Sound is a pressure wave. When you speak, your vocal cords compress air molecules; those compress neighboring air molecules; the wave propagates through the room until it reaches another person's ear drum. Here's the physics from start to finish.
What sound is
Sound is a longitudinal wave — a pressure pulse traveling through a medium. Unlike light (which can travel through vacuum), sound needs molecules to vibrate.
The wave consists of:
- Compressions: regions where molecules are pushed together, slightly higher pressure.
- Rarefactions: regions where molecules are spread apart, slightly lower pressure.
The wave moves; the molecules just oscillate in place. They don't travel with the wave.
Speed of sound in different media
| Medium | Speed (m/s) | Speed (mph) |
|---|---|---|
| Air at 0°C | 331 | 740 |
| Air at 20°C | 343 | 767 |
| Air at 40°C | 355 | 794 |
| Helium at 20°C | 972 | 2,175 |
| Hydrogen | 1,310 | 2,930 |
| Water at 20°C | 1,498 | 3,350 |
| Wood | 3,500–5,000 | ~9,000 |
| Concrete | 3,200 | 7,160 |
| Glass | 4,540 | 10,150 |
| Steel | 5,960 | 13,300 |
| Diamond | 12,000 | 26,800 |
Pattern: denser, more rigid materials transmit sound faster. Steel transmits 17× faster than air.
Why sound is faster in denser materials
Counterintuitive: in fluids, you might expect denser to mean slower (water vs air). But:
Speed of sound = √(stiffness / density)
Stiffness (or "bulk modulus" in fluids) increases faster than density does going from air to water to steel. So sound speeds up despite higher density.
Steel's bulk modulus is about 100 million times that of air, while density is "only" 6,500× higher. Net effect: 17× faster sound.
Why helium voices sound funny
Sound speed in helium is 972 m/s vs 343 in air — about 2.83× faster. When you breathe helium and speak:
- Vocal cord vibration frequency is unchanged.
- The resonance of your throat shifts. Higher harmonics dominate, making your voice sound higher.
The vocal cords aren't producing higher-pitch sound; the resonating cavity emphasizes higher frequencies. Your voice's "timbre" changes.
Sulfur hexafluoride (SF₆) does the opposite — denser than air, sound travels at 130 m/s. Voice sounds much lower.
Speed of sound and temperature
Sound speed in air increases with temperature:
v = 331 + 0.6 × T (where T is Celsius)
- 0°C: 331 m/s
- 20°C: 343 m/s
- 40°C: 355 m/s
This is why a thunderclap on a hot day reaches you slightly faster than on a cold one. The difference is small but measurable.
Why sound can't travel in vacuum
No molecules = no medium = no propagation. In a perfect vacuum, no sound exists.
Outer space isn't a perfect vacuum, but molecules are so spread out (a few per cubic meter) that pressure waves don't propagate. You couldn't hear an explosion in space.
Movie soundtracks ignore this for dramatic effect. The Star Wars laser sounds wouldn't work in real space.
Frequency and wavelength
For sound: speed = frequency × wavelength.
- 20 Hz (low rumble): wavelength 17 m in air.
- 440 Hz (concert A): wavelength 78 cm.
- 1000 Hz (mid-range): wavelength 34 cm.
- 10 kHz (high pitch): wavelength 3.4 cm.
- 20 kHz (top of human hearing): wavelength 1.7 cm.
Big speakers (subwoofers) for low frequencies. Small drivers for high. The wavelength roughly matches the speaker size for efficient radiation.
Doppler effect
Moving sources change the observed frequency:
- Source moving toward you: waves bunch up; frequency rises.
- Source moving away: waves stretch; frequency drops.
An ambulance siren rising as it approaches and falling as it passes — that's the Doppler effect in action.
The same physics applies to light (used to measure how fast galaxies are moving away from us). The "redshift" of distant galaxies is the cosmic Doppler effect.
The sonic boom
An object moving faster than sound creates a shock wave. The pressure waves it makes can't escape ahead of it; they pile up into a single dense wavefront.
Mach 1 = the speed of sound at that altitude/temperature. At ground level, Mach 1 ≈ 767 mph.
When a supersonic aircraft passes over you, you hear:
- First, nothing (the plane is approaching faster than sound).
- Then, the sonic boom (single sharp pop) — the shock wave.
- Then, the engine noise (catching up).
The boom can rattle windows, set off car alarms, cause structural damage in extreme cases. This is why supersonic flight over land is restricted.
Sound levels and decibels
Loudness is measured in decibels:
- 0 dB: threshold of hearing
- 30 dB: whisper
- 60 dB: conversation
- 85 dB: dangerous for prolonged exposure (OSHA threshold)
- 110 dB: rock concert
- 140 dB: jet engine close up (instant damage)
- 180 dB: rocket engine (eardrum rupture)
The decibel scale is logarithmic — every 10 dB is 10× the energy. 80 dB has 100× the energy of 60 dB.
Echolocation
Bats and dolphins use sound to locate objects. They emit a pulse, listen for the echo, and use the delay to calculate distance.
Modern technology does this too:
- Sonar (ships, submarines)
- Ultrasonic medical imaging
- Distance sensors in cars (parking)
- Underwater communication
Sound is excellent for finding objects underwater because water transmits sound very well; light is absorbed quickly.
Calculate wave properties
Our wave frequency calculator uses v = f × λ. Enter wave speed (343 m/s for air) and either frequency or wavelength to get the other.