英语听力文摘 English Digest 274、天然扩音器

　　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。

1 scatter   

vt.撒，驱散，散开；散布/播；vi.分散，消散

参考例句：

• You pile everything up and scatter things around.你把东西乱堆乱放。
• Small villages scatter at the foot of the mountain.村庄零零落落地散布在山脚下。

2 abrupt   

adj.突然的，意外的；唐突的，鲁莽的

参考例句：

• The river takes an abrupt bend to the west.这河突然向西转弯。
• His abrupt reply hurt our feelings.他粗鲁的回答伤了我们的感情。

3 wavelength   

n.波长

参考例句：

• The authorities were unable to jam this wavelength.当局无法干扰这一波长。
• Radio One has broadcast on this wavelength for years.广播1台已经用这个波长广播多年了。

4 amplifying   

放大，扩大( amplify的现在分词 )； 增强； 详述

参考例句：

• Often they use borrowed funds, amplifying their gains and losses. 他们通常会用借贷的资金交易，从而放大收益或损失。
• An amplifying type (or analog) device, as opposed to digital device. 放大器类（或模拟）器件，相对于数字器件而言的。

5 wavelengths   

n.波长( wavelength的名词复数 )；具有相同的/不同的思路；合拍；不合拍

参考例句：

• I find him difficult to talk to—we're on completely different wavelengths. 我没法和他谈话，因为我们俩完全不对路。 来自《简明英汉词典》
• Sunlight consists of different wavelengths of radiation. 阳光由几种不同波长的射线组成。 来自辞典例句