摘要 為瞭解音波於土壤介質中傳遞的特性，本研究藉著電容式微音器的高靈敏度及優良的頻率反應，於大型土壤試驗槽中，控制不同鋼球落距使落於砂土表面作為音源。分別量測不同深度聲速、不同位置及深度之均方根音壓及頻率分佈。最後以鋼球撞擊不同材質塊體產生音源，探討其在砂土中音波訊號之異同。 综合實驗結果發現，於淺層疏鬆砂土中，音波傳遞速度介於170~220m/s之間，且隨深度的增加而遞增，但遞增的速度有逐漸變緩的趨勢。控制不同鋼球落距撞擊砂土表面，所產生於砂土中之音波訊號，均方根音壓方面，會隨著距音源距離增加而降低。其中水平方向在距離2m之內，有急劇降低的趨勢，而2m之外均方根音壓降低的趨勢卻變得非常緩慢。頻率方面，距音源水平距離1m以內分佈在300Hz以下，主要頻率頻帶在30Hz以下。而水平距離1m以外分布在30~300Hz之間，主要頻率頻帶在30~100Hz之間。改變鋼球落距條件，不會改變均方根音壓於空間中衰減的趨勢及音波於砂土中的頻率分佈。另外，改變鋼球撞擊材質所產生之音波訊號，主要的差異在於距音源水平距離1m內30Hz以下頻帶中，主要頻率及所對應之音壓大小明顯不同。 Abstract To realize the properties of acoustic waves propagating in soil medium, this research rely on the high sensitivity and excellent frequency response of condenser microphone to control the drop height of a steel ball dropping to the surface of sand as a sound source at the large scale soil testing pit. The purposes of this research are to investigate the propagation speed of sound, the distribution of root mean square of sound pressure and the variation of sound pressure in frequency domain in the soil medium. Finally, acoustic responses in the sand layer by dropping a steel ball on different materials were compared. From the results of experiments, it was revealed that the propagation speed of acoustic waves is about 170~220m/s and it is getting faster with depth, but the growth of speed is getting slower. Acoustic responses of dropping a steel ball onto the surface of sand layer by different drop heights make the root mean square of sound pressure attenuate with the increase of distance between sound source and microphone. The variation of root mean square of sound pressure is very large between 0~1m in horizontal direction from sound source, and very small besides 0~1m. In another aspect, frequency distributes below 300Hz and it mainly distributes below 30Hz within 1m in horizontal direction from sound source. Frequency occupy the domain of 30~300Hz and mainly occupy the domain of 30~100Hz outside of 1m from sound source. Changing the drop height of a dropping steel ball did not change the attenuation trend of root mean square of sound pressure and the frequency distribution of acoustic signals in sand. In addition, the major differences of the acoustic signals of changing the material of knocked blocks is that the change of the dominant frequency and corresponding sound pressure inside of 1m distance from sound source in horizontal direction and below 30Hz in frequency domain.
