摘要

就其目的而言，號角在聲學上的應用大致上可以分成兩類：一種是為了增加聲場的方向性而在聲源處加上號角，另一種則是藉由改變號角的幾何而調整輸出聲場的自然頻率。所有號角的工作品質皆與其聲學阻抗有著密不可分的關係，而號角的幾何參數（例如其截面變化）對其聲學阻抗頻譜有決定性的影響。
因此，在本文中我們利用適當的理論模型與邊界條件，並配合數值計算結果，闡明以幾何（截面積）修正手段來調整號角之聲學阻抗頻譜的可行性，對應號角不同的應用特性，嘗試各種不同的幾何修正方式並比較其結果優劣。結果發現號角自然頻率可藉由控制幾何調整的參數來達到微幅調整的目的，而號角低頻截止頻率在不影響號角頻譜均勻性的考量下也可經由幾何調整手段得到下降的效果，故本文所提出的方法應可用於號角之幾何設計與調音維修上。

Abstract

Acoustic horns usually are used to increase the directivity of the acoustic field, or to adjust the natural frequencies of the instruments to which they are attached. The quality of all acoustic horns, however, depends upon their acoustic impedance spectrum, which, in turn, is determined by the geometry (especially the cross-sectional area variation) of the horns.

Here, using appropriate theoretical models and boundary conditions, and through numerical computations, we demonstrate the possibility of adjusting the acoustic impedance spectrum of horns by modifying their cross-sectional area. Specifically, it is shown in this thesis that the natural frequency distribution of acoustic horns can be precisely adjusted to some extent by local cross-sectional area modification of small amplitude. Furthermore, the cut-off frequency of acoustic horns can also be significantly lowered by the same means. These findings are expected to be useful for the design and tuning of acoustic horns.

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