Last reviewed 30 Sept 2026 · 7 min read
What is sound?
Sound is a form of energy produced by vibrating bodies (a vibrating string, a tuning fork, vocal cords). It needs a material medium (solid, liquid or gas) to travel — sound cannot travel through a vacuum. This is why an astronaut on the Moon cannot hear another person directly, and why a bell in an evacuated jar cannot be heard.
- Sound travels as a longitudinal wave: particles of the medium vibrate parallel to the direction of the wave, producing alternate compressions (regions of high pressure) and rarefactions (low pressure).
- Sound in air is a mechanical, longitudinal wave (unlike light, which is electromagnetic and transverse). In solids sound can also travel as a transverse wave.
Wave terms
| Term | Meaning | Unit |
|---|---|---|
| Amplitude () | maximum displacement of a particle from its mean position | m |
| Wavelength () | distance between two successive compressions (or rarefactions) | m |
| Frequency () | number of vibrations (cycles) per second | hertz (Hz) |
| Time period () | time for one complete vibration; | s |
| Wave speed () | distance travelled by the wave in one second | m/s |
Frequency depends only on the source and remains the same when the wave passes from one medium to another; the speed and wavelength change.
Speed of sound
- The speed depends on the medium and its temperature; it does not depend on the loudness or frequency.
- Solids > liquids > gases: about 5000 m/s in steel/iron, about 1500 m/s in sea water, about 1480 m/s in fresh water, and about 343 m/s in air at 20 °C (331 m/s at 0 °C).
- In air, the speed increases by about 0.6 m/s for each °C rise. It is larger in humid air than in dry air (water vapour is lighter). It is affected by pressure only through density, and in a gas at constant temperature does not change with pressure.
- Sound is slower than light; in a thunderstorm we see lightning first and hear the thunder later. Distance to lightning ≈ speed of sound × time delay.
Worked ExampleExample — thunder
The thunder is heard 6 s after the lightning is seen. If the speed of sound is 340 m/s, the lightning is away.
Characteristics of sound
| Characteristic | Depends on | Notes |
|---|---|---|
| Pitch | frequency | high frequency = shrill (high pitch, e.g. a woman's or child's voice, a whistle); low frequency = grave (a man's voice, a drum) |
| Loudness | amplitude (and the sensitivity of the ear) | measured in decibels (dB); the sound intensity level is logarithmic |
| Quality (timbre) | waveform / harmonics | lets us tell two instruments or two people apart even if pitch and loudness are the same |
- Intensity is the energy flowing per second through unit area (W/m²). Loudness is a subjective measure of intensity.
- Audible range for human beings: 20 Hz to 20,000 Hz (20 kHz).
- Infrasonic sound: below 20 Hz (earthquakes, whales, elephants); ultrasonic sound: above 20 kHz.
- Typical levels: whisper about 30 dB; normal conversation about 60 dB; heavy traffic about 80 dB; a loud horn about 110 dB; the threshold of pain is about 120–130 dB. Prolonged exposure above about 85 dB can damage hearing.