How sound is made
Every sound begins with a vibration. When you pluck a guitar string, tap a drum or speak, an object vibrates back and forth very quickly. These vibrations push and pull the particles of the material around them, creating a sound wave that spreads outward in all directions.
A sound wave carries energy from the vibrating source to your ear. It does not carry the particles along with it — each particle only moves a little and then returns to its resting place, passing the energy to the next particle.
Key idea
Sound is a form of energy produced by vibration. It must travel through a medium (a solid, liquid or gas) and cannot travel through a vacuum such as outer space.
A medium is needed
Because sound needs particles to pass the vibration along, it cannot travel through empty space. That is why astronauts in space use radios to talk — there is no air to carry their voices. Sound generally travels fastest in solids, slower in liquids, and slowest in gases, because particles are packed most closely in solids.
| Medium | Speed of sound (about) |
| Air | 340 m/s |
| Water | 1500 m/s |
| Steel | 5000 m/s |
Two main characteristics
A sound wave can be described by its amplitude and its frequency. Amplitude is the size of the vibration — a bigger amplitude means a louder sound. Frequency is the number of vibrations per second, measured in hertz (Hz) — a higher frequency means a higher pitch.
Example
Tap a table gently and it makes a soft sound (small amplitude). Tap it hard and it makes a loud sound (large amplitude) — the harder tap gives the particles a bigger vibration.
Remember
- Sound is produced by vibration.
- Sound needs a medium and cannot travel through a vacuum.
- Frequency is measured in hertz (Hz).