Sound
by Chris Woodford. Last updated: August 23, 2011.
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!
Photo: Sound is energy we hear made by things that vibrate. Photo by William R. Goodwin courtesy of US Navy.
What is sound?
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.
How sound travels
When you hear an alarm clock ringing, you're listening to energy making a journey. It sets off from somewhere inside the clock, travels through the air, and arrives some time later in your ears. It's a little bit like waves traveling over the sea: they start out from a place where the wind is blowing on the water (the original source of the energy, like the bell or buzzer inside your alarm clock), travel over the ocean surface (that's the medium that allows the waves to travel), and eventually wash up on the beach (similar to sounds entering your ears). If you want to learn more about how sea waves travel, read our article on surfing science.
Picture: Energy moves outward! In that respect, sounds coming from a drum are no different from the light bursting from this supernova (exploding star). Photo courtesy of NASA Jet Propulsion Laboratory (NASA-JPL).
There is one crucially important difference between waves bumping over the sea and the sound waves that reach our ears. Sea waves travel as up-and-down vibrations: the water moves up and down (without really moving anywhere) as the energy in the wave travels forward. Waves like this are called transverse waves. That just means the water vibrates at right angles to the direction in which the wave travels. Sound waves work in a completely different way. As a sound wave moves forward, it makes the air bunch together in some places and spread out in others. This creates an alternating pattern of squashed-together areas (known as compressions) and stretched-out areas (known as a rarefactions). In other words, sound pushes and pulls the air back and forth where water shakes it up and down. Water waves shake energy over the surface of the sea, while sound waves thump energy through the body of the air. Sound waves are compression waves. They're also called longitudinal waves because the air vibrates along the same direction as the wave travels.
To get the difference between transverse and longitudinal waves clear in your mind, take a look at these two little animations on Wikimedia Commons:
- Animation of a transverse wave (how light and water waves travel).
- Animation of a longitudinal wave (how sound waves travel).
More about sound waves
If you've ever got time on your hands while you're lazing on the beach, try watching the different ways in which waves can behave. You'll notice that waves traveling on water can do all kinds of clever things, like smashing into a wall and reflecting straight back with more or less the same intensity. They can also spread out in ripples, creep their way up the beach, and do other clever stuff. What's happening here with water waves doesn't actually have anything to do with the water: it's simply the way energy behaves when it's carried along by waves. Similar things happen with other kinds of waves—with light and with sound too.
You can reflect a sound wave off something the same way light will reflect off a mirror or water waves will bounce off a sea wall and go back out to sea. Stand some distance from a large flat wall and clap your hands repeatedly. Almost immediately you'll hear a ghostly repeat of your clapping, slightly out of step with it. What you hear is, of course, sound reflection, better known as an echo: it's the sound energy in your clap traveling out to the wall, bouncing back, and eventually entering your ears. There's a delay between the sound and the echo because it takes time for the sound to race to the wall and back (the bigger the distance, the longer the delay).
Sound waves lose energy as they travel. That's why we can only hear things so far and why sounds travel less well on blustery days (when the wind dissipates their energy) than on calm ones. Much the same thing happens on the oceans. Crisp water waves can sometimes travel vast distances across the ocean, but they can also be messed up when squally weather dissipates their energy over shorter distances.
Sound waves are like light and water waves in other ways too. When water waves traveling long distances across the ocean flow around a headland or into a bay, they spread out in circles like ripples. Sound waves do exactly the same thing, which is why we can hear around corners. Imagine you're sitting in a room off a corridor and, much further up the corridor, there's an identical room where someone is practicing a trumpet inside. Sound waves travel out from the trumpet, spreading out as they go. They ripple out down the corridor, race along it, ripple through the doorway into your room and eventually reach your ears. The tendency waves have to spread out as they travel and bend around corners is called diffraction.
