Every sound starts with a vibration
Put your fingers on your throat and hum. You’ll feel a buzz. That buzz is a vibration, a rapid back-and-forth motion. Every sound begins with something vibrating: a guitar string, a drumhead, a speaker cone, or your vocal cords.
How sound travels
When an object vibrates, it pushes and pulls on the air particles next to it.
- When the object moves outward, it squeezes particles together, making a compression.
- When it moves back, it leaves a space where particles spread out, called a rarefaction.
- These compressions and rarefactions pass from particle to particle, like a push passing along a line of people.
This makes sound a longitudinal wave: the particles move back and forth in the same direction the wave travels.
The air itself doesn’t travel from the speaker to your ear. Only the energy does.
Sound needs a medium
Sound must travel through a medium: a solid, liquid or gas. In empty space there are no particles to vibrate, so sound can’t travel. Space is silent.
In 1660, Robert Boyle placed a ringing bell in a glass jar and pumped out the air. The ringing faded until it could no longer be heard.
| Medium | Speed of sound |
|---|---|
| Air (0 °C) | 331 m/s |
| Air (20 °C) | 343 m/s |
| Water | About 1480 m/s |
| Wood | About 3500–4000 m/s |
| Steel | About 5900 m/s |
Sound is fastest in solids because particles are packed tightly and pass vibrations along quickly. Sound is slightly faster in warm air than cold air.
Sound vs. light speed
Light travels about 300 000 000 m/s, nearly a million times faster than sound. During a thunderstorm, you see lightning almost instantly but hear thunder later.
To estimate distance: count the seconds between the flash and the thunder, then divide by 3. That’s roughly the distance in kilometres.
Properties of sound
| Property | Depends on | Example |
|---|---|---|
| Pitch | Frequency (vibrations per second) | A whistle is high; a tuba is low |
| Loudness | Amplitude (size of vibrations) | Shouting vs. whispering |
| Quality (timbre) | The mix of frequencies | A piano and a violin playing the same note sound different |
Loudness in decibels
Loudness is measured in decibels (dB). The scale is unusual: every increase of 10 dB means the sound is 10 times more intense.
| Sound | Decibels |
|---|---|
| Rustling leaves | 20 dB |
| Normal conversation | 60 dB |
| Vacuum cleaner | 70 dB |
| Heavy traffic | 85 dB |
| Snowmobile | 100 dB |
| Rock concert | 110 dB |
| Fireworks up close | 140–150 dB |
Long exposure to sounds above 85 dB can cause permanent hearing loss. Use earplugs or ear protection at loud events and keep headphones at a moderate volume.
How we hear
| Part of the ear | Job |
|---|---|
| Outer ear | Collects sound and funnels it into the ear canal |
| Eardrum | A thin membrane that vibrates when sound waves hit it |
| Middle ear bones (hammer, anvil, stirrup) | Pass the vibrations along and make them stronger; the stirrup is the smallest bone in your body |
| Cochlea | A snail-shaped tube of fluid with tiny hair cells that turn vibrations into nerve signals |
| Auditory nerve | Carries signals to the brain |
Your brain then interprets the signals as voices, music or noise.
Animal hearing
| Animal | Hearing range |
|---|---|
| Humans | 20–20 000 Hz |
| Dogs | About 40–45 000 Hz |
| Cats | About 45–64 000 Hz |
| Bats | Up to about 200 000 Hz |
| Elephants | As low as about 14 Hz |
Sounds too high for humans are ultrasound; sounds too low are infrasound.
Common mistakes
- Thinking sound travels through space.
- Thinking air travels from the source to your ear. Only energy does.
- Thinking sound is faster in air than in solids.
Try it yourself
- Stretch rubber bands of different thicknesses over a box. Pluck them and compare the sounds.
- Put your ear on a table while a friend taps the far end gently. Then listen through the air. Which is louder?
- Sprinkle rice on plastic wrap stretched over a bowl. Bang a pot nearby. Watch the rice jump.
Quick recap
- Sound is made by vibrations.
- It travels as a longitudinal wave of compressions and rarefactions.
- Sound needs a medium and travels fastest in solids.
- Pitch depends on frequency; loudness depends on amplitude and is measured in decibels.
- Your ear turns vibrations into signals your brain understands.