Sound is a longitudinal wave of squeezes and stretches in the air (or in water, or a solid). Like any wave it has a frequency and an amplitude, and those are exactly what your ears hear as pitch and loudness. It travels at a speed set by the material.
Key idea
Pitch comes from frequency: higher frequency, higher note. Loudness comes from amplitude: bigger squeezes, louder sound. Sound needs a medium; it travels at about 343 m/s in air at 20 °C and cannot cross empty space.
Why it works
Your eardrum is pushed in and out by the air's squeezes and stretches. Faster squeezes (more per second) make it vibrate faster, which your brain hears as a higher note. Bigger squeezes push it harder, which you hear as louder. Pitch and loudness are separate dials: a whisper can be high and a drum can be loud and low.
Worked example: Two notes
Note A is 440 Hz with a small amplitude. Note B is 220 Hz with a large amplitude. Which is higher, and which is louder?
- Pitch follows frequency: A (440 Hz) is higher.
- Loudness follows amplitude: B is louder.
Answer: A is higher; B is louder
Worked example: How far was the lightning?
You see a lightning flash and hear the thunder 6 s later. About how far away was the strike?
- The light arrives almost instantly; the sound takes 6 s.
- $d = v\,t = 343\,\mathrm{m/s} \times 6\,\mathrm{s} \approx 2{,}060$ m, about 2 km.
- Rule of thumb: about 3 seconds per kilometer.
Answer: About 2 km
Common mistake
Linking pitch to loudness, as if louder sounds were higher. Turning up the volume on a song doesn't change its notes. Check: pitch goes with frequency, loudness with amplitude.
| Medium | Speed of sound (about) |
|---|---|
| Air at 20 °C | 343 m/s |
| Water | 1,500 m/s |
| Steel | more than 5,000 m/s |
| Empty space | none: sound can't travel |
People hear frequencies from about 20 Hz to 20,000 Hz. Dogs hear higher; bats and dolphins use sounds far above our range.