【英语时差8,16】扩音器（下）（在线收听）

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 abrupt change bounces some of the sound energy backward, where it’s absorbed by your mouth, reducing the overall volume. A cone-shaped megaphone creates a more gradual transition from your mouth to the open air. By reducing the amount of sound that 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 wavelength of the sound it’s amplifying. Human voices have wavelengths up to several feet long, so a professional megaphone, like a cheerleader might use, is also several feet long.