• The Journal of the Acoustical Society of Korea
• /
• v.27 no.3
• /
• pp.140-148
• /
• 2008

In this study, a numerical method for a time-domain acoustic wave backscattering analysis is established based on a physical optics and a Fourier transform. The frequency responses of underwater targets are calculated based on physical optics derived from the Kirchhoff-Helmholtz integral equation by applying Kirchhoff approximation and the time-domain signals are simulated taking inverse fast Fourier transform to the obtained frequency responses. Particularly, the adaptive triangular beam method is introduced to calculate the areas impinged directly by acoustic incident wave and the virtual surface concept is adopted to consider the multiple reflection effect. The numerical analysis result for an acoustic plane wave field incident normally upon a square flat plate is coincident with the result by the analytic time-domain physical optics derived theoretically from a conventional physical optics. The numerical simulation result for a hemi-spherical end-capped cylinder model is compared with the measurement result, so that it is recognized that the presented method is valid when the specular reflection effect is predominant, but, for small targets, gives errors due to higher order scattering components. The numerical analysis of an idealized submarine shows that the established method is effectively applicable to large and complex-shaped underwater targets.

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

• The Journal of the Acoustical Society of Korea
• /
• v.28 no.8
• /
• pp.806-814
• /
• 2009

This paper reviews a study about sound reflection control using an active sound absorber. An active sound absorber includes sound transmitting and receiving piezocomposite sensor layers molded by water tight epoxy, and connected with a feedback controller. The multi-layer sensors and the controller consists a closed feedback loop, whose intrinsic characteristics shows excellent impedance matching performance within specified frequency band, and consequently, minimizes reflection waves. Multilayer sound transmission model is derived based on one dimensional model, and its performance is verified with experiment using a pulse tube setup.

• Journal of the Institute of Electronics and Information Engineers
• /
• v.49 no.11
• /
• pp.110-117
• /
• 2012

As the demand for underwater systems providing pollution monitoring, marine ecosystem observation, surveillance monitoring is increased, acoustic modem for short-range underwater communication is spotlighted as one of significant research topics. Typically, in shallow water, it is so hard to analyze acoustic wave which undergoes spreading, absorption, reflection and scattering through transmission that there are limited advanced results. Furthermore, in order for the modem to be loaded in a fixed node or a moving vehicle in shallow water, its size should be small enough. In this paper, we address underwater acoustic channel model and design and implement an efficient micro acoustic modem which is adequate for short-range underwater communication. The developed modem is verified in a lake by varying working range and data rate up to 500 meters and 2 kbps, respectively.

• International Journal of Aeronautical and Space Sciences
• /
• v.18 no.4
• /
• pp.797-806
• /
• 2017

In this study, the reflection coefficient (RC) and the flame transfer function (FTF) were measured by applying acoustic excitation to a duct-type model combustor and were used to predict the frequency of the combustion instability (CI). The RC is a value that varies with the excitation frequency and the geometry of the combustor as well as other factors. Therefore, in this study, an experimentally measured RC was used to improve the accuracy of prediction in the cases of 25% and 75% hydrogen in a mixture of hydrogen and methane as a fuel. When the measured RCs were used, an unstable condition was correctly predicted, which had not been predicted when the RCs had been assumed to be a certain value. The reason why the CI occurred at a specific frequency was also examined by comparing the peak of the FTF with the resonance frequency, which was calculated using Helmholtz's resonator analysis and a resonance frequency equation. As the CI occurred owing to the interaction between the perturbation in the rate of heat release and that in the pressure, the CI was frequent when the peak of the FTF was close to the resonance frequency such that constructive interference could occur.

• The Journal of the Acoustical Society of Korea
• /
• v.20 no.4
• /
• pp.5-11
• /
• 2001

Acoustic characteristics of the wedge-shaped anechoic tiles, used as absorbing lining materials for an anechoic water tank, were investigated for different wedge-apex angles. The anechoic tile base has the dimensions of 400mm x 385mm x 15.5mm. In order to investigate anechoic effect, the wedge-apex angles 30° and 60° were selected by using a ray-tracing method. The reflection loss of the anechoic tiles with and without wedges were experimentally studied at normal incident sound waves in water. In this experiment, the reflection loss of wedge-shaped anechoic tiles with the optimum wedge-apex angle 30° is larger than one with the angle 60° and one without wedges. The experimental results show that the wedge-shaped anechoic tiles with the wedge-apex angle 30°, optimized by using ray-tracing method, turn out better absorbing lining materials of an anechoic water tank.

• The Journal of the Acoustical Society of Korea
• /
• v.43 no.3
• /
• pp.324-334
• /
• 2024

The Wavefront Curvature Ranging (WCR) is an estimation method for a source range from the wavefront curvature of acoustic waves. The conventional method uses trigonometry to estimate the source range by assuming the sound speed as a constant. Because of this assumption, range error occurs in the ocean environment where the bottom reflection is clearly separated. In order to reduce the range error, Matched Wavefront Curvature Ranging (MWCR) was proposed applying the sound speed structure in the ocean environment and Maximum Likelihood Estimation (MLE). The range error was reduced in the results of the simulation on the proposed method. In the future, this method will be applicable to the sonar system if the reliability of ranging is confirmed by measured signal.

• The Journal of the Acoustical Society of Korea
• /
• v.39 no.4
• /
• pp.364-370
• /
• 2020

Rocket noise generated from the exhaust plume produces the enormous acoustic loading, which adversely affects the integrity of the electronic components and payload (satellite) at liftoff. The prediction of rocket noise consists of two steps: the supersonic jet exhaust is simulated by a method of the Computational Fluid Dynamics (CFD), and an acoustic transport method, such as the Helmholtz-Kirchhoff integral, is applied to predict the noise field. One of the difficulties in the CFD step is to remove the boundary reflection artifacts from the finite computation boundary. In general, artificial damping, known as a sponge layer, is added nearby the boundary to attenuate these reflected waves but this layer demands a large computational area and an optimization procedure of related parameters. In this paper, a cost-efficient way to separate the reflected waves based on the two microphone method is firstly introduced and applied to the computation result of a laboratory-scale supersonic jet noise without sponge layers.

• The Journal of the Acoustical Society of Korea
• /
• v.38 no.5
• /
• pp.494-510
• /
• 2019

Underwater acoustic communication channel parameters consist of impulse response, delay spreading, scattering function, coherence bandwidth, frequency selective fading, coherence time and time variant magnitude fading statistics on which communication system modem and channel coding are designed. These parameters are influenced by sound velocity profile, platform motion and sea surface roughness in given acoustical oceanography condition. In this paper, channel model based on phasor, channel simulator, measurement and analysis method of channel parameters are given in a fixed source-to-receiver system and the parameters are analyzed using shallow water experimental data. For two different source-to-receiver ranges of 300 m and 600 m, the parameters are characterized by three multipaths such as a direct, a surface reflection path with time variant scattering and a bottom reflection path. The results present a channel modelling method of a fixed source source-to-receiver system, channel parameters measurement and analysis methods and a system design and performance assessment method in shallow water.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.8 no.1
• /
• pp.117-123
• /
• 1998

We investigated the effect of mechanical backside damage in Czochralski grown silicon wafer. The intensity of mechanical damage was evaluated by minority carrier recombination lifetime by laser excitation/microwave reflection photoconductivity decay method, photo-acoustic displacement method, X-ray section topography, and wet oxidation/preferential etching methods. The data indicate that the higher the mechanical damage intensity, the lower the minority carrier lifetime, and the photo-acoustic displacement values increased proportionally, and it was at Grade 1: Grade 2:Grade 3 = 1:19.6:41 that the normalized relative quantization ratio of excess photo-acoustic displacement in damaged wafer was calculated, which are normalized to the excess PAD from sample Grade 1.