Theodore F. Argo

Sound propagation in water containing large tethered spherical encapsulated gas bubbles with resonance frequencies in the 50 Hz to 100 Hz range

The efficacy of large tethered encapsulated gas bubbles for the mitigation of low frequency underwater noise was investigated with an acoustic resonator technique. Tethered latex balloons were used as the bubbles, which had radii of approximately 5 cm. Phase speeds were inferred from the resonances of a water and balloon-filled waveguide approximately 1.8 m in length. The Commander and Prosperetti effective-medium model [J. Acoust. Soc. Am. 85, 732-746 (1989)] quantitatively described the observed dispersion from well below to just below the individual bubble resonance frequency, and it qualitatively predicted the frequency range of high attenuation for void fractions between 2% and 5% for collections of stationary balloons within the waveguide. A finite-element model was used to investigate the sensitivity of the waveguide resonance frequencies, and hence the inferred phase speeds, to changes in individual bubble size and position. The results indicate that large tethered encapsulated bubbles could be used mitigate low frequency underwater noise and that the Commander and Prosperetti model would be useful in the design of such a system.

Measurement of the resonance frequency of single bubbles using a laser Doppler vibrometer

The behavior of bubbles confined in tubes and channels is important in medical and industrial applications. In these small spaces, traditional means of experimentally observing bubble dynamics are often impossible or significantly perturb the system. A laser Doppler vibrometer (LDV) requires a narrow (<1 mm diameter) line-of-sight access for the beam and illumination of the bubble does not perturb its dynamics. LDV measurements of the resonance frequency of a bubble suspended in a small tank are presented to illustrate the utility of this measurement technique. The precision of the technique is similar to the precision of traditional acoustic techniques.

Acoustic radiation patterns of mating calls of the túngara frog (Physalaemus pustuosus): Implications for multiple receivers

In order for a signal to be transmitted from a sender to a receiver, the receiver must be within the active space of the signal. If patterns of sound radiation are not omnidirectional, the position as well as the distance of the receiver relative to the sender is critical. In previous measurements of the horizontal directivity of mating calls of frogs, the signals were analyzed using peak or root-mean-square analysis and resulted in broadband directivities that ranged from negligible to a maximum of approximately 5 dB. Idealized laboratory measurements of the patterns of acoustic radiation of the mating calls of male tungara frogs (Physalaemus pustulosus), along axes relevant to three receivers in this communication network, female frogs in the horizontal plane, and frog-eating bats and blood-sucking flies above the ground, are reported. The highest sound pressure level was radiated directly above the frog, with a 6 dB reduction radiated along the horizontal direction. Band-limited directivities were significantly greater than broadband directivities, with a maximum directivity of 20 dB in the vertical plane for harmonics near 6 kHz. The implications with regard to mating and predator-prey interactions are discussed.

Mitigation of low‐frequency underwater sound using large encapsulated bubbles and freely‐rising bubble clouds.

Low-frequency anthropogenic noise may affect marine life, motivating the need to minimize its potential impact. Bubbles cause significant dispersion and attenuation of underwater sound at frequencies near the individual bubble resonance and can potentially be used to abate this noise. Such effects have been reported for large encapsulated bubbles with resonance frequencies below 100 Hz, and significant attenuation due to bubble resonance phenomena and acoustic impedance mismatch was observed in a tank experiment [J. Acoust. Soc. Am. 127:2015 (2010); J. Acoust. Soc. Am. 128:2279 (2010)]. Both of these mechanisms were found to significantly reduce down-range radiated acoustic pressure, as much as 40 dB, at low frequencies (60 to 1000 Hz) in a series of lake experiments where a sound source was surrounded by an array of tethered resonant toroidal air bubbles, a cloud of freely-rising sub-resonant bubbles, and various combinations of the two. Hydrophones were placed at various depths and ranges to determine the effect of the bubbles on the radiated field. The effects of void fraction and bubble size variation on the spectrum of the radiated sound were also investigated. [Work supported by Shell.]

Attenuation of low-frequency underwater sound using bubble resonance phenomena and acoustic impedance mismatching

Air bubbles can be a significant source of attenuation in underwater sound propagation, but such effects have not been experimentally verified for low frequencies in part due to the difficulty in creating large stable bubbles. This work is in part an extension of a previously reported study on the acoustic effects of large resonant encapsulated air bubbles in a 1‐D waveguide [J. Acoust. Soc. Am. 127, 2015 (2010)]. Now, both bubble resonance effects and impedance contrast effects are investigated in a large (2600 m3) tank. Both of these mechanisms are shown to attenuate sound at low frequencies (50–200 Hz) through experiments in which a sound source is surrounded by a column of freely rising sub‐resonant bubbles, a matrix of tethered resonant air balloons, and a combination of the two. Experiments with a matrix of thicker‐shelled encapsulated bubbles demonstrate decreased attenuation due to weaker resonant interaction. The use of a polydipserse versus monodisperse encapsulated bubble size distribution demo...

Sound speed in water-saturated glass beads as a function of frequency and porosity

Sound propagation in water-saturated granular sediments is known to depend on the sediment porosity, but few data in the literature address both the frequency and porosity dependency. To begin to address this deficiency, a fluidized bed technique was used to control the porosity of an artificial sediment composed of glass spheres of 265 μm diameter. Time-of-flight measurements and the Fourier phase technique were utilized to determine the sound speed for frequencies from 300 to 800 kHz and porosities from 0.37 to 0.43. A Biot-based model qualitatively describes the porosity dependence.

Implications of the presence of shell hash on the speed and attenuation of sound in water-saturated granular sediments

The propagation of sound through water-saturated granular sediments has been widely studied, yet there is no consensus on an expected wave speed and attenuation in these materials owing to variation in both physical properties and test methods used for their interrogation. One aspect confounding model predictions is the presence of inhomogeneities such as rocks, bubbles, and biological organisms in an otherwise homogeneous sediment. In a laboratory setting, it is possible to control both manufacture of artificial sediments with specific inclusions and the measurement method used to observe their properties. To study the effect of inclusions on the speed of sound and attenuation in an otherwise homogeneous sediment, shells were systematically added to a sand sediment. The volume fraction of shells relative to sand grains was varied and the speed of sound and attenuation was measured using a time of flight technique from 200 to 800 kHz. Results will be compared to both sediment acoustic models and scatterin...

Laboratory measurements of sound speed and attenuation in water‐saturated sand and glass beads.

The exact nature of sound propagation in water‐saturated granular sediments, across the range of frequencies of interest in underwater acoustics, remains insufficiently understood. Well‐controlled laboratory measurements are useful for validation and continued development of predictive models. Toward this end, a time‐of‐flight technique operating in the 250 kHz–750 kHz range was used to determine the sound speed and attenuation in a variety of artificial sediments including sand and glass beads of varying grain size and varying levels of homogeneity. Measurements will be presented and compared to existing model predictions.

Acoustic behavior of large encapsulated gas bubbles with resonance frequencies in the 50 to 100 Hz range

A one‐dimensional resonator technique was used to investigate the acoustic behavior of water containing large spherical and toroidal encapsulated air bubbles. Tethered balloons and inner tubes, respectively, were used to create the bubbles, which had effective spherical radii of approximately 5 cm. The number of balloons or inner tubes varied from 3 to 6, which resulted in void fractions from 1.5% to 5%. Effective mixture sound speeds for both shapes were inferred from the resonances of a water and bubble‐filled waveguide 1.8 m in length. For the spherical bubbles, the Commander and Prosperetti (CP) model [J. Acoust. Soc. Am. 85, 732–746 (1989)] quantitatively described the measured sound speed dispersion approaching the individual bubble resonance frequency and qualitatively predicted the frequency range of high attenuation, even in this regime of high‐void fractions and discrete bubble distribution within the waveguide. Qualitative agreement was found between the CP model and the toroidal bubble measure...

Low‐frequency laboratory measurements of sound speed in water‐saturated granular sediments

Accurate knowledge of the acoustic behavior of water‐saturated granular sediments is required for effective sonar operation in certain environments. Acoustic dispersion at frequencies below about 20 kHz has been the topic of much recent study and debate. While the understanding of real ocean‐bottom sediments in their full complexity is the ultimate goal, laboratory measurements are significantly less difficult and subject to less uncertainty. This plays an important role in sediment acoustics since the difference between the predictions of competing propagation models is only a few percent. A new laboratory impedance tube apparatus, which operates in the 0.5–10‐kHz frequency range, has been developed to address this issue. The device can be operated as a traditional acoustical impedance tube, in which water‐borne sound waves interact with the water‐saturated sediment. The device can also be operated as a driving‐point impedance instrument, in which the mechanical input impedance of a column of water‐satur...