Sound is a form of energy. From the source of sound this energy is propagated through the surrounding medium in the form of waves. When the vibration of the source has a frequency between 20Hz and 20,000Hz these compressional waves are able to cause a sensation of hearing and are referred to as sound waves.

In the previous section, we have seen that sound is a mechanical longitudinal wave. Thus when the source of sound vibrates, it makes compressions and rarefactions which spread out through medium, which may be a solid, liquid or gas. Sound cannot travel through a vacuum because it needs particles to vibrate.

Key terms

• Source of sound: A vibrating body that could produce sound.
• Compressions: the particles are close together (high pressure region).
• Rarefactions: the particles are spread apart (low pressure region).