本實驗主要是研究閉管駐波的發聲原理。何謂「閉管駐波」？就是一個管子在相同長度下，用不同的力道吹，會有不同音高的聲音產生，這些音被稱為「諧音」。原管長所能發出的最低頻率稱作「第一諧音」，第二低的聲音稱作「第三諧音」，依此類推。在簫的演奏上，只要按住同樣的孔，用較大的力量吹，也同樣會發出較高的音；同樣地，在曲笛的演奏技巧上，有平吹、急吹等分別。為什麼吹越用力，音就越高呢？如果現在拿一個大吸管吹（要裝活塞），你會發現，只有在特定的位置（角度）下，才能吹出聲音。那麼，角度對於聲音也有影囉？這些現象的幕後黑手，就是在管口產生的「渦流」，渦流頻率也會隨著風速而增加；而且，渦流的頻率在特定風速下，會有特定的範圍。經由實驗可以大略歸納出，影響閉管駐波的三個主要變因，分別是「風速」、「風吹角度」、及「吹口至管口的距離」。吹得越急，風速就越快，渦流頻率越高，越易使諧音躍遷；吹的角度越小，越易產生渦流，亦易引發聲音；吹的距離越小，渦流越不?定，越易產生其他的擾動。以上就是本實驗的概略。This project is aimed to fine out how the closed tube can produce a sound. We know what harmonics are. When we hold a big straw and blow with increasing strength (the bottom should be in water), it will generate a higher sound. The high sound is called “harmonic”. The lowest sound it can make is “the first harmonic”, the second lowest sound is “the third harmonic”, and so forth. Likewise, when we press the same key on vertical bamboo flute with increasing strength, it’ll also produce a higher sound. But why do we use the strong air stream to blow the tube to cause the tone to transfer? Now let’s blow a straw flute. You will find that you need to blow in the particular position, and then the sound will be produced. So, is there any relationship between the blowing angle and the frequency? Actually, all these sound are produced by “vortex in the mouthpiece.” The frequency of vortex will increase with the wind speed. Moreover, the frequency of vortex has a range. In sum, the higher the wind speed is , the higher the frequency of the vortex is , and leads to the higher frequency of the sound. The smaller the blowing angle is, the easier the vortex will be produced; the easier the frequency will be made. The smaller the distance between the blowing angle and the frequency is, the more unstable the frequency will be. The above is the most important research in this project.
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