Sound—it's almost impossible to imagine a world without it. It's probably the first thing you experience when you wake up in the morning—when you hear birds chirping or your alarm clock bleeping away. Sound fills our days with excitement and meaning, when people talk to us, when we listen to music, or when we hear interesting programs on the radio and TV. Sound may be the last thing you hear at night as well when you listen to your heartbeat and drift gradually into the soundless world of sleep. Sound is fascinating—let's take a closer look at how it works!
Sound is the energy things produce when they vibrate (move back and forth quickly). If you bang a drum, you make the tight skin vibrate at very high speed (it's so fast that you can't usually see it), forcing the air all around it to vibrate as well. As the air moves, it carries energy out from the drum in all directions. Eventually, even the air inside your ears starts vibrating—and that's when you begin to perceive the vibrating drum as a sound. In short, there are two different aspects to sound: there's a physical process that produces sound energy to start with and sends it shooting through the air, and there's a separate psychological process that happens inside our ears and brains, which convert the incoming sound energy into sensations we interpret as noises, speech, and music. We're just going to concentrate on the physical aspects of sound in this article.
Sound is like light in some ways: it travels out from a definite source (such as an instrument or a noisy machine), just as light travels out from the Sun or a light bulb. But there are some very important differences between light and sound as well. We know light can travel through a vacuum because sunlight has to race through the vacuum of space to reach us on Earth. Sound, however, cannot travel through a vacuum: it always has to have something to travel through (known as a medium), such as air, water, glass, or metal.
The first person to discover that sound needs a medium was a brilliant English scientist known as Robert Boyle (1627–1691). He carried out a classic experiment that you've probably done yourself in school: he set an alarm clock ringing, placed it inside a large glass jar, and while the clock was still ringing, sucked all the air out with a pump. As the air gradually disappeared, the sound died out because there was nothing left in the jar for it to travel through.
Robert Boyle's alarm clock experiment demonstrates that sound needs a medium through which to travel.
Artwork: Robert Boyle's famous experiment with an alarm clock.
Put a ringing alarm clock inside a large glass case with a valve on top. Close the valve so no air can get in.
You can easily hear the clock ringing because the sound travels through the air in the case and the glass, before continuing to your ears.
Switch on the vacuum pump and remove the air from the case. As the case empties, the ringing clock sounds fainter and fainter until you can barely hear it at all. With little or no air in the case, there's nothing to carry the sound to your ears.
Switch off the pump. With the clock still ringing, open the valve on top of the case. As air rushes back in, you'll hear the clock ringing once again. Why? Because with air once again inside the case, there's a medium to carry the sound waves from the ringing clock to your ears