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研究生(外文):Chuan-Hsuan Chao
論文名稱(外文):Underwater Acoustic Characteristics of the Towing Tank at National Taiwan University
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量測部分包含環境噪音、吸音係數、餘響時間。環境噪音量測為短、長時間兩部份,分別研究整體水槽之聲場於地域性和時域性上的變化,並同時記錄水溫和氣溫。吸音係數是以水為本體四周的邊界聲強損耗(如:牆壁、與空氣接觸的表面等)作為吸音係數的量測,而水槽內邊界使用脈衝方法(Tone burst)分別量測訊號直接聲壓和反射聲壓,並以此比值去計算吸音係數。餘響時間是觀察單頻正弦波於水槽之聲壓衰減行為,以衰減曲線平均法(Decay curve average method,DCAM)降低窄頻訊號於能量衰減曲線的波動,並使用包絡線平均(Envelope mean)平滑化衰減曲線,得出餘響時間參數早期衰減時間(Early decay time,EDT)、T_20、T_30。模擬部份則是使用軟體COMSOL Multiohysics內的有限分析方法去計算得出餘響時間參數值與實驗量測值比較。
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.
第一章 緒論 10
1.1 研究背景 10
1.2 論文目的 12
1.3 文獻回顧 13
1.4 論文架構 16
第二章 理論介紹 17
2.1 有限元素分析法 17
2.2 簡正模態(Normal mode) 19
第三章 實驗數據與分析 23
3.1 實驗場地介紹 23
3.2 儀器設備 24
3.3 環境噪音 26
3.4 吸音係數 34
3.5 餘響時間 41
第四章 COMSOL模擬分析 53
4.1 COMSOL模擬建模與流程 53
4.2 模擬參數、網格和計算設定 55
4.3 模擬結果 58
4.4 實驗與模擬比較結果 69
第五章 結論與建議 76
參考文獻 78
附錄一 擴散聲學理論 81
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