The speed of sound in air
Sound can travel in air at approximately 332 metres per second. This is fast but not nearly as fast as light which travels at 300 000 kilometres per second. This difference in speeds enables us to appreciate that sound does take time to travel. When we see lightning the sound it produces at exactly the same time is often heard as thunder a few seconds later by an observer a few miles away. It is, however, possible to watch a sound being made at some distance away and to detect a slight delay in hearing the sound. Exploding fireworks, the click of a ball on a cricket bat and a child bashing a dustbin lid at the other end of a playing field will all provide this opportunity.
Sound travels in solids and liquids too
Sound waves travel faster and more effectively in liquids than in air and travel even more effectively in solids. This concept is particularly hard to believe since our general experiences lead us to hear reduced or garbled sounds in water or behind a solid door. There are reasons for this. Most of our everyday experiences are when a sound travels first through air and then through water or a solid. When the sound wave transfers from air into a solid some of it is reflected back into the air and some may be absorbed by the new medium. The noise coming from a room will be reduced if an observer outside closes the door. Sound from within the room will travel to the closed door and start it vibrating. The vibrating door will set the air on the outside vibrating too and a little of the original sound will be transmitted to the observer. However, some of the sound arriving at the door will have been reflected back into the room (actually making the noise inside the room louder!). Also some of the sound energy will have been used up in making the door begin to vibrate so we can say some of the sound has been absorbed by the door.
Hearing sounds through solids
If the sound is made directly within the solid and this travels directly to the ear then both reflection and absorption are reduced or eliminated. Thus putting an ear to a desk and making a quiet sound at the other end will demonstrate how well the sound will travel. (Compare this with listening to the same sound through the air.) Further examples of sounds travelling effectively through solids include listening to the central heating pump by placing a (protected) ear to a radiator, listening to a string telephone and putting an ear to the ground to hear the approach of horses hooves.
