Sound waves

Sound waves are longitudinal waves. They cause particles to vibrate parallel to the direction of wave travel. The vibrations can travel through solids, liquids or gases. The speed of sound depends on the medium through which it is travelling. When travelling through air, the speed of sound is about 330 metres per second (m/s). Sound cannot travel through a vacuum because there are no particles to carry the vibrations.

The ear

The human ear detects sound. Sound waves enter the ear canal and cause the eardrum to vibrate. Three small bones transmit these vibrations to the cochlea. This produces electrical signals which pass through the auditory nerve to the brain, where they are interpreted as sound.

Properties of sound

The frequency of a sound wave is related to the pitch that is heard:

  • high frequency sound waves are high pitched
  • low frequency sound waves are low pitched

The amplitude of a sound wave is related to the volume of the sound:

  • high amplitude sound waves are loud
  • low amplitude sound waves are quiet
Trace one: peak heights half unit, peak to peak distance 1.5 units. Trace two: peak heights one unit, peak to peak distance 1.5 units. Trace three: peak heights one unit, peak to peak one unit.

Oscilloscope traces showing the following sounds:

  1. quiet, low pitch sound
  2. loud, low pitch sound
  3. loud, high pitch sound

The cochlea is only stimulated by a limited range of frequencies. This means that humans can only hear certain frequencies. The range of normal human hearing is 20 Hertz (Hz) to 20,000 Hz (20 kHz).

A graph charting increasing sound frequencies. The range of human hearing is between 20 hertz (low pitch) and 20 kilohertz (high pitch). Frequencies above 20 kilohertz are classed as ultrasound.