Sound
Why would nobody hear you if you shouted on the Moon?
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Explain
Sounds are made by shaking.
Air carries sound to your ears.
All sounds start with vibrations. When you speak, your vocal cords shake.
Vibrations shake the air next to them, and the air passes the shake along, until it reaches your ear.
Sound needs something to travel through. There is no air on the Moon, so there is no sound.
Sound is a longitudinal wave of squashed and stretched air. Its speed in air is about 343 m/s.
Sound travels faster in water, about 1500 m/s, and faster still in solids, since particles are closer together.
Worked example: a 200 Hz sound has λ = 343 / 200 ≈ 1.7 m.
- Something vibrates
- Air passes it on
- Your ear hears it
Want the full story?
The speed of sound depends on temperature, and 343 m/s is at 20 °C. The sim slows the sound down enormously, so that you can see it. Humans hear from about 20 Hz to 20 000 Hz, and the top limit falls with age.
The Formula
wavelength = speed of sound ÷ frequency
- the wavelength, measured in metres (m)
- the speed of sound in air, about 343, measured in metres per second (m/s)
- the frequency, measured in hertz (Hz)
Try the numbers
Wavelength: 1.72 m
Try it at home
You need a ruler, a table edge.
You need: a ruler, a table edge.
Hum and feel your throat shake.
- Hold a ruler on a table edge with part sticking out.
- Twang it.
- Slide it to change the length, and twang it again.
What's happening: the vibrating ruler makes the air vibrate, and a shorter ruler vibrates faster.
Physics is life
Your body
Your throat shakes when you talk.
Daily life
Music comes from vibrations.
Nature
Whales sing under water.
Space
Space is silent.
Quick check
Question 1 of 3
Words to know
Sound: Vibrations that travel through air or other materials.