5.3 Sound Waves

Sound waves are longitudinal waves. They are caused by vibrations.

Because they are longitudinal, they show areas of compression (high pressure) and rarefaction (low pressure).

Sound is a mechanical wave, which means it must have a medium (a substance) to travel through (like air, water, or a solid). It cannot travel through a vacuum.

The speed of sound depends on the medium. It travels fastest in solids, then liquids, then gases.

Humans can hear sounds with frequencies between 20 Hz and 20,000 Hz.

• The frequency of a sound wave determines its pitch (a high frequency = a high pitch).
• The amplitude of a sound wave determines its loudness (a high amplitude = a loud sound).

Ultrasound is sound with a frequency above human hearing (higher than 20,000 Hz).

It is used in:

• Medical Imaging: Used for pre-natal (baby) scans. The ultrasound waves reflect off the boundary between different tissues (like fluid and bone) to build an image.
• Industrial Use: Finding cracks in metal or pipes.

Infrasound is sound with a frequency below human hearing (lower than 20 Hz).

It is produced by large-scale natural events like earthquakes and volcanic eruptions. Some animals, like elephants, use it to communicate over long distances.