Sound and ultrasound - Higher
Sound is caused by the vibration of particles but not all vibrations can be heard as sound. Common ideas about sound come from the limited range of vibrations that human ears can detect.
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
Oscilloscope traces showing the following sounds:
- quiet, low pitch sound
- loud, low pitch sound
- 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).