Underwater Acoustic Characteristics of the Towing Tank at National Taiwan University

• Main Authors: Chuan-Hsuan Chao, 趙傳軒
• Other Authors: 陳琪芳
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/j9n3w3

碩士 === 國立臺灣大學 === 工程科學及海洋工程學研究所 === 107 === In addition to open water, underwater acoustic testing is conduced in a tank or indoor swimming pool, as testing in open water costs considerable manpower, money, and other resources. So, if there is no requirement to test in open water experiments, the Underwater Acoustic Laboratory (UAL) from Department of Engineering Science and Ocean Engineering at National Taiwan University (NTU) usually does underwater acoustic testing in a shipping modal testing tank. As the original use was not for acoustical testing, interference from many environmental factors exists. This research project will help one understand the acoustic properties of the tank, and it gives a series of tests and simulations to verify findings. The items of measurement includes ambient noise, sound absorption coefficient and reverberation time. The ambient noise measurement was separated into two parts: short time and long time. The measurement in short time is spatially varying, and the measurement in long time is time varying while concurrently recording temperature in air and water. The sound absorption coefficient measures the sound intensity of the boundaries around the tank(such as walls, surfaces in contact with air, etc.), while measuring both the direct sound pressure and the reflected sound pressure of the signal by the Tone burst method. The reverberation time shows the attenuation behavior of pure tone energy in the tank, where the fluctuating signal on the energy attenuation curve is reduced using the decay curve average method (DCAM), and the envelope mean is used to smooth the decay curve, which eventually results in reverberation time parameters like early decay time (EDT), T_20and T_30. The simulation utilizes room acoustic concepts in the water, and the calculation module is based on statistical acoustics theory, and is called the diffusion equation. Its boundary condition is found by adding the measured sound absorption coefficient to the Eyring absorption model. Finally, verifications is through comparing the theoretical reverberation time with the measured value. The results showed that, due to the body of water being big enough, the air temperature does not affect the water temperature in short time, and the tank exists within the 60 Hz octave band. The sound absorption coefficient of the side walls at frequencies less than 4 kHz is up to 0.974, with other frequencies being 0.6 ~ 0.7. All of the reverberation time parameter EDT, T_20, T_30 is within 0.5 seconds, The results display that it may have coupled room effect caused by multiple spaces existing in the tank, which makes the decay curve not have linear attenuation, resulting in extension of the reverberation time. The experimental data show that EDT is better than T_20, T_30 in 4kHz ~ 10kHz.