• The Journal of the Acoustical Society of Korea
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• v.38 no.3
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• pp.266-274
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• 2019

As can be seen in the statistics on domestic performance facilities, small or large scale of acoustic renovations seem to be unavoidable in many of those facilities due to its senility. However it is hard to find case studies which can be referred to for its acoustic renovation process. This study tried to provide a case study example of acoustic renovation for a high school auditorium which has many restrictions as a cultural heritage building. Requirements from the client and acoustic problems found through a field measurement of existing space motivated acoustic design solutions and alternatives which can improve the acoustics of the space. The whole process from the initial stage of diagnosing acoustic problems to the final stage of a post acoustic evaluation after the completion of construction was described. The result from the post acoustic evaluation suggested that reverberation characteristics of the space as well as definition and clarity have been greatly improved by applying acoustic design solutions and alternatives. However, loudness of sounds in the space didn't reach the aimed level, which is due to the limited capability of slat ceiling structure for providing strong early reflections.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.4
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• pp.28-37
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• 2002

Acoustic behavior in combustion chamber with acoustoc cavity is numerically investigated by adopting linear acoustic analysis. Helmholtz-type resonator is employed as a cavity model to suppress acoustic instability passively. The tuning frequency of acoustic cavity is adjusted by varying the sound speed in acoustic cavity. Through harmonic analysis, acoustic pressure responses of chamber to acoustic oscillating excitation are shown and the resonant acoustic modes are identified. Acoustic damping effect of acoustic cavity is quantified by damping factor. As the tuning frequency approaches the target frequency of the resonant mode to be suppressed, mode split from the original resonant mode to lower and upper modes appears and thereby damping effect is degraded significantly. Considering mode split and damping effect as a function of tuning frequency, it is desirable to make acoustic cavity tuned to maximum frequency of those of the possible splitted upper modes.

• Journal of the Korean Society of Propulsion Engineers
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• v.16 no.5
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• pp.29-36
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• 2012

3-D linear acoustic analysis has been performed to elucidate damping characteristics of large Helmholtz acoustic cavities in FASTRAC thrust chamber. Acoustic impedance concept has been introduced to quantify combustion stabilization capacity. For a given acoustic cavity, sonic velocity in the cavity to achieve an optimal tuning has been determined and satisfactory agreement with the previous results has been obtained. Feasible estimation of sonic velocity in the acoustic cavity has been devised. Results show similar trends without significant deviations, which can be used in the procedure of design and verification of acoustic cavity. From the satisfactory results, investigation of other thrust chambers with acoustic cavities which have shown combustion instabilities will be done as future works.

• Aerospace Engineering and Technology
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• v.5 no.2
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• pp.195-204
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• 2006

A linear acoustic analysis has been performed to elucidate damping characteristics of acoustic cavities in a liquid rocket combustor. Results have shown that resonant frequencies of acoustic cavity obtained by classical theoretic approach and by the present linear analysis are somewhat different with each other. This difference is attributed to the limitation of the simplified classical theory. To quantify the damping characteristics, acoustic impedance has been introduced and resultant absorption coefficient and conductance have been evaluated. Satisfactory agreement has been achieved with previous experiment. Finally the design procedure for an optimal tuning of acoustic cavity has been established.

• Journal of Ocean Engineering and Technology
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• v.36 no.1
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• pp.61-81
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• 2022

The available underwater acoustic spectrum is limited. Therefore, it is imperative to avoid frequency interference from overlapping frequencies of underwater acoustic equipment (UAE) for the co-existence of the UAE. Cognitive technology that senses idle spectrum and actively avoids frequency interference is an efficient method to facilitate the collision-free operation of multiple UAE with overlapping frequencies. Cognitive technology is adopted to identify the frequency usage of UAE to apply cognitive technology. To this end, we investigated two principle underwater acoustic sources: UAE and marine animals. The UAE is classified into five types: underwater acoustic modem, acoustic positioning system, multi-beam echo-sounder, side-scan sonar, and sub-bottom profiler. We analyzed the parameters of the frequency band, directivity, range, and depth, which play a critical role in the design of underwater acoustic cognitive technology. Moreover, the frequency band of several marine species was also examined. The mid-frequency band from 10 - 40 kHz was found to be the busiest. Lastly, this study provides useful insights into the design of underwater acoustic cognitive technologies, where it is essential to avoid interference among the UAE in this mid-frequency band.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.26 no.7
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• pp.836-843
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• 2016

This study investigates the vibro-acoustic characteristics of an automotive brake drum given free boundaries using the hybrid approach combining numerically obtained structural properties with analytical solution for acoustic radiation. Structural vibrations of the drum are investigated with the finite element method, and modal displacements on the outer surfaces of the drum are idealized as simple mathematical expressions. Based on the expressions, modal sound radiations of the drum are calculated using the Rayleigh integral method. Structural and acoustic responses of the drum for a harmonic excitation are obtained from above results using the modal expansion technique. The results are confirmed with numerical analyses using the boundary element method. Based on these results, it can be concluded that the vibro-acoustic characteristics of a brake drum can be accurately investigated with the process used in this study. Also, many noise and vibration problems in drum brake can be addressed using the procedure proposed in this study.

• The Journal of the Acoustical Society of Korea
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• v.19 no.2
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• pp.73-82
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• 2000

This paper presents a significant improvement achieved in the acoustic characteristics of a conference room of I-company originally designed and built without consideration for room acoustic characteristics. The reverberation time of the room was excessive, which resulted in poor 'Definition' for speech. Thus the improvement of room acoustic characteristics was required. In this study, experiments and simulations using the principles of geometrical acoustics were executed to improve the acoustic characteristics in the room. By performing a series of simulation, it has become possible to predict acoustic parameters for an improvement plan. And the improvement plan of room acoustic characteristics was established. And for the verification of the simulation results, the experimental data were compared and evaluated against those from the simulation. Additionally, the proposed method in this paper demonstrated its effectiveness by successfully improving the room acoustic characteristics resulted from repair construction.

• Journal of the Korea Institute of Military Science and Technology
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• v.23 no.2
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• pp.115-124
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• 2020

Due to the increasing threats of submarines from North Korea and other countries, ROK Navy should improve the detection capability of submarines. There are two ways to detect submarines : acoustic detection and non-acoustic detection. Since the acoustic-detection way has limitations in spite of its usefulness, it should have the complementary way. The non-acoustic detection is the way to detect submarines which are operating mast sets such as periscopes and snorkels by non-acoustic sensors. So, this paper proposes a new submarine non-acoustic detection model using an ensemble of Convolutional Neural Network models in order to automate the non-acoustic detection. The proposed model is trained to classify targets as 4 classes which are submarines, flag buoys, lighted buoys, small boats. Based on the numerical study with 10,287 images, we confirm the proposed model can achieve 91.5 % test accuracy for the non-acoustic detection of submarines.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.8
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• pp.1302-1307
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• 2001

This study identifies the major coupling path by examining the variation of the coupled acoustic modal frequencies and modes. A 1/2-size acryl compartment model is designed and manufactured for the measurement and analysis of coupled acoustic modes. Due to acoustic coupling, a new acoustic mode appears in a low frequency range. This coupling affects only longitudinal acoustic modes of compartment. Experimental result shows that the package tray holes contribute to the coupling much more than the back seat and hole size is an important design factor to control low frequency acoustic modes in the coupled system.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.16 no.7 s.112
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• pp.729-738
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• 2006

Knowledge of acoustic transmission loss of acoustic window material has a great importance for the sonar performance in ship. The purpose of this study was to investigate the measurement and analysis method for the acoustic transmission loss of the acoustic window materials for sonar dome. The measurement and analysis were carried out in water with GRP material. Transmission losses were calculated based on integrated direct and transmitted signals. The experimental setup enabled to vary the angle of incidence. Thus the transmission loss data could be expressed as the function of frequency and angle of rotation. In this paper, diffraction effect of incident angle, size of specimen with test material, transmission analysis method and multiple waves as incident acoustic signal were discussed.