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Recent measurements of laboratory-generated noise by breaking waves exhibit an increase in sound-pressure levels for salt water compared to fresh water over a broader range of frequencies [A. R. Kolaini, J. Acoust. Soc. Am. 103, 300-308 (1998)]. The surprise increase in sound-pressure levels inspired the study of the effects of salt on the sound radiation by single bubbles released from various size needles. A needle assembly and a false tank (2×2×4 ft), with acoustically transparent walls placed in the middle of an anechoic tank (12×12×8 ft), were used to study the acoustical characteristics of bubbles released from needles. The false tank was filled with water that contained various percentages of sodium chloride (NaCl). In this paper, the results of experiments to examine the variations in radiated sound pressure and the change in the damping coefficient of bubbles as the salinity level increased are reported. The effects of a surfactant agent such as Triton™ 100-X and other mineral acids such as HCl on the bubble acoustic radiation are discussed. The observations show that the change in the local surface tension may not alter the acoustic radiation, whereas the local influence of the ions on water structure, possibly related to the electric repulsion, may play a dominant role in altering the sound pressure and significantly reducing the quality factor of the bubble sound. © 1999 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

A recent article [A. R. Kolaini and L. A. Crum, J. Acoust. Soc. Am. 96, 1755-1765 (1994)] reported the measurements of the ambient sound generated by laboratory breaking waves over the range 100-20 000 Hz in fresh water. Those observations from both spilling- and plunging-type breakers have been repeated in the same manner and wavemaker/anechoic tank with water that had 25[per_thousand/salinity] salt in its content. The observations in salt water, just like those in fresh water, reveal that the sources of sound in laboratory spilling breakers are due primarily to single bubble oscillations that can have frequencies lower than a few hundred Hertz. In the case of weak spilling breakers, the sound spectra level in fresh water was due primarily to single bubble oscillation, while the same breakers in salt water have introduced smaller size bubbles with large density. The relatively high-density populated bubble cloud generated by weak breakers shows the evidence of the onset of collective oscillation that was absent for the same breakers in the fresh water. In the case of moderate spilling and plunging breakers, it appears that both individual bubbles and bubble clouds can contribute to the acoustic emissions in fresh and salt water. The average sound spectra reveal that the peak frequencies of the spectra shift from a few kHz (weak, spilling breaker) to few hundred Hz (plunging breaker), and the high-frequency portions have slopes approximately 5-6 dB/oct, which are the slopes observed from the noise spectra of the ocean. Besides the high bubble density and smaller bubble sizes in salt water, all breakers experienced a significant increase in sound-pressure level in all observed frequency range. The ionic structure of the medium alters the sound radiation from bubbles. In this paper some of the observed acoustic signatures from breaking waves are discussed and a plausible explanation of how salt can effect the sound radiation from bubbles is given. © 1998 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

The acoustical characteristics of bubbles injected into a fully developed turbulent flow field are studied. By injecting an 'adult' bubble into a flow, generated by a submerged axisymmetric horizontal water jet, the acoustic reexcitation of the bubble with and without breakup may occur in the shear-induced flow region. Bubbles of various sizes were introduced into jets of various speeds by means of interchangeable hypodermic needles. Results of the role of the turbulent flow characteristics in determining the acoustic bubble response are discussed. The characterizations of both the acoustical and the dynamics of the bubbles encountering a turbulent flow field depend upon the estimated integral and microlength scales, the corresponding Reynolds numbers, and the critical Weber numbers for both bubble distortion and breakup. A critical Weber number was estimated to be 0.52 for an 'adult' bubble acoustic reexcitation without breakup and 1.10 for bubble breakup. A simple model is given to account for the bubble acoustic radiation. The Rayleigh-Plesset equation was altered by incorporating the turbulent pressure fluctuation of the flow as a driving force of the bubble. The turbulent energy spectrum obtained experimentally was used to predict the monopole acoustic emission by the bubble. Some conclusions are given about the nature of the turbulent flow field and conditions under which the bubble acoustic reexcitation may occur. © 1997 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

The results of an experiment to characterize the underwater sound field radiated by various breaking waves intensities in fresh water in the range from 0.1 to 20 kHz are described. Waves are generated by a computer-controlled plunging-type wave maker and propagate along a 12.7-m-long channel where they are made to break at the mid-surface of a 3-×3-×2.5-m anechoic water tank. The individual bubbles and bubble clouds entrained by the breaking wave provide a mechanism for sound production. Using high-speed cinephotography, correlations were established between the hydrodynamic evolution of the cloud and the radiated acoustic emissions. The bubble size distributions inside the cloud were measured with the aid of a high-speed video camera and a fiber optic cable. These measurements indicate that single bubbles with radii as large as 7-8 mm may be entrained in this fresh-water system by moderate spilling breakers. Detailed measurements of the bubble size distribution of the bubble cloud enabled us also to obtain a measurement of the average void fraction in the cloud. These observations reveal that the sources of sound in laboratory spilling breakers is due mostly to single bubble oscillations that can have frequencies as low as 400 Hz; in the case of plunging breakers, it appears that both individual bubbles and bubble clouds can contribute to the acoustic emissions. The acoustic radiation from bubble clouds is the result of collective oscillations of the bubbles, stimulated by large scale vortices created by the plunging breaker. The sound spectra, averaged over 100 breakers, reveal the following observations. First, the peak frequencies of the average sound spectra shifts from few kHz (weak, spilling breaker) to few hundred Hz (plunging breaker). Second, the sound pressure levels increase in all frequency bands with increasing breaker severity. Lastly, the high-frequency portions of the sound spectra have slopes of about 5-6 dB/oct, which are the slopes observed for the noise spectra of the ocean. These results provide considerable insight into the likely source mechanisms for ocean ambient noise. [ABSTRACT FROM AUTHOR]

The impact of a jet of water onto a still water surface results in the entrainment of large amounts of air and the eventual formation of a bubble plume. Densely populated bubble plumes are generated by dropping a fixed volume of water, held in a cylindrical container, onto a still-water surface. The detached bubble plume, which is roughly spherical in shape, then undergoes volume pulsation and radiates relatively large-amplitude, low-frequency sound. The results of a laboratory study of the noise produced by this process were reported previously by Kolaini et al. [J. Acoust. Soc. Am. 94, 2809-2820 (1993)]. In this paper we report the results of a field study of noise produced by this process in both fresh water (Lake Washington, WA) and salt water (Puget Sound, WA). Studies of acoustic emissions from transient bubble plumes as a function of cylinder parameters are described, with specific attention devoted to a comparison of the results obtained in salt and fresh water. The measurements, which exhibit good agreement with laboratory study, indicate that there is a correlation between the acoustic intensity radiated from bubble plumes and the total potential energy of the water jet. [ABSTRACT FROM AUTHOR]

In the absence of whitecapping, other physical mechanisms may contribute to the generation of high-frequency ambient noise. It has been suggested [Longuet-Higgins, in NATO Advanced Research Workshop on Sound Generation Mechanisms at the Open Surface (NATO, Geneva, 1987)] that capillary waves, with surface profiles that are peaked downward in the troughs and are relatively flat at the crests, can inject acoustically active bubbles into the ocean, and thus contribute to the ambient noise background. It has been demonstrated in the laboratory that bubble injection can be generated at the trough of capillary-gravity, short-fetched waves by blowing air over water contained in a long, narrow tank. Simultaneous in situ acoustic and high-speed video monitoring of the capillary-gravity waves demonstrate that these waves can produce acoustically active bubbles. The generation of capillary waves depends principally upon the surface tension, which can be changed by adding surface-active agents to the water. The bubble production rate per unit area of these capillary-gravity waves was measured, as well as the dependence of this rate on wind speed, laboratory wind fetch, and surface tension. It was determined that an increase in water salinity and a reduction in surface tension increases the bubble production rate. The spectra of radiated frequencies ranges from 1 kHz to over 100 kHz with a broadband peak located around 4 kHz. The measured spectral densities were weakly related to wind speed. The wind-speed threshold value for bubble production was determined to be approximately 8.6 m/s (14.6 m/s at 10-m level) in fresh water and salt water, which decreased to 8.1 m/s (13.8 m/s at 10-m level) with a surface tension of 40.5 dyn/cm. [ABSTRACT FROM AUTHOR]

The impact of a jet of water onto a still-water surface results in the entrainment of large amounts of air and the eventual formation of a bubble plume. Results from an experimental study of the noise produced by this process is presented. Preliminary results of this study were reported previously by Kolaini et al. [J. Acoust. Soc. Am. 89, 2452-2455 (1991)]. The densely populated bubble plumes were generated by dropping a fixed volume of water, held in a cylindrical container, onto a still-water surface. High-speed video images reveal the formation of a cylindrical bubble plume with a very high void fraction which grows in size until all the water is injected into the tank. As the leading end of the plume advances, a section of the plume separates near the crater region formed by the jet. After detachment, the separated plume, which is roughly spherical in shape, undergoes volume pulsations, and radiates relatively large-amplitude, low-frequency sound. The nature of the acoustic emissions from bubble plumes depends on the height of the water in the container, the container's radius, and the velocity of the impacting jet. The natural frequency of oscillation of an individual bubble plume is inversely proportional to the radius of the plume and ranges from a few tens of Hz to over 100 Hz depending upon the void fraction of air contained within the plume. Results obtained with salt water as well as with rough jets are also discussed. The high-speed video observations reveal that immediately following the bubble plume detachment, there is evidence of an axial jet directed downward into the bubble plume and an opposing jet directed upward into the crater formed by the impact. This jet appears to be the physical mechanism that drives the cloud into oscillation. Measurements indicate that the acoustic intensity radiated from bubble plumes correlate with the total potential energy of the water jet. [ABSTRACT FROM AUTHOR]

Toroidal bubbles can be formed by injecting a small volume of air impulsively through a nozzle placed underwater. As the air leaves the nozzle, the front face (which is initially moving with a large velocity) slows down within a short distance and time, and a jet of air moving from the back of the bubble at high speed then penetrates the front face, thus forming a toroidal bubble. These bubbles then move upwards with their plane perpendicular to the direction of motion. The ring radius increases while the cross-sectional area of the air core decreases; simultaneously, the fluid velocity on the surface of the toroid slows down due to viscous effects. The combination of these two effects causes the toroidal bubbles eventually to become unstable and to break into a number of small bubbles. This phenomenon can be observed for a number of conditions. During the formation of the toroid, low-frequency, relatively high-amplitude damped acoustic emissions were observed, with a shift to higher frequencies as a function of time. During toroid breakup, a number of bubbles within a fairly narrow size distribution were observed which emitted sound at higher frequencies and lower amplitudes than the toroid-formation sound. In this presentation some preliminary results are shown of the acoustic emissions of toroidal bubbles formed using a variety of nozzle diameters, air injection velocities and air volumes. [Work supported by ONR and ONT.] [ABSTRACT FROM AUTHOR]

The acoustical characteristics of an ''adult'' bubble encountering a turbulent flow are studied both exprimentally and theoretically. By injecting an ''adult'' bubble in a flow, generated by a submerged axisymmetric water jet, the acoustic reexcitation of the bubble with and without breakup may occur in the shear-induced flow region. Bubbles of various sizes were introduced into jets of various speeds by means of interchangeable hypodermic needles. Results of the role of the turbulent flow characteristics in determining the acoustic bubble response are discussed. The characterizations of both acoustical and the dynamics of bubbles encountering the turbulent flow field depend upon the estimated integral and microlength scales, the corresponding Reynolds numbers, and the cirtical Weber numbers for both bubble distortion and breakup. These parameters are examined both in fresh and salt water. A simple model is given to account for bubble acoustic radiation. The Rayleigh-Plesset equation was altered by incorporating the turbulent pressure fluctuation of the flow as a driving force of the bubble. Some conclusions are given about the nature of the turbulent flow field and conditions under which the bubble acoustic reexcitation may occur. [Work supported by ONR.] [ABSTRACT FROM AUTHOR]

Computer-controlled gravity waves were generated to break in an anechoic tank. A sequence of 15 incident bursts was directed at the bubble cloud entrained by each breaking wave. A burst has seven cycles of a sine wave of frequency ranging from 10 to 15 kHz. The interval between adjacent bursts is set at 0.1 s in order to avoid overlapping the bursts. By analyzing the data of the 15 sound scattering events, the fluctuation was significantly reduced. The bubble clouds were observed to be roughly semicylindrical. The bubble size distribution in an entrained cloud was obtained from video images of a bubble cloud. This experimental information of the shape, size, and bubble size distribution of a bubble cloud was employed as the input into the theory adapted from Sarkar and Prosperetti [J. Acoust. Soc. Am. 93, 3128-3138 (1993)] to estimate the theoretical scattering strength. A comparison between the experimentally measured and the theoretically estimated scattering strengths will be shown. [Work supported by ONR.] [ABSTRACT FROM AUTHOR]

The results of a laboratory experiment to characterize the backscattered sound field from the bubble cloud generated by spilling breaking waves are described. Gravity waves were generated by a computer controlled plunging-type wavemaker along the length of a 12.7-m-long channel where they were made to break in a 3.6 m×3.6 m×2.4 anechoic tank. An underwater F42A transducer with a special parabolic reflector was used to generate incident bursts of sound ranging from 15 to 40 kHz. This procedure was conducted in the presence of breaking and nonbreaking gravity waves in order to isolate the acoustic scattering strength of the bubble clouds from surface roughness. Detailed measurements of the average void fraction of the bubble cloud at the instant the sound was incident were made. These observations show that there is a significant increase in backscattering strength from the bubble cloud when compared to surface roughness. The scattering strength of the bubble cloud, with an average void fraction of 0.33%, is also shown to increase as the incident frequency approaches the resonant frequency of the individual bubbles in the cloud. A simple theory will be given to predict some of these observations. [ABSTRACT FROM AUTHOR]

Results of a laboratory experiment to characterize the underwater acoustic noise radiated from breaking waves in fresh and salt water are described. The underwater sound field radiated by various breaking waves intensities in fresh water in the range from 0.1 to 20 kHz were reported earlier [A. R. Kolaini and L. A. Crum, J. Acoust. Soc. Am. (in press); 92, 2349(A) (1992)]. These waves were generated by a computer-controlled plunging-type wavemaker and propagated along a 12.7-m-long channel where they were made to break at the midsurface of a 3.6-×3.6-×2.4-m anechoic tank. Specific attention is given to a comparison of the results obtained in salt and fresh water. The sources of acoustic radiation from bubble clouds, the average sound-pressure level, bubble clouds shape, bubble concentration, and size distribution in these medium are discussed. These results may provide considerable insight into the likely source mechanisms for ocean ambient noise. [ABSTRACT FROM AUTHOR]

Results of a laboratory experiment to characterize the sound propagation through a bubble screen are described. A bubble screen of various void fractions was positioned in the middle of an anechoic water tank. The cloud was driven using a broadband source and the acoustic pressure was measured on both sides of the layer. The results have revealed that there is significant attenuation near the resonance frequency of the bubbles contained within the screen. When the source frequency is near that of one of the ''collective-oscillation'' frequencies of the screen, sound pressure amplification may occur. In this paper the new findings corresponding to the collective mode amplification, high-frequency attenuation, as a function of the screen's physical properties are discussed. [Work supported by ONR.] [ABSTRACT FROM AUTHOR]

The impact of a jet of water onto a still water surface results in the entrainment of large amounts of air and the eventual formation of a bubble plume. The densely populated bubble plumes were generated by dropping a fixed volume of water, held in a cylindrical container, onto a still-water surface. The detached bubble plume, which is roughly spherical in shape, undergoes volume pulsations and radiates relatively large-amplitude, low-frequency sound. The results of laboratory study of the noise produced by this process were reported previously by Kolaini et al. [J. Acoust. Soc. Am. 89, 2452-2455 (1991)]. In this presentation, a field study of noise produced by this process in both fresh water (Lake Washington) and salt water (the Puget Sound) will be described. Studies of acoustic emissions from transient bubble plumes as a function of cylinder parameters will be described, with specific attention devoted to a comparison of results obtained in salt and fresh water. The measurements indicate that there is a correlation between the acoustic intensity radiated from bubble plumes and the total potential energy of the water jet. [Work supported by ONR.] [ABSTRACT FROM AUTHOR]

An experiment was designed to investigate the characteristics of sound produced by the turbulent excitation of bubbles. By injecting a bubble into a fully developed turbulent flow, generated by a submerged axisymmetric horizontal water jet, the excitation of bubbles and shear-induced fission/fusion may occur. In the case of shear-induced bubble distortion by a turbulent jet, nonlinear coupling between volume pulsation and surface oscillations occur at exactly the same frequency as the radial breathing mode of the bubble. Particle tracking velocimetry (PTV) is used to characterize the turbulent flow field. Two high-speed Kodak Ekta-Pro video cameras are used to capture, simultaneously, several 180-×180-×10-mm illuminated regions along the jet. An 8-W argon ion laser is utilized for illumination. The method provides not only a visualization of the various patterns and structures of the turbulent flow, but also yields quantitative instantaneous velocity data of the flow containing the bubble. The role of the turbulent flow characteristics in determining the acoustical bubble response is discussed. [Work supported by ONR.] [ABSTRACT FROM AUTHOR]

Results of a laboratory experiment to characterize the underwater sound field radiated from breaking waves are described. Waves are generated by a computer controlled plunging-type wave maker that propagate along a 40-ft-long channel and break at the surface of 12×12×8-ft anechoic water tank. The wave-maker parameters are controlled to produce spilling-type breakers with different breaker intensity (i.e., bubble cloud size). At smaller wave-maker amplitudes, a ''gentle'' spilling breaker can be produced to create a discrete number of bubbles. The number of bubbles and the sounds emitted by them can be measured and their type can be identified [H. Medwin and A. C. Daniel, J. Acoust. Soc. Am. 88, 408-412 (1990)]. Increasing the amplitude of the wave maker causes the breaker intensity to increase. The evolution of the bubble cloud size and its acoustic emissions are monitored by a high-speed video camera. The acoustic noise emitted at different breaker intensities by individual bubbles and by bubble clouds is presented and scaled to ocean breakers. Backscattering from bubble clouds was also observed using a parametric source. The scattered signal level was measured in the presence and absence of breaking waves as the insonifying frequency approached the collective oscillation frequencies of the bubble clouds. [Work supported by ONR.] [ABSTRACT FROM AUTHOR]