Definition
A sound wave is a ripple that spreads outward when a voice or an instrument makes a vibration that pushes and pulls on the air, water, or other material around it. Our ears pick up the tiny changes in pressure as the air shakes, and we hear them as all kinds of sounds.
A Shake That Travels Like Dominoes
When you hit a drum, the tight drumhead shakes back and forth very fast. That shaking shoves the air particles touching the drumhead. A shoved particle bumps the one next to it, then swings back to where it started.
The air particles themselves do not fly from a speaker's mouth to a listener's ear. Think of people standing in a line, tapping the next person's shoulder to send a signal. Only the energy of the shaking in place gets passed along, one particle after another.
So regions where air particles are packed tightly and regions where they are spread thin take turns, and the pattern spreads in every direction. The material wobbles back and forth in the same direction the wave travels, so science calls this a "longitudinal wave" (a parallel wave).
Hold one end of a jump rope and shake it up and down. A ripple runs along the rope to the other end. A wave like this, where the shaking direction and the travel direction are at right angles, is called a "transverse wave" (a perpendicular wave). Sound is different: it is a wave of pressure, where the air is squeezed and stretched again and again along the direction of travel. By the way, ocean waves are a special case. The water particles move in circles, so they have traits of both transverse and longitudinal waves.
A Bit More Precisely
To be more precise, a sound wave can only exist if there is a medium (a material to carry it) that acts like stepping stones for the shaking. In outer space, or inside a glass jar with the air pumped out, there are no particles at all. Even a huge explosion would not send out any sound.
How fast sound travels depends a lot on the material around it. In air, where particles fly around freely without gripping each other, sound moves about 340 meters per second. In water, where particles hold on to each other much more strongly, it moves about 1,500 meters per second.
Inside solids like a steel pipe or a rail track, sound spreads at over 5,000 meters per second. The particles in a solid grip each other tightly, and the more quickly they spring back when pushed, the faster they hand the vibration to their neighbors. Sound is fast not because the material is heavy, but because it is stiff.
In old Western movies, there is a scene where the hero presses an ear to the rail to catch a train coming from far away. It was a smart bit of science. He was trying to catch the fast vibration in the solid rail, which races ahead of the whistle traveling through the air.
The Secret of Pitch and Loudness
Why does a mosquito sound thin and sharp, while a big drum sounds deep and grand? The secret is frequency, which tells how many times per second the air particles shake. The more times per second the air shakes, the higher the pitch. The slower it shakes, the lower and heavier the sound.
Human ears can usually hear only sounds that shake from about 20 to 20,000 times per second. A sound that shakes faster than 20,000 times per second, too fast for people to hear, is called "ultrasound." Bats and dolphins send out ultrasound and listen to the echoes that bounce back, and that is how they find food and obstacles right in front of them.
Loudness depends on how wide and strong the air particles swing. The bigger the swing, the harder the sound hits your eardrum, so you feel that the sound is loud.
When you whisper, the air particles sway gently and weakly in place. When you shout, they move back and forth with a huge amplitude, and with great force, sending strong energy out into the surroundings.
Common misconceptions
You hear a voice because air particles from the speaker's mouth fly all the way to your ear.
Air particles only wobble back and forth in place. What moves forward is not the particles themselves but the "vibration energy" they pass along.
Where you meet it
If you put a candle in front of a speaker, the flame flickers back and forth when the music gets loud.
Sound travels more than four times faster in water than in air, so divers find it hard to tell which direction a sound is coming from.
You hear thunder a few seconds after a flash of lightning because sound travels more slowly than light.
In one sentence
A sound wave is a wave in which the particles of a material shake in place and pass energy to their neighbors.