探討地聲訊號與粒徑之關係

Abstract

本研究以地聲檢知器(Geophone)於室內實驗環境下測量不同粒徑乾砂受撞擊時產生之振動訊號，利用快速傅立葉轉換(FFT)對振動訊號進行頻譜分析，並以頻譜能量平均值為門檻值，將高過門檻值的頻段視為特徵頻率，找出了乾砂受到外力撞擊時沿撞擊方向會產生一寬頻訊號，垂直撞擊方向則會有一低頻訊號，該低頻訊號能量大小會隨不同敲擊點位而有明顯變化。 除了地聲檢知器以外，本實驗亦使用次聲麥克風來量測土砂撞擊下空氣中的聲波訊號，並拿來與地聲訊號進行比較，發現僅在80~120Hz之間兩種系統量測到的訊號能量分布較為相近，若將次聲麥克風埋設於土砂中時，則收到的聲壓訊號與第聲訊號頻譜較為接近。

In this study, we use geophones to investigate the vibration signals generated by the collision of different sizes of dry gravel in the laboratory. Afterwards, we use the fast Fourier transform (FFT) to analyze the vibration signals. Then we take the average value of the spectrum energy as the threshold, and regard the frequency band higher than the threshold as the characteristic frequency. We found that it will generate a wide-band frequency signal along the direction of the collision, and generate a low-frequency signal in the vertical direction of the collision. The energy of the low-frequency signal will change significantly with different collision points. Besides the geophones, we also use infrasound microphone to investigate the sound wave signal propagating in the air. Then we compare the spectrum of the microphone with the spectrum of the geophone, and find that the spectrum of two different system is similar only between 80~120Hz. If we install the infrasound microphone underground, the spectrum of the microphone will be much similar to the spectrum of the geophone.