Acoustic Field In Water Research Articles

The fluid bottom approximation and its implications for sound attenuation in underwater waveguides

The fluid-bottom model makes forward and inverse problems more amenable to theoretical analysis and numerical simulations but has some well-known limitations. The accuracy and weaknesses of the fluid approximation are elucidated in this paper by quantifying the effects of weak shear rigidity on acoustic normal modes. Mathematically, appearance of finite shear rigidity and attendant slow shear waves is a singular perturbation of the fluid-bottom model, which standard perturbation theories fail to capture correctly. The problem is addressed here by combination of an asymptotic technique and an analysis of exact solutions for idealized seabed models and placed in the context of earlier studies of the effects of shear rigidity. Due to coupling between compressional and shear waves at the seafloor and within the seabed, shear rigidity affects phase and group speeds of normal modes. By transferring the energy from compressional to shear waves, wave coupling makes a significant contribution to sound attenuation from low up to mid-frequencies. It is found that the effects of shear rigidity on the acoustic field in water are magnified by seabed stratification and especially by density variations. Implications of the results for seabed parameterization in geoacoustic inversions will be discussed. [Work supported by ONR.]

Frequency Response Characterization of a Sonoluminescence Acoustic Resonator by Using a FPGA-Based Instrumentation

Sonoluminescence (SL) is a physicochemical phenomenon that is observed when a single or multiple air bubbles, excited by an acoustic field in water, collapse and emit light. Until nowadays, it is still an outstanding phenomenon in which acoustic energy is (partially) transformed into light. Along the years, some models have been proposed to explain sonoluminescence and a lot of experimental setups have been built in an attempt to reproduce and study it. Since the flash of light is forced to occur inside a resonant cavity (i.e., a glass flask filled with water) by using external circuitry, many efforts have been made to study the relationship between the phenomenon and the electrical and mechanical quantities involved in the experimental apparatus. Following this field of study, this paper proposes an electronic instrumentation system based on a FPGA device for performing automatic monitoring of the input signal applied to the resonator as well as the output response signal produced by it. The objective of using the automated setup is to collect data for characterizing the resonator frequency response in order to identify its input/output behavior. The key idea is to find out whether there is a linear circuit which might have analogous input/output characteristics that may be used to simulate the phenomenon in future studies.

The effect of viscosity, applied frequency and driven pressure on the laser induced bubble luminescence in water–sulfuric acid mixtures

Production and oscillation of sonoluminescence bubbles by laser pulse in the presence of acoustic field in water and different concentrations of sulfuric acid are investigated. In the presence of acoustic field, the laser causes variable speed of sound, surface tension and density; and the host liquid acts as a compressible one and strongly affects the bubble's dynamics equations. The effect of various concentrations of sulfuric acid as a host liquid on the oscillation of bubble radius, bubble wall velocity and bubble interior temperature is studied. Furthermore, the effect of applied frequency on LI-SCBL in the presence of the acoustic field is investigated and an optimum sound wave frequency for the bubble oscillation and bubble interior temperature in pure water and SA is introduced. Based on the modification of RP equation, by applying the optimum frequency, the results indicate that the maximum bubble radius for LI-SCBL in the presence of the acoustic field is increased up to 7×10−4 m as this article presents, which is more than 40% improvement. This amount results in interior temperature of more than three times, from almost 5000 K in the previous works to almost 16 000 K in the present report. This is very similar to the experimental measurements for bubble radius induced by laser. Furthermore, the effects of driving pressure amplitudes on the bubble radius, the bubble interior temperature and the bubble wall velocity in different host liquids and in optimum frequency are investigated.

Quasiadiabatic approach for laser-induced single-bubble sonoluminescence

The luminescence parameters of laser-induced bubble in the presence of an acoustic field in water are studied. A comparison is made between parameters such as bubble radius, interior temperature, and pressure of the bubble induced by laser and an acoustic field influenced by different driving pressure amplitudes. It is found that the bubble volume induced by laser at the collapse instant is more than 10(6) times larger than the one induced by an acoustic field. It is also noticed, by increasing the driving pressure amplitude, the bubble radius decreases in both cases, however, the bubble interior pressure and temperature increase.

Partial Quenching of Sonoluminescence by Monoatomic Gases

Abstract In a solitary bubble trapped at the antinode of the standing acoustic field in water, the density increase of n-butanoic acid in the bubble quenched the sonoluminescence in consuming the acoustic energy by sonochemical reaction. In the bubbles filled with inert gases (He, Ar, and Xe), the quenching was obvious in Xe, but in He. These observations suggest that He locating at the bubble center reduces the sonochemical reactivity.

Pressure and temperature dependence of fiber Bragg grating for acoustic sensing.

Dependence of fiber-Bragg-grating (FBG) characteristics on static pressure around an FBG is analyzed in conjunction with the operation of an FBG as an underwater acoustic sensor. As the pressure increases from 0 to 69 MPa, the reflection and transmission spectra of an FBG shifts to a shorter wavelength while keeping the spectral shape unchanged. The rate of shift of the Bragg reflection wavelength is about -0.006 nm/MPa and therefore the pressure more than 0.17 MPa can be detected with an instrument available today. However, the sensitivity is too small to detect sound pressure of our interest that is less than 140 dB re 1μPa. It is consequently considered at the moment that the only practical method for measuring an acoustic field in water with an FBG is to make use of the intensity modulation of a laser light transmitting through or reflecting back from an FBG of which optical wavelength is at the slope of the transmission or reflection spectrum curve of the FBG.

Experimental verification of the theory for sound propagation in a wedge with a shear supporting bottom

Laboratory experiments have been carried out to investigate underwater sound propagation over a sloping bottom that can support shear. The sound field was measured in both the across slope and down slope direction for many different frequencies and bottom slopes. The experimental results have been compared with the source image theory for sound propagation in a penetrable wedge [G. B. Deane, J. Acoust. Soc. Am. 91, 2390 (A) (1992)] and have shown very good agreement for both down slope and across slope propagation. In particular the effect of shear waves on across slope propagation has been demonstrated both theoretically and experimentally. Further the effect of a sediment layer has been successfully incorporated into the source image theory for comparison with the experimental measurements at low frequencies. One of the problems with laboratory scale measurements of underwater sound propagation is the determination of the bottom parameters with sufficient accuracy for acoustic calculations. In this case the model of the sea floor consisted of a sediment layer (modeled experimentally by epoxy) over a hard basement made of concrete requiring 11 variables to describe the geo-acoustic features of the bottom. These were obtained by inverting measurements of the acoustic field in water of constant depth over the same model. The results are therefore geo-acoustically calibrated. [Work supported by ONR.]

Surface statistics and the spatial coherence of surface‐scattered energy

The facet‐ensemble method [W. A. Kinney et al., J. Acoust. Soc. Am. 73, 183–194 (1983)] was used to determine how the statistics of a rough surface relate to the spatial coherence of energy scattered from it. Two ideal water surfaces were studied. One had a sawtooth cross section derived from the sum of the first four terms of a Fourier sine series representation. The cross section of the other surface was derived from the sum of the same terms but with each having random phase. This resulted in two surfaces each with the same rms roughness (0.2 cm ), spatial wavelength (16 cm), and spatial wavenumber components, but different cross‐sectional shapes. With each surface, the acoustic field in water was computed at two receiver locations for both backscatter and forward scatter for a grazing angle of 35 (with respect to the surface mean), 100 kHz, and a total travel distance of 421 cm. For backscatter, the pressure amplitudes and coherence values differ considerably between the two surfaces, and the differen...