Sound
Good vibrations
- Every sound starts with something vibrating — a guitar string, a drum skin, your vocal cords.
- In space films, explosions are silent for a real reason: sound needs a material to travel through.
- Let's see how sound moves.
Sound is a longitudinal wave
- A vibrating object pushes the air into compressions (particles squeezed together) and rarefactions (particles spread apart).
- This makes sound a longitudinal wave — the air vibrates along the direction of travel.
- Because it needs particles, sound cannot travel through a vacuum (empty space).
Sound travels as a series of:
A vibrating source squeezes and stretches the air into compressions and rarefactions — a longitudinal wave.
Sound can travel through a vacuum (empty space).
Sound needs particles to pass the vibration along, so it cannot travel through a vacuum.
Speed and hearing range
- Humans can hear from about $20\ \text{Hz}$ to $20\,000\ \text{Hz}$.
- Sound travels at about $340\ \text{m/s}$ in air — far slower than light.
- It travels faster in liquids and solids than in gases, because their particles are closer together.
The approximate range of human hearing is:
Most people hear from about 20 Hz (very low) up to about 20 000 Hz (very high).
Sound travels fastest through:
The closer the particles, the faster the vibration is passed on — so sound is fastest in solids, slowest in gases.
Echoes
- A reflected sound is an echo.
- You can measure the speed of sound by timing an echo over a known distance.
- The sound travels to the wall and back, so it covers twice the distance.
A boy shouts at a cliff $660\ \text{m}$ away and hears the echo $4.0\ \text{s}$ later. What is the speed of sound, in m/s?
The sound travels there and back: $2 \times 660 = 1320\ \text{m}$ in $4.0\ \text{s}$, so $v = \dfrac{1320}{4.0} = 330\ \text{m/s}$.
You've got it
- sound is made by vibrations and is a longitudinal wave (compressions + rarefactions)
- it needs a medium — no sound in a vacuum
- human hearing: about $20\ \text{Hz}$ to $20\,000\ \text{Hz}$; speed ≈ $340\ \text{m/s}$ in air
- an echo is reflected sound — remember it travels there and back