Sound Waves — How Sound Travels, Simply Explained
Sound is a longitudinal pressure wave. Speed of sound, pitch, loudness, echoes, and why there is no sound in space.
Quick answer
Sound is a traveling pattern of air compressions and rarefactions — a longitudinal wave. Your eardrum vibrates in step with these pressure oscillations, and your brain reads the frequency as pitch and the amplitude as loudness.
Unit: Hz for pitch (frequency), dB for loudness
Example: Clap your hands: the sudden air compression races outward at ~343 m/s. A wall 170 m away returns an echo after about 1 second (there and back).
What you will learn
- Describe sound as a longitudinal pressure wave
- State the speed of sound and what affects it
- Distinguish pitch from loudness
- Explain echoes, ultrasound and the vacuum silence
Sound is moving air
Clap once. What races outward is not air itself but a pattern: molecules bunch together (compression), then spread apart (rarefaction), then bunch again. Each molecule only jiggles back and forth — the pattern does the traveling.
Because the jiggling is along the travel direction, sound is a longitudinal wave. Your eardrum is simply a membrane that vibrates in sympathy with these pressure swings, and your auditory system decodes them.
Speed of sound
In dry air at 20 °C, sound travels at about 343 m/s — a million times slower than light, which is why you see lightning long before hearing thunder.
The speed isn't fixed: it grows with temperature (hotter air molecules move faster and hand off the disturbance quicker) and varies hugely by medium — roughly 1500 m/s in water, 5000 m/s in steel. Denser, stiffer media transmit the jiggle faster.
Pitch and loudness
Two independent properties, two independent sensations:
- Pitch ← frequency. More oscillations per second = higher note. Human hearing spans roughly 20 Hz to 20,000 Hz; a piano's highest key is about 4186 Hz.
- Loudness ← amplitude (intensity). Bigger pressure swings = louder sound, measured in decibels.
A piccolo and a tuba can play the same pitch at different loudness, or the same loudness at different pitches — the two knobs turn separately.
Why there is no sound in space
No air, no sound. The Moon's surface is silent no matter how violently you strike it, because there are (practically) no molecules to carry compressions. Astronauts talk by radio — electromagnetic waves, which need no medium.
This also explains an old puzzle: the Sun is unimaginably violent, yet we hear nothing. Its roar dies in the vacuum between us.
Sound = medium + vibration. Remove the medium and even the loudest event is silent. Light has no such requirement — that's why we see the distant universe but never hear it.
Echoes and ultrasound
- Echoes: shout at a distant wall and time the return. Distance = 343 × time / 2 — the divide-by-2 accounts for the round trip.
- Ultrasound: frequencies above ~20 kHz are inaudible but reflect beautifully off tissue boundaries — the basis of medical imaging and fetal scans.
- Infrasound: below ~20 Hz, elephants and whales communicate across kilometers; humans feel it more as vibration than sound.
Variables and units
| Symbol | Name | Unit | Meaning |
|---|---|---|---|
| Speed of sound | m/s | How fast sound travels; ~343 m/s in air at 20 °C | |
| Frequency | Hz | Oscillations per second; perceived as pitch | |
| Amplitude | Pa (pascals) | Size of pressure swings; perceived as loudness |
Common mistakes
Speed depends on the medium, not the amplitude. A whisper and a shout cross a room in the same time — the shout just carries more energy.
At 343 m/s, sound takes about 3 seconds per kilometer. Thunder's delay after lightning is literally a distance measurement: count seconds, divide by 3, get kilometers.
Pitch is frequency (how fast the air oscillates); loudness is amplitude/intensity (how hard it oscillates). A high quiet note and a low loud note differ in both.
Real-world applications
Sonar maps the seafloor by timing echoes — distance = speed × time / 2.
Medical ultrasound images organs and monitors pregnancies using reflected high-frequency sound.
Noise-cancelling headphones sample ambient sound and broadcast its exact inverse, erasing it by destructive interference.
Frequently asked questions
What is the speed of sound?
About 343 m/s in air at 20 °C. It rises with temperature (~0.6 m/s per °C) and is far higher in water (~1500 m/s) and steel (~5000 m/s).
Is there really no sound in space?
Yes. Sound needs a medium — air, water, solids. The vacuum of space has (practically) no molecules to compress, so sound can't cross it. Explosions in movies are pure artistic license.
What is a decibel?
The decibel (dB) scale for loudness. It's logarithmic: +10 dB means ten times the intensity. A whisper is ~30 dB, normal speech ~60 dB, a jet engine ~140 dB.
Can sound bend around corners?
Yes — diffraction. Long-wavelength bass bends around doorways easily; short-wavelength treble is blocked more. That's why you hear the thump of a neighbor's music but not the lyrics.