• The Journal of the Acoustical Society of Korea
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• v.27 no.1
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• pp.33-39
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• 2008

Void fraction of air bubble in the seagrass bed by photosynthesis was estimated with sound speed dispersion. A field experiment was conducted at Seagrasss bed of which bottom type is sandy mud and 120 kHz CW waveform was transmitted to obtain backscattered signals from seagrass bed. The differences of the arrival time of received signal from seagrass bed were observed between day and night. The diurnal variation of arrival time was caused by sound speed dispersion of air bubble generated by photosynthesis of seagrass.

• The Journal of the Acoustical Society of Korea
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• v.28 no.3
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• pp.208-212
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• 2009

Ground wave is an acoustic wave propagating at a sediment sound speed in the case that sediment sound speed is constant with depth, which is explained by modal dispersion effects. In this paper, the ground wave in time domain is simulated using the ray-based approach, which is possible because the modal dispersion can be explained by the guiding of energy caused by reflection and refraction in the waveguide geometry. For a Pekeris waveguide, the ground wave can be interpreted as a sequence of head waves, called a head wave sequence [Choi and Dahl, J. Acoust. Soc. Am. 119, 3660-3668 (2006)]. The ground wave is simulated by convolution of the source signal with a channel impulse response of the head wave sequence, which is compared with simulated signals obtained via a Fourier synthesis of a complex parabolic equation (PE) field.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.17 no.2 s.119
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• pp.168-176
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• 2007

The mixture sound speed in bubbly fluids is highly dispersive due to differences of the density and compressibility between bubbles and fluids. The dispersion range in bubbly fluids expands to a higher frequency than the resonance frequency of an air bubble. A theoretical model was developed to compute the reduction of radiation noise that is generated by a force applied on an infinite flat plate using a bubble layer as a compliant baffle. For evaluating the effectiveness of a bubble layer in reducing the structure-borne noise of an infinite elastic plate, the noise reduction levels for various parameters such as the thickness of bubble layers, the volume fractions and the distribution types of bubbly fluids are calculated numerically. The noise reduction effect of an air bubble layer on an infinite flat plate is considerable level and similar to the tendency of dispersion of bubbly fluids. It is recommended that the thickness of a bubble layer should be increased with keeping an appropriate volume fraction of an air bubble for the most effective reduction of the radiation noise.

• The Journal of the Acoustical Society of Korea
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• v.28 no.3
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• pp.198-207
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• 2009

Distribution of cavitation bubbles relative to change of the sound speed and attenuation in the water was estimated using acoustic signal from 20 to 300 kHz in two cases that cavitation bubbles exist and do not exist. To study generation and extinction property of cavitation bubble, bubble distribution was estimated in three cases: change of rotation speed (3000-4000 rpm), surface area of blade ($32-98\;mm^2$) and elapsed time (30-120 sec). As a result, the radii of the generated bubbles ranged from 10 to $60{\mu}m$, and bubble radius of $10-20{\mu}m$ and $20-30{\mu}m$ was accounted for 45 and 25% of the total number of cavitation bubbles, respectively. And generation bubble population correlated closely with the rotating speed of the blades but did not correlate with the surface area of blade. It was observed that 80% of total bubble population disappeared within 2 minutes. Finally, acoustic data of bubble distribution was compared with optical data.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.111.2-111.2
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• 2011

While many astrophysical disks are vertically stratified and obey a polytropic equation of state, most studies on gravitational instability (GI) of flattened systems consider isothermal, razor-thin disks by taking vertical averages of disk properties. We investigate local GI of rotating pressure-confined polytropic disks with resolved vertical stratification by performing linear stability analysis. We find that the GI of vertically-stratified disks is in general a combination of conventional razor-thin Jeans modes and incompressible modes. The incompressible modes that dominate in the limit of the maximal disk compression require surface distortion and are an unstable version of terrestrial water waves. Disks with a steeper equation of state are found to be more Jeans unstable because they tend to have a smaller vertical scale height as well as a steeper temperature gradient corresponding to lower pressure support. GI depends more sensitively on the vertical temperature than density distribution. The density-weighted, harmonic mean, rather than the simple mean, of the adiabatic sound speed well describes the dispersion relation of horizontal modes, and thus is appropriate in the expression for Toomre Q stability parameter of razor-thin disks. We generalize Q into vertically-stratified disks, and discuss astrophysical application of our work.

• The Journal of the Acoustical Society of Korea
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• v.30 no.7
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• pp.402-407
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• 2011

The necessary and sufficient condition for causality of a physical system can be expressed as Kramers-Kr$\ddot{o}$nig (K-K) relation. K-K relation for acoustic wave is a Hilbert transforms pair between dispersion equations of phase speed and attenuation. In this study, we quantitatively compare the acoustic measurements in water-saturated glass beads for the frequency ranges from 400 kHz to 1.1 MHz with the predictions of differential form of K-K relation obtained by Waters et al. For media with attenuation obeying an arbitrary frequency power law, acoustic measurements show good agreements with the predictions of Kramers-Kr$\ddot{o}$nig relation.