• Korean Journal of Fisheries and Aquatic Sciences
• /
• v.42 no.2
• /
• pp.165-170
• /
• 2009

Recently, wide spread distribution of the giant jellyfish, Nemopilema nomurai, has occurred in the East China Sea. This increased distribution has caused serious problems in inshore and offshore fisheries in Korea and Japan. As a result, it is necessary to evaluate the damage caused to the fisheries by jellyfish. Accordingly, several hydroacoustic studies have been conducted to estimate the target strength (TS) of the giant jellyfish. However, the effects of fluctuation in the acoustic scattering characteristics on swimming patterns have not yet been elucidated. Therefore, in this study, we theoretically estimated the effects of changes in the acoustic scattering pattern on the swimming behavior of jellyfish using the Distorted Wave Born Approximation (DWBA) model. The results confirmed that acoustic scattering of jellyfish results in a significant change in their swimming pattern. Specifically, our theoretical estimation indicated that the TS of giant jellyfish (d=40 cm) fluctuated until 8.5 dB at 38 kHz, 13.8 dB at 70 kHz, and 15.1 dB at 120 kHz based on changes in their swimming patterns.

• Korean Journal of Fisheries and Aquatic Sciences
• /
• v.43 no.4
• /
• pp.380-385
• /
• 2010

Several theoretical acoustic scattering models were applied to estimate the target strength (TS) for assessing the biomass of zooplankton, to overcome the difficulty of direct measurements. Acoustical scattering characteristics of copepods were estimated using three theoretical models, and the application of the models was evaluated for four frequencies of a scientific echo sounder. The scattering directivity by the body shapes of copepods at 200 kHz and 420 kHz was significantly affected by TS patterns. Averaged TS, however, were similar at higher frequencies. Consequently, a low frequency model, such as a truncated fluid sphere model, provides a good acoustical biomass estimation. The regressions of TS and 30 logL were < $TS_{200\;kHz}$ >= 30logL-118.4 ($R^2=0.716$) and < $TS_{420 kHz}$ > =30 logL-108.8 ($R^2=0.758$), respectively.

• Journal of the Korean Society for Nondestructive Testing
• /
• v.30 no.6
• /
• pp.548-557
• /
• 2010

An ultrasonic resonance scattering spectroscopy technique is developed and applied for reconstructing elastic constants of a transversely isotropic cylindrical component. Immersion ultrasonic measurements are performed on tube samples made from a boron/aluminum composite material to obtain resonance frequencies and dispersion curves of different guided wave modes propagating in the tube. Theoretical analysis on the acoustic resonance scattering from a transversely isotropic cylindrical tube is also performed, from which complete backscattering and resonance scattering spectra and theoretical dispersion curves are calculated. A sensitive change of the dispersion curves to the elastic properties of the composite tube is observed for both normal and oblique incidences; this is exploited for a systematic evaluation of damage and elastic constants of the composite tube samples. The elastic constants of two boron/aluminum composite tube samples manufactured under different conditions are reconstructed through an optimization procedure in which the residual between the experimental and theoretical phase velocities (dispersion curves) is minimized.

• Transactions of the Korean Society for Noise and Vibration Engineering
• /
• v.23 no.6
• /
• pp.563-573
• /
• 2013

The ship radiated noise level fluctuates by the interference between direct and reflected paths. The effect of sea surface reflection path on interference depends strongly on sea surface roughness. This paper describes error characteristics of ship acoustic signature estimation by sea surface scattering effect. The coherent reflection coefficient which explains a magnitude of sea surface scattering and its resultant interference acoustic field is analyzed quantitatively as a function of a grazing angle, effective surface height, frequency, source-receiver range and depths of source and receiver. Theoretical interference acoustic field is compared with experimental result for two different sea surfaces and five different frequencies by changing source-receiver range. It is found that both matches well each other and a magnitude of interference acoustic field is decreasing by increasing a grazing angle, effective surface height, frequency, and depths of source and receiver and decreasing source-receiver range. For given experimental conditions, the transmission anomaly which is a bias error of ship acoustic signature estimation, is about a range of 1~3 dB. The bias error of an existing ship radiated noise measurement system is also analyzed considering wind speed, source depth and frequency.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
• /
• 2007.05a
• /
• pp.1096-1099
• /
• 2007

Hydrophone system has necessarily additional structures to resist installation and operation for military usage. In this paper, we assume that the hydrophone is a rigid circular cylinder and the neighborhood structure is a perfect reflector. Scattering fields by hydrophone and neighborhood structure are investigated to use a boundary element analysis program, SYSNOISE, which has an acoustic analysis capability. The pressure fields around circular cylinder with respect to the angle are evaluated.

• Proceedings of the Acoustical Society of Korea Conference
• /
• spring
• /
• pp.347-348
• /
• 2004

Nonlinear scattering of difference frequency acoustic wave in a water-saturated sandy sediment was investigated. Difference frequency acoustic wave was observed to be scattered due to the nonlinearity of water-saturated sandy sediment when the collinear acoustic waves with two different fundamental frequencies are incident on the sediment. The pressure level of the difference frequency acoustic wave was 6 dB higher than the background noise level. It seems very useful to evaluate the nonlinear parameter of water-saturated sandy sediment without disturbing the sediment. Such nonlinear acoustic response of water-saturated sandy sediment can be used as background acoustic data for estimating the gas void fraction in marine gassy sandy sedimen.

• Journal of the Optical Society of Korea
• /
• v.14 no.4
• /
• pp.403-408
• /
• 2010

The acoustic properties of anti-inflammatory egonol were investigated by using micro-Brillouin scattering spectroscopy, by use of a 6-pass tandem Fabry-Perot interferometer and an optical microscope specially modified for spectroscopic purposes. The measured Brillouin spectrum was composed of a central peak centered at zero and a Brillouin doublet arising from the longitudinal acoustic waves, i.e. propagating density fluctuations. For the first time, the glass transition of egonol was identified to be about $5^{\circ}C$ at which the Brillouin peak position and the half width showed abrupt changes. The substantial damping of acoustic phonons of egonol near the glass transition temperature indicated that the contribution of internal relaxation processes such as small-amplitude librations of side chains to the damping of acoustic phonons may be substantial depending on the internal structure of molecules.

• The Journal of the Acoustical Society of Korea
• /
• v.22 no.4
• /
• pp.250-260
• /
• 2003

Since a bubble in water is a highly nonlinear acoustic scatterer, the acoustic scattered waves from underwater bubbles show highly nonlinear acoustic properties. These acoustic scattered waves can be observed at the second or higher harmonics as well as at the fundamental primary frequency of incident acoustic wave. When two primary acoustic waves of different frequencies are incident on a bubble, the acoustic scattered waves can be also observed at the sum and the difference frequencies of the primary waves. In this study, when the two primary acoustic waves were incident on a bubble screen in water, we observed that the amplitude of difference frequency wave was amplified by the bubble nonlinearity and its directivity was oriented in the propagation directions of primary waves. The directivity of scattered difference frequency wave was analyzed as a coherent scattering for virtual source by using the directivity of the primary acoustic wave.

• Bulletin of the Korean Mathematical Society
• /
• v.33 no.2
• /
• pp.177-186
• /
• 1996

Colton and Wendland [1] have considered acoustic wave propagations in a spherically symmetric medium. They used constructive method for in a spherically symmetric medium. They used constructive method for solving the exterior Neumann problem. Jones [2] has derived an integral equation for the exterior acoustic problem. Jones has also discussed analytical and numerical solution of the acoustic problem.

• The Journal of the Acoustical Society of Korea
• /
• v.22 no.1
• /
• pp.69-77
• /
• 2003

Measurements of backscattered intensity were made over a shallow water using 300 ㎑and 1200 ㎑ bottom mounted ADCP (Acoustic Doppler Current Profiler) to determine the temporal variability of vertical distribution of high-frequency volume scattering strength (Sv). The variability of Sv in relatively deep water column(85 m and 113 m was due to the daily vertical migration, probably of larger zooplankton. However it was not found with 1200㎑ data at shallow water column. From the empirical orthogonal function (EOF) analysis using 1200㎑ data, the vertical distribution of the first mode eigenvectors of Sv is characterized by the presence of the maximum values near the bottom of the water.