Sound: vibrations on the move
How sound travels through air, water and metal, why it is faster in solids, what pitch and loudness are, and why space is silent. With a string-telephone experiment.
Read
Sound is made when something vibrates. A vibrating guitar string pushes the air next to it, squashing it together (a compression) and then letting it spread out (a rarefaction). These squashes and stretches travel outwards as a sound wave. When they reach your ear, they make your eardrum vibrate.[1]
Sound needs something to travel through: a gas, a liquid or a solid. In air at 20 °C it travels at about 343 metres per second. In water it is much faster, about 1480 m/s, and in steel about 5960 m/s.[2]
Most of space is almost completely empty, a vacuum. With no air or anything else to carry the vibrations, sound cannot travel there.[5]
How high or low a sound is, its pitch, depends on how many vibrations happen every second: its frequency, measured in hertz (Hz). People can hear from about 20 Hz (a very deep rumble) up to about 20,000 Hz (a very high whine).[2]
How loud a sound is, is measured in decibels (dB). A whisper is about 20 dB and normal talking about 60 dB. Around 120 dB, as at a loud rock concert, sound starts to hurt, and long or very loud exposure damages hearing.[3]
The loudest sound in recorded history came from the volcano Krakatoa in 1883. People on the island of Rodrigues, about 4,800 km away, heard it like distant gunfire. Its pressure wave went around the whole Earth several times.[4]
Try it: The string telephone
You need
- Two paper or plastic cups
- About 5–10 m of string
- A sharp pencil (to make the holes, with an adult)
- Two paper clips
Safety
Let an adult make the holes. Don't pull the string near anyone's face, and never shout into the cup when someone is listening.
What to do
- Make a small hole in the bottom of each cup (ask an adult).
- Thread the string through both holes and tie each end to a paper clip inside the cup, so it cannot slip out.
- Walk apart until the string is tight. One person talks quietly into a cup; the other holds the other cup to their ear.
- Now let the string go slack. Can you still hear?
- Try again with the string touching a door frame or a hand. What happens?
What happens, and why?
Your voice makes the bottom of the cup vibrate. A tight string carries those vibrations to the other cup, which vibrates and pushes the air into the listener's ear. Sound travels well through the solid string. A slack string, or a finger touching it, stops the vibrations, so the sound fades.[1][2]
Play: Can sound get through?
Can sound travel through this? Decide yes or no.
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 15–16