一般在海洋環境中利用聲波以為探測工具，其運用原理為：將一聲源藉由換能器(可同時為訊號發射器及接收器)，將聲能注入海洋水體中，繼而經由聲波在海水中之傳播將聲能傳達目標物，由於目標物本身對聲波的反射與散射現象，會將部份聲能經由反射路徑傳回換能器而為其所吸收，最後經由訊號處理方式達到目標辨識(identification) 的目地。聲納系統源自於軍事的需求,如果水下潛體欲避免敵方的聲納系統偵測出，乃試圖改變其反射波或穿透波之聲能與聲場，致使聲納系統所接受的訊號，不足以判讀或造成誤判，達到其隱蔽之目的。在水中聲學領域中，為達其隱蔽目的，乃應用吸音機制裝置於目標物表面上。一般而言，若欲將反射波與穿透波的聲能降低，常使用之方法有二：其一為主動式消音控制 ，旨在研究如何將入射聲波以破壞干涉之方式，令其聲波相位反置達到聲能降低之目的。另一則為被動式減音技術，乃藉吸音材料之吸音特性將聲能消耗於材料內部，降低反射聲能，令使其聲波不足以傳回偵測點。是故，應用此方法之吸音材料，其吸音係數需事先估算規劃之。雖言主動式消音控制之效果優於被動式減音技術，然而由於壓電複合材料之開發技術所限，主動式消音控制技術在水中吸音市場上並不多見。再者，其在安裝及維護成本上，所費不貲，且困難度皆高於被動式減音技術。鑑於此，本論文乃針對被動式減音技術之吸音材料特性為研究對象。

根據德國科學家Alberich對水中吸音材料被覆層之設計經驗中，發現吸音特性會隨水中壓力與溫度變化，而產生吸音特性之變化。基此動機，本論文旨在研究水中吸音材料於各種水深中，亦即不同壓力作用下，其吸音特性之變化情形。在實驗過程中，需建構一套量測系統及尋求量測標準，以為量測實驗方法之依據。並探討量測參數之設定，以求實驗結果之精確。本實驗之試片材質，經χ光繞射儀分析，判定為丁基橡膠與鋸木屑之合成物，編號為A1、A2、A3、A4、及A5共五片。在尚未進行水中加壓實驗之前，先行量測在空氣中之物理性質及吸音特性。結果為，此些試片其聲阻抗皆與水之聲阻抗匹配，而其在空氣中之聲學特性屬為隔音效果。

在水中實驗量測中，為減低反射、繞射及散射之影響，聲源採用脈衝波訊號，並以門檻系統(gating system)來擷取所需訊號。本實驗量測頻率11 kHz 至30 kHz之回聲衰減量(echo reduction)及插入損失量(insertion loss)，以估算試片之聲學特性。實驗結果顯示，試片在水中未加壓(1 bar)時，其回聲衰減量約為10 dB，插入損失量約為15 dB；於加壓5 bar時, 其回聲衰減量約減少3 dB， 插入損失量約減少3 dB；在加壓10 bar時,其回聲衰減量約減少8 dB，插入損失量約減少5 dB。此現象印證水中吸音材料，在不同水壓作用下，會影響其聲學特性。

In general the acoustic wave is used as a detecting tool in the ocean, its application placing a sound source into ocean, then the sound may impinge involves the target by wave propagation in the ocean. Due to the reflection and scattering effect of target, part of acoustic energy will be received by transducer through the path of reflection. The goal of target identification can be achieved by signal processing finally. If a submarine wish to avoid the detection by sonar system , it should attenuate the acoustic energy . Therefore the reflected signal can not be analyzed and distinguished by sonar system .The area of underwater acoustic attenuation has been researched for camouflaging submarine purpose for many years. There are two acoustic energy attenuation methods to reduce the reflective wave and transmitted wave. One is active attenuation control, which is to understand how the destructive interference of incident acoustic wave could be achieved for acoustic energy attenuation purposes. The other one is passive acoustic attenuation technique, which rely on the attenuation performance of underwater acoustic material to reduce the acoustic energy of incident wave. To be evaluated the acoustic absorption efficiency of material. Although the efficiency of active attenuation control is better compared with passive acoustic attenuation technique, the development of active attenuation control have not been highly pursued in the commercial market for underwater application, due to the limitations in piezo-composite technology. The cost of installation and maintenance is also higher in active control. This thesis studied the acoustic absorptive material based on passive acoustic attenuation technique . It could be attenuated the acoustic energy and spectrum of reflection and transmitted wave. Therefore, the signal can not be analyzed and distinguishing by sonar system.

According to Alberich acoustic absorption coating, their designs have the inherent problem of degradation under hydrostatic pressure and temperature. Thus, the objective of this thesis is to study the characteristics of the acoustic absorptive material at various water depth where the hydrostatic pressure are different. To measure the characteristics of acoustic material, an experimental system is setup, and the standard measuring method and criterion is also studied for future experimental reference. Furthermore, the different measurement parameters are discussed for accuracy of experimental results. There are five specimens tested in this experiment. The specimens are mainly made of neoprene and sawdust mixture and marked as A1、A2、A3、A4、and A5 respectively. The composites of these specimens are analyzed by x-ray diffraction meter. The physical properties and the acoustic absorption in airborne were measured before underwater hydrostatic pressure applied on these specimens. The physical properties show that the impedance of these specimens is very close to acoustic impedance of the water. Therefore, the specimen may be considered an acoustic isolator in the air.

To reduce the boundaries interference, such as reflection, diffraction and scattering signal. The pulse sound is used as sound source in this underwater experiment. Moreover, the gating system is applied to capture the proper signals for analysis. The echo reduction and insertion loss are measured in the 11 to 30 kHz frequency region for acoustic absorption evaluation in this experiment. The performance of experiment is found that specimen has the echo reduction about 10 dB and the insertion loss about 15 dB at 1 bar hydrostatic pressure. But when the hydrostatic pressure was increased to 5 bar, the echo reduction and insertion loss were both decreased by 3 dB. In addition, when the hydrostatic pressure was loaded at 10 bar, the echo reduction was decreased by 8 dB, and the insertion loss was decreased by 5 dB. It became evident that the efficiency of acoustic absorption is degraded under the higher hydrostatic pressure.

Chapter 1 INTRODUCTION
1.1 History of underwater acoustics 1
1.2 Development of active acoustic control 1
1.3 Evolution of acoustic absorptive material 3
1.4 Goals of research 7

Chapter 2 METHODOLOGY
2.1 Echo reduction 8
2.1.1 Pulse and gating method 12
2.1.2 Interference technique 13
2.2 Insertion loss 14

Chapter 3 EXPERIMENT
3.1 Acoustic material properties 16
3.1.1 Impedance comparison 17
3.2 Acoustic absorption in airborne 18
3.2.1 Methods for absorption coefficients
measurement 18
3.2.1.1. Reverberation room method 18
3.2.1.2 Standing wave method 20
3.2.1.3 Alternate method of acoustic
absorption 21
3.2.2 Results and summary 35
3.3 Underwater acoustic measurement setup and
procedure 38
3.3.1 High pressure water chamber 38
3.3.2 Measurement setup 38
3.3.3 Pulse sound 43
3.4 Experiment results 44
3.4.1 Source level 45
3.4.2 Echo reduction 46
3.4.3 Insertion loss 52

Chapter 4 CONCLUSIONS AND DISCUSSION
4.1 Conclusion 58
4.2 Discussions 59

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