英语时差：扩音器

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 megaphones amplify 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 scatter 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.