the array. When the wavenumber‐frequency region of interest is evanescent, transmitting arrays previously constructed have been found to generate fields contaminated with harmonics, acoustic wavenumbers, and nonacoustic wavenumbers from the excitation of antisymmetric Lamb waves. An array that greatly reduces contamination has recently been constructed using a sheet of polyvinylidene fluoride (PVDF) with independent rectangular electrode stripes. The array operates in the frequency range of 500 Hz to 2 kHz and generates evanescent waves with phase speeds between 30 and 150 m/s. Contamination due to the excitation of antisymmetric Lamb waves is eliminated by shifting the phase speed of the Lamb wave out of the region of interest. This is accomplished by bonding the thin sheet of PVDF directly to a thick plate of LEXAN. Contamination from harmonics and acoustic wavenumbers is eliminated by driving the electrode stripes with suitably chosen shading coefficients generated by a simple‐source numerical algorithm. Measured pressure fields and phase velocities compare favorably with numerical calculations.

ISSN:0001-4966    

DOI:10.1121/1.399046

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP