# Speed of sound

Sound is a mechanical wave - it propogates (moves along) by using the vibrations of particles in matter.

The speed of sound depends upon how easy it is to pass the energy on to neighbouring particles. Sound travels quickest through solids with a regular structure of particles as it is easy to set yp vibrations in neighbouring particles. It travels slowest through gases... and doesn't travel at all through a vacuum as there are no particles through which to pass on the vibrations.

• Sound travels much slower than light does.

Question: If there is a ten second gap between you seeing a flash of lightening and hearing the thunderclap. How far away is the storm centre?

Light travels so fast that the time interval between the lightening happening and you seeing the flash is negligible.

Sound travels at 330 m/s therefore in 10 seconds it travels 3300 metres. The storm is therefore 3.3km away.

The time taken for the light to reach you is actually :-

Speed = distance/time

time = distance/speed

=  3300/300,000,000

= 0.000011s

(rather small , eh?
THAT's why we can ignore it !!!)