時頻域方法應用於洋香瓜敲擊響應之分析

Abstract

農產品非破壞性品質檢測方法中，利用敲擊農產品取得自然頻率藉此判斷品質，是文獻中常使用之方式。此法是利用不同型式的鐘擺敲擊農產品，記錄其聲音或振動響應，再利用快速傅利葉轉換得到自然頻率。雖然實際上生物材料大都為黏彈性體，但在水果敲擊的理論研究中，都假設水果為彈性體。此論文嘗試使用時頻域方法分析洋香瓜的敲擊響應，同時記錄洋香瓜的敲音與振動訊號，再對訊號作短時間傅利葉轉換並畫出聲譜圖。聲譜圖可隨著時間軸呈現出自然頻率與能量的關係。利用此改進方法作分析，可在洋香瓜的儲存期間，輕易看出整體主頻的頻率降低行為，並且辨識出不同主頻的能量大小。論文中對於敲音與振動響應都有個別之討論，在聲譜圖中可以得到每ㄧ個自然頻率與時間的關係，且洋香瓜敲擊響應所得之聲譜圖中，低頻能量緩慢消減的現象與Maxwell model 的應力鬆弛現象相似。因此建議可將時頻域分析方法作為研究水果黏彈性現象的新工具。

Acoustic impulse response method for measuring natural frequency of an impacted fruit is a nondestructive textural quality evaluation method extensively studied in the literature. The method used various types of pendulum to hit the studied object and to record the acoustic or vibrating responses. The recorded data were then processed by means of digital Fourier transform to obtain the natural frequency of the object. The main theory behind is assuming fruit to be an elastic object that response to the impaction, although fruit fresh is viscoelastic in reality. The objective of this thesis was to use time-frequency method to analysis impulse response for a melon in an impaction test. Both acoustic and vibrating responses were recorded and processed with a short time Fourier transform program developed in this study to give spectrographs for each impulse response data obtained in experiment. The spectrograph gave a better resolution for the natural frequencies also with the time history of the power spectrum. By this improved analysis tool the changes of mechanical properties of melon during short storage were easily depicted in decreasing of major nature frequencies and variations of power in different wave bands. Those were two main parameters separately discussed in the literature. Abstracting information from the spectrograph, the time history of each natural frequency was obtained. Skew curves were observed for low frequency signal, the graphics were similar to those in stress relaxation curves for the viscoelastic materials. Along with other evidences that suggested time-frequency method might be a new tool for studying viscoelastic properties of fruit.