超音波聲速藉由一端傳感器發射超音波再由另一端傳感器接收聲波，計算從發射聲波的時間點到接收聲波的時間點得知飛行時間（Time-of-Flight,以下簡稱TOF），並利用已知的距離與TOF推估可得到聲波在該介質中的傳遞速率。超音波需要透過介質來傳遞，然而在空氣中，聲速可能被環境因素如溫度、濕度、大氣壓力所影響，導致推算平均聲速結果誤差大。現今已經有的聲速判斷方法如脈衝最大值以及訊號調變包絡方波法，經TOF分析過後得知聲速，但並未考量溫度不均勻的情況下，聲波會因為聲阻抗之不同產生的反射和折射使波形偏移，讓TOF產生誤差，導致聲速推算錯誤。為了提高聲速估計的空間分辨率，如何在不受真實環境影響下提高聲速計算的準確性正是需要克服的難題。
在真實環境中，聲波傳遞過程中可能因為衰減、反射以及回波干擾等因素，導致聲速判斷不準確，因此在本研究透過演算法以及實驗架構的改善增加聲速判斷準確性。在實驗架構中，利用架設12個超音波傳感器形成一個圓形區域，對於圓中心而言，每間隔30°就放置一個傳感器，彼此間距離相等且互相皆可當作發射器與接收器，並且在四周圍都放上吸音棉用以降低回波反射影響主訊號。在量測路徑不同時，可以藉由演算法得到訊號到達的接收器的時間點來進行TOF的計算，然後推算各路徑上的聲速。
本研究透過超音波傳感器發射與接收的訊號進行分析，找出在非均勻氣體溫度分佈下的能夠正確判斷聲速的方法。根據系統得到之聲波訊號，不受聲波傳遞經過溫度分佈不同的區域產生的反射與折射所造成之干涉的影響，正確計算出該路徑之聲速。當接收器收到聲波訊號，此時訊號就會產生脈衝波形，利用雜訊強度的大小以及Schmitt-trigger的概念並加入判斷式，使訊號分析在各路徑上的接收訊號中得到主訊號接收時間點，使聲速計算更加準確且適用於溫度分佈不均勻的氣體中量測。

In real-world industrial application, knowledge of temperature field distribution plays an important role. Acoustic tomography (AT) offers advantages like non-contact to measure and visualize the temperature distribution. AT technique reconstructs the temperature distribution from the sound speeds of multiple paths between transmitting transducers (Tx) and receiving transducers (Rx). In maximum peak method, a signal is transmitted from Tx to Rx, and the time-of-flight (TOF) between the transmitted signal and received signal is calculated by the time difference between the maximum peak value of the transmitted signal and the received signal. Next, the TOF and the known distance between the Tx and Rx are used to estimate the sound speed in the medium. However, when the ultrasound wave travel through the non-uniform temperature medium, the TOF estimation using the maximum peak method is inconsistent.
Therefore, the aim of this research is to find an appropriate method to estimate the sound speed under the non-uniform gas temperature distribution of continuous heating by measuring and analyzing the signals. In the proposed sound speed method, the TOF is calculated using the concept of Schmitt-trigger along with two additional judgements. The proposed method is compared against the maximum peak method. The temperature data measured by 37 thermocouples are used to reconstruct the temperature distribution, and the sound speed data of each path in the temperature distribution map is used to verify the sound speed obtained by the proposed method. The sound speed results show that when the heat source is turned off, the sound speed calculated by the proposed method is similar to the sound speed of each path from reconstructed temperature distribution, while the standard deviation of the sound speed can reach within ±4m/s in the heating state, which verifies the stability of the sound speed acquired by the proposed algorithm about estimating sound speed.

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