AQA Physics (8463) 4.6.1.4 · Process
Sound waves and hearing
A sound is only squeezes and stretches travelling through the air. How does your ear turn that into something you hear, and why are some sounds silent to you but not to a dog?
Triple Science · Higher tier
Start here
A whistle that only the dog can hear.
A trainer blows a dog whistle. Across the park, the dog’s head snaps round. The trainer, with the whistle at her lips, hears nothing at all.
If you had to guess, does she hear nothing because the whistle is too quiet, or because its note is too high?
Sound in air is a longitudinal wave: a pattern of compressions, where the air is squeezed together, and rarefactions, where it is spread out. It needs particles to travel through, so it cannot cross a vacuum. To be heard, that pattern has to be turned into something your body can sense. Follow one note from a loudspeaker into the ear, and predict each step before you see it.
From air to ear · step 1 of 4
A note leaves the loudspeaker
The loudspeaker cone moves out and back 500 times a second. What do the air particles between the loudspeaker and the ear do?
Your turn · build the path
From the speaker to the brain
A phone plays a voice message. Put the steps from the phone's speaker to the listener's brain in order.
5 links belong. 2 do not.
The links
Your chain · 0 of 5
Turning sound waves into vibrations of a solid is not special to the ear. Any solid that sound reaches is pushed and pulled by the compressions and rarefactions, so it vibrates at the sound's frequency. It works the other way round too: a vibrating solid pushes and pulls on the air next to it and sends out sound waves.
Describe, with examples
Which way is the conversion?
Each card is a solid vibrating. Did sound waves make it vibrate, or is its vibration making sound waves?
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
When sound passes into a new material, its frequency stays the same: each compression still arrives once per cycle, which is why the ear drum vibrates at the sound's frequency. The speed changes, so the wavelength changes with it. Sound usually travels fastest in solids and slowest in gases, because the particles in a solid are closer together and more strongly linked, so each vibration is passed on more quickly: about 340 m/s in air and 1500 m/s in water.
Equations
v = f λ
wave speed (v, metres per second) = frequency (f, hertz) × wavelength (λ, metres)
λ = v ÷ f · f = v ÷ λ
Units first
Frequency must be in hertz. 1 kHz = 1000 Hz: multiply a kilohertz value by 1000 before you insert it.
Command words in this lesson
- State
- Give the fact and nothing more. A range needs both limits, each with its unit.
- Describe, with examples
- Say what happens, in order, and name real solids that vibrate.
- Explain
- Link each step to the next with a reason: sound wave, vibration, and why the range is limited.
- Calculate
- Show every line. Convert kHz to Hz first. The unit earns its own mark.
Key fact
Hearing is a conversion: sound waves make the ear drum and the parts behind it vibrate. In a human ear, the conversion only works from about 20 Hz to 20 kHz.
Key note · AQA 4.6.1.4 (8463)
Sound waves and hearing
- Sound: longitudinal, mechanical, needs medium.
- Sound waves travelling through a solid make the solid vibrate.
- In the ear, sound waves make the ear drum and the parts behind it vibrate, causing the sensation of sound.
- The conversion only works over a limited range of frequencies. Human range: 20 Hz–20 kHz.
- Speed: usually solid > liquid > gas.
- Entering a new material, the frequency stays the same; the speed and wavelength change (v = f λ).
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Not sure how a sound wave becomes something you hear?
The ear is a simplified diagram, not to scale. In it, the ear drum responds fully from 20 Hz to 20 kHz and not at all outside that range; a real ear's response fades towards both limits, differs between people, and its upper limit falls with age. The air particles move far more slowly than in a real sound, so you can follow them.