• The Journal of the Acoustical Society of Korea
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• v.10 no.3
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• pp.13-18
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• 1991

In this study, the analysis technique of fracture stress using the reflection coefficient of SAW reflected from a brittle solid with surface crack has been studied. Fracture stress of brittle solid with surface crack has been obtained by the function of the critical stress intensity factor and the maximum normalized stress intensity factor of the crack in the body. And the maximum normalized stress intensity factor of a surface crack can be inferred from a measurement of reflection coefficient of SAW. In experiment, the surface cracks ranging from 0.5mm to 0.9mm in crack depth has been made at the center of each Pyrex disc, and the SAW wedge transducer has been set up for the pitch-catch mode. It has been compared the theoretical values of the fracture stress calculated from the reflection coefficient of SAW with the values of the fracture stress measured from UTM.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.23 no.5
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• pp.456-462
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• 2013

Eco-friendly material applied to building would be one of the materials which is must developed for global environmental conservation and reduction of carbon dioxide. For development of eco-friendly material, a cellulose absorber has been developed with waste paper through adjustment of various mix proportions. The developed cellulose absorber has been tested for its acoustic properties such as absorption coefficient and dynamic stiffness. The absorption coefficient was evaluated by developing six samples and using impedance tube and reverberation chamber. As a result of the evaluation, 0.64(NRC) was secured in absorption coefficient and 4.7 $MN/m^3$ was indicated in dynamic stiffness. Also, for practical use of developed absorbers as inner heartwood in drywall, comparison test of sound reduction index was performed with existing glass wool absorbers and constructed drywall of gypsum board. The results have shown 55 dB(Rw) of sound reduction index in glass-wool wall and 46 dB(Rw) in cellulose.

• The Journal of the Acoustical Society of Korea
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• v.34 no.5
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• pp.335-342
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• 2015

Phononic crystals are composite materials consisting of a periodic arrangement of scattering inclusions in a host material. One of the most important properties of phononic crystals is the existence of band gaps, i.e., ranges of frequencies at which acoustic waves cannot propagate through the structure. The present study aims to investigate theoretically and experimentally the acoustic band structures in two-dimensional (2D) phononic crystals consisting of periodic square arrays of stainless steel solid cylinders with a diameter of 1 mm and a lattice constant of 1.5 mm in water. The theoretical dispersion relation that depicts the relationship between the frequency and the wave vector was calculated along the ${\Gamma}X$ direction of the first Brillouin zone using the finite element method to predict the band structures in the 2D phononic crystals. The transmission and the reflection coefficients were measured in the 2D phononic crystals with 1, 3, 5, 7, and 9 layers of stainless steel cylinders stacked in the perpendicular direction to propagation at normal incidence. The theoretical dispersion relation exhibited five band gaps at frequencies below 2 MHz, the first gap appearing around a frequency of 0.5 MHz. The location and the width of the band gaps experimentally observed in the transmission and the reflection coefficients appeared to coincide well with those determined from the theoretical dispersion relation.

• The Journal of the Acoustical Society of Korea
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• v.33 no.5
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• pp.291-299
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• 2014

To probe the temperature elevation effect caused by ultrasound, a tissue mimicking phantom was newly suggested. A carrageenan gel was adopted to realize not only the required transparency for visualization but also acoustic characteristics similar to human tissue. To visualize the temperature elevation inside phantom, thermochromic film with a critical temperature of discoloration was introduced. Acoustic characteristics of the tissue mimicking phantom were examined when the concentrations of carrageenan and sucrose changed. As the results, the attenuation coefficient of the phantom could be controlled in the range of 0.44~0.49 dB/cm/MHz, and the acoustic impedance in the range of 1.52~1.77 Mrayls. We could control the acoustic characteristics of the phantom by different concentration of carrageenan and sucrose, and it was possible to examine the temperature elevation caused by ultrasound in the phantom. The suggested method was verified by noninvasively visualizing the temperature elevation due to planar and focused ultrasound using the fabricated phantom.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.22 no.2
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• pp.253-260
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• 2018

The acoustic communication channel in shallow underwater is typically shown as time-varying multipath fading channel characteristics. The received signal through channel transmission cause inter-symbol interference (ISI) owing to multiple components of different time delay and amplitude. To compensate for this, several techniques have been used, and one of them is acoustic equalizer. In this study, we used four equalizers - feed forward equalizer (FFE), decision directed equalizer (DDE), decision feedback equalizer (DFE) and combination DDE with DFE to compensate ISI. And we applied two adaptive algorithms to adjust coefficient of equalizers - normalized least mean square algorithm and recursive least square algorithm. As result, we found that it has a significant performance improvement over 6 dB on SNR in nonlinear equalizer. By combination of DFE and DDE has almost best performance in any case.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.48 no.5
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• pp.798-804
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• 2015

The distributions of demersal fish along the coast of the South Sea, Korea, were estimated from a hydroacoustic survey and bottom trawl catches in April of 2015. Acoustic data were collected at 38 kHz, and converted into the nautical area scattering coefficient (NASC, m²/n·mile²) for 0.1 n∙mile along eight transects during day and nighttime. Various demersal fish species were collected and the dominant species comprised no more than half of the catch. The estimated acoustic density of demersal fish compared well with the cumulative catch weight from bottom trawl catches in daytime, but showed a low correlation at nighttime. The NASC of pelagic fish determined for anchovy was correlated with water temperature. The combined hydroacoustic and trawl method can be used to understand the spatial/temporal structure and estimate the density of demersal fish in coastal areas.

• Ocean and Polar Research
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• v.28 no.3
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• pp.225-236
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• 2006

Seagrass meadows are considered as critical habitats for a wide variety of marine organisms in coastal and estuarine ecosystems. In many cases, studies on the spatial/temporal distribution of seagrass have depended on direct observations using SCUBA diving. As an alternative method fur studying seagrass distribution, an application of hydroacoustic technique has been assessed for mapping seagrass distribution in Dongdae Bay, on the south coast of Korea, in September 2005. Data were collected using high frequency transducer (420 kHz split-beam), which was installed with towed body system. The system was linked to DGPS to make goo-referenced data. Additionally, in situ seagrass distribution has been observed using underwater cameras and SCUBA diving at four stations in order to compare with acoustic data. Acoustic survey was conducted along 23 transects with 3-4 blot ship speed. Seagrass beds were vertically limited to depths less than 3.5m and seagrass height ranged between 55 and 90cm at the study sites. Dense seagmss beds were mainly found at the entrance of the bay and at a flat area around the center of the bay. Although the study area was a relatively small, the vertical and spatial distributions of the seagrass were highly variable with bathymetry and region. Considering dominant species, Zostera marina L., preliminary estimation of seagrass biomass with acoustic and direct sampling data was approximately $56.55g/m^2$, and total biomass of 104 tones (coefficient variation: 25.77%) was estimated at the study area. Hydroacoustic method provided valuable information to understand distribution pattern and to estimate seagrass biomass.

• Ocean and Polar Research
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• v.30 no.2
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• pp.181-191
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• 2008

This study aims at assessment of spatio-temporal distribution of demersal fish aggregations near the west coast of Jeju Island using hydroacoustic survey. A 200 kHz split beam transducer attached to a small towed body was used for all acoustic investigations. The received acoustic data were in situ acoustic target strength (TS, dB) for all pings and nautical area scattering coefficient(NASC, $m^2/mile^2$) for 0.1 mile along 12 acoustic transects. Demersal fish aggregations are distributed around the coastal slope having 20 to 30 m depth throughout all seasons. The concentration is higher during the summer season. With regard spatial distribution, higher demersal fish aggregations have been detected near the West coast of Shinchang and especially near Chagwi-do. Pelagic fish aggregations were higher to the south of Chagwi-do during the spring season. Additionally, standing stock of demersal fish aggregations from the NASC data, TS function, and length-weight function of dominant species was estimated as follows: 3.2 ton (CV 21.8%) in December 2006, 17.9 ton (CV 21.6%) in April 2007, 30.8 ton (CV 17.8%) in June 2007, and 22.5 ton (CV 24.2%) in October 2007. The application of hydroacoustic methods offers a new approach to understanding spatiotemporal structure and estimate the biomass of demersal fish aggregations in the coastal area. And the results can be made up limitations of qualitative analysis through net and diving for fisheries resources survey in coastal area.

• Journal of the Korean Society of Laryngology, Phoniatrics and Logopedics
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• v.13 no.1
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• pp.28-32
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• 2002

Background and Objective : Classical singing students who have received in systematic voice training appeared distinctive voice characteristics compared to normal people who have not received in systematic voice training. The purpose of this study was to determine the correlation between acoustic parameters and Electroglottography(EGG) parameters in two groups(ordinary college students vs. classical singing students group). Materials and Methods : The 80 ordinary college students and 65 classical singing students participated in this study by utilizing Dr. speech program to obtain acoustic measurements and physiologic measurements simultaneously. The Pearson correlation coefficient was used to find the correlation between acoustic parameters and EGG parameters in two groups(ordinary college students group and classical singing students group). Results : The results of the study were as follows : First, there was no correlation between Jitter and EGG Jitter in ordinary college students group, but there was strong correlation between Jitter and EGG Jitter in classical singing students group. Second, there was no correlation between Shimmer and EGG Shimmer in ordinary college students group, but there was strong correlation between Shimmer and EGG Shimmer in classical singing students group. Third, there was no correlation between Harmonic to Noise Ratio(HNR) and EGG HNR in ordinary college students group, but there was strong correlation between HNR and EGG HNR in classical singing students group. Finally, there was no correlation between Normalized Noise Energy(NNE) and EGG NNE in two groups.

• The Journal of the Acoustical Society of Korea
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• v.33 no.1
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• pp.1-9
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• 2014

Coastal water including estuaries has distinctive environmental characteristics where sediments are transported and deposited by flowing river water, providing an environment in which fluid mud layers can be formed. Acoustic method is mostly used to detect or monitor the fluid mud layer. However, since sound propagating in this layer suffers severe attenuation, it is important to estimate the accurate attenuation coefficient for various concentrations of fluid mud layer for the successful use of the acoustic method. In this paper, measurement results of attenuation coefficient for 3.5, 5, and 7.5 MHz ultrasounds were presented. The measurements were made in a small-size water tank in which suspended sediment samples with various sediment concentrations were formed using kaolinite powder. The results were compared to the model predictions obtained by attenuation coefficient model in which the mean grain size (called as Mass-median-diameter, D50) was used as input parameter. There were reasonable agreements between measured attenuation coefficients and model outputs predicted using the particle range of D50 ${\pm}20%$. The comparison results imply that although the suspended sediments consist of various-sized particles, sound attenuation might be greatly influenced by amount of particle with a size which has a larger attenuation than that of any particle in the suspended sediments for the frequency used.