英语听力文摘 English Digest 274、天然扩音器

（单词翻译）

　　Megaphones
If you want to call to a friend half a block down a crowded street, you might cup your hands around your mouth to better aim your voice. Have you ever wondered why simple megaphonesamplify your voice so well?
Actually, they work for two different reasons.  First, they direct your voice.  As sound emerges from your mouth, it travels outward in all directions, going left, right, up, and down, as well as straight ahead.  Using a megaphone is a lot like using a funnel–it directs more sound toward your target, and allows less to scatter¹ to the sides.

　　This is only half the story, though.  Megaphones don’t simply aim the sound that you’re already making, they can actually draw more volume from your mouth in the first place. This is because whenever a sound wave moves suddenly from a narrow space to a more open one, some of the sound is reflected backward.  This is exactly what happens when the sound of your voice moves from the narrow confines of your mouth into the wide, open air.  The abrupt² change(突变；陡变) bounces some of the sound energy backward, where it’s absorbed by your mouth, reducing the overall volume.
A cone-shaped(锥形的) megaphone creates a more gradual transition(渐变) from your mouth to the open air.  By reducing the amount of sound that (1)bounces back into your mouth, this allows more volume to come out.  For full effect, a megaphone should be at least as long as the wavelength³ of the sound it’s amplifying⁴.  Human voices have wavelengths⁵ up to several feet long, so a professional megaphone, such as a cheerleader might use, is also several feet long.
Notes:
(1)bounce back 反弹；迅速恢复活力
When I kicked the football against the post, it bounced back.
我把足球往柱上踢去，它就弹了回来。