Sound is all around us; everywhere we go (something like that line). But what is sound? Aristotle (384 -322 BC) understood that sound was caused by a change in air pressure. When an object vibrates, it causes air molecules around the object to move, thus causing the sound to travel away from the object. When air particles vibrate they either push or pull. When a layer is pushed away it causes a subsequent increase in pressure of the region in front of it, these are called compressions. The pulls on the other hand are the opposite causing a decrease in pressure and are called rarefactions. This process repeats as far as the vibrations can go causing the sound to travel. This was further explored by Galileo (1564-1642) who discovered that sound travels as a wave. Due to this property sound require a medium for propagation i.e. water, air and cannot travel through a vacuum, as there are no particles to vibrate.