The flash of lightning due to collision of clouds is seen much before the thunder, although both occur simultaneously. This happens because the velocity of light is greater than the velocity of sound. The speed of sound depends on (1) elasticity and (2) density of the medium through which it propagates.
Sound waves are emitted from a vibrating source and transmitted through air. The human ear can hear sound waves in the range 20 Hz and 20 kHz. This range is known as audible range. The sound waves having frequencies above the audible range are known as ultrasonic waves and it is usually referred as ultrasound. The sound waves having frequencies less than the audible range are called infrasonic waves.
