Bok Kyoung Choi

Snapping Shrimp Sound Measured Under Laboratory Conditions

The typical temporal waveforms and spectra of the sounds produced by the three species of snapping shrimp with different claw shapes and almost the same claw lengths were investigated under laboratory conditions. The sound spectra generated by one species of snapping shrimp were also investigated for various claw lengths. For the three species of snapping shrimp, their typical temporal waveforms were similar, but their sound spectra and pulse durations differed. This difference was related to the size of the single cavitation bubble, which was generated by the high-speed water jet emitted from the snapping shrimp claw. The range of difference in the times between the snapping shrimp claw closure and the collapse of the single cavitation bubble for the three species of snapping shrimp seemed to be determined by the difference in the claw shape. The collapse time, the equilibrium radius, and the maximal radius of the cavitation bubble for each species were estimated from the first peak frequency component in the snapping shrimp sound spectrum. For one species of snapping shrimp, the peak frequency components in the sound spectra were observed for various claw lengths and their superposition could be considered as the cause that the broad peak frequency components were variously observed in the averaged snapping shrimp sound spectra, which were measured in many shallow water areas.

Acoustic Characteristics of the Snapping Shrimp Sound Observed in the Coastal Sea of Korea

The typical temporal waveforms, spectra, and water-temperature dependence of the snapping shrimp sound in the coastal sea of Korea were investigated. The peak-to-peak source levels of the snapping shrimp sound at three sites in the coastal sea, at water depths of 8, 10, and 40 m, were also investigated. The waveform and dominant frequency response band of the sound were similar to those observed under laboratory conditions. The broad peak frequency components of the normalized average snapping shrimp sound spectra measured in nine different coastal sea areas were differently determined by superposition of the peak frequency components of individual snapping shrimp spectra for the each area. The sound did not significantly affect the ambient noise level at a low seawater temperature (<10 °C). The mean peak-to-peak source levels of the snapping shrimp sound at the sites with water depths of 8 and 10 m were similar, with estimated values of 176 ±3 and 175 ±3 dB (re 1 µPa at 1 m), respectively. However, the estimated mean peak-to-peak source level at the site with a water depth of 40 m was 184 ±2 dB (re 1 µPa at 1 m). The differences in the mean peak-to-peak source levels at the three sites could be caused by the differences in the ambient pressure at the sea bottoms of the sites. For water depths below 10 m, the mean peak-to-peak source levels of the snapping shrimp sound were much lower than those estimated in a previous study, in which the water depth at the experimental site was 6.1 m. On the other hand, the mean peak-to-peak source level at the site with a water depth of 40 m was similar to that estimated in a previous study. This study shows that the mean peak-to-peak source levels of the snapping shrimp sound may change as water depth is largely varied.

Acoustic bubble counting technique using sound speed extracted from sound attenuation

Sound attenuation has been solely used to estimate bubble size distributions of bubbly water in the conventional acoustic bubble sizing methods. These conventional methods are useful for the void fraction around 10/sup -6/ or lower. However, the change of compressibility in the bubbly water also should be considered in bubble sizing for the void fraction around 10/sup -5/ or higher. Recently the sound speed as well as sound attenuation was considered for acoustic bubble size estimation in bubbly water. In this paper, the sound speed estimated from sound attenuation in bubbly water by an iterative method is used for a bubble counting. This new iterative inverse bubble sizing technique is numerically tested for bubble distributions of single-size Gaussian, and power-law functions. The numerical simulation results are in agreement with the given bubble distributions even for the high void fractions of 10/sup -4/-10/sup -3/. It suggests that the iterative inverse technique can be a very powerful tool for practical use in acoustic bubble counting in the ocean.

Effect of the Geoacoustic Parameters on the Range-Frequency Interference in Shallow-Water Waveguide

In this study, range-frequency interference is analyzed in the propagation of acoustic waves in shallow-water waveguides. The interference pattern is examined using image processing techniques to produce the waveguide-invariant parameter β. The variability of the β-distributions of four geoacoustic parameters in a sediment layer and four typical sediment types is characterized by skewness and kurtosis. The result shows that a low value of skewness and a high value of kurtosis appear in the hard-sediment case and a high value of skewness and a low value of kurtosis appear in the soft-sediment case. This analysis technique will be useful in geoacoustic parameter inversion.

Seawater Temperature and Wind Speed Dependences and Diurnal Variation of Ambient Noise at the Snapping Shrimp Colony in Shallow Water of Southern Sea of Korea

The seawater temperature and wind dependences and diurnal variation of the ambient noise at the snapping shrimp colony in shallow water of the southern sea of Korea were investigated. The ambient noise levels are significantly affected by the snapping shrimp sound, when the bottom seawater temperature increases and the wind speed decreases. However, they are not exceptively almost affected by the snapping shrimp sound when the wind speed decreases at low seawater temperatures (<10 °C). In diurnal variation, the ambient noise levels are also significantly affected by the snapping shrimp sound in the morning and night time zones. This study shows that the activity of the snapping shrimp affecting the variation in ambient noise level in shallow water can be related to the wind speed as well as the seawater temperature. This study also shows that the snapping shrimp in diurnal activity can be more active in the morning and night time zones.

Active and passive acoustic roles of bubbles in the ocean

The agitation of the ocean surface wave action produces bubbles of various sizes in the upper layers and then forms bubble clouds down to tens of meters by Langmuir circulation, turbulence and other mechanisms in the ocean. Such bubbles may act collectively as well as individually as ambient noise sources and strong acoustic scatterers. Recently the production of bubble clouds was verified to be a more-likely mechanism of low-frequency ambient noise generation in the ocean. In the present paper we discuss sound speed variation in bubbly mixtures due to compressibillty change. Active and passive acoustic roles of bubbles in the ocean are also discussed reviewing experimental measurements of bubble cloud oscillations and sound propagation through bubbly mixtures.

Gas Void Fraction Estimation in Gas-Bubble-Contained Sands with Difference Frequency Waves

A nonlinearity parameter can be used for the estimation of the gas void fraction in gassy sediment. If two primary acoustic waves of different frequencies are incident on gassy sediment, nonlinear acoustic waves can be generated at the difference frequency. In the present study, the parametric acoustic array theory for the difference frequency wave was employed to estimate the nonlinearity parameters of gas-bubble-contained sands under laboratory conditions. The nonlinearity parameters of the sands were estimated between 103 and 104. Then, the estimated gas void fractions in the sands were between 10-6 and 10-4. They were similar to those estimated through the sound speed variation method. However, this linear acoustic method could not be applied to gas-bubble-contained sand with a low gas void fraction, because the sound speed variation was not observed in the sand. This study suggests that the nonlinear acoustic method seems very feasible to estimate the gas void fraction in gassy sediment with a low gas content.

Study of underwater sound propagation in the East China sea at spring 2015

In this study, we investigated the effect of water temperature inversion layer on the propagation of acoustic wave in the western coastal sea of Jeju island in April 2015. When the acoustic source and receiver are simultaneously located within the water temperature inversion layer depth, the long range propagation of acoustic wave is confirmed by numerical modeling. This is caused by the duct effect due to the water temperature inversion phenomenon. For the experimental area without the water temperature inversion layer, when the acoustic source and receiver are simultaneously located below thermocline depth, the long range propagation of acoustic wave is also confirmed. This is generally caused by the seasonal water temperature profile.

Effects of water temperature inversion layer on underwater sound propagation in the East China Sea

In the northern side of East China Sea it exists Tsushima Current into the East Sea through the Korea Strait that a branch of Kuroshio Current moving from the Western Kyushu in Japan are coming up to the north. The others pass through the western coastal sea of Jeju island, performing Yellow Sea Warm Current into the Yellow Sea. A part of Yellow Sea Warm Current turns lockwise round in the western coastal sea of Jeju island. This becomes to Jeju Warm Current passing to the Korea Strait[1]. The complicated ocean processes are existed in East China Sea on account of large fresh water inflow from landside[1]. The area to meet a different water mass is called coastal upwelling[2], and near these area the boundaries show complicated ocean physical characteristic such as temperature front. This study was analyzed the temperature inversion phenomenon by tidal current and Jeju Warm Current of the western coastal sea of Jeju island into the complicated northern coastal sea of East China Sea, and researched the underwater sound propagation by the temperature inversion.

Real-time wave height measurements using a cable type wave monitoring system in shallow waters

We developed a cable type wave monitoring system for real-time observation of wave height in the coastal zone, comprising nine pressure sensors and a total cable length of 500 m. The resulting wave height observations were discontinuous. Thus, we developed a new interpolation algorithm using shallowwater wave speed. The resulting temporal and spatial variation in wave height was very realistic, but the algorithm is applicable only between sensors spaced ≤ 100 m apart.