What is a sound?
Definition: Sound is a form of energy caused by vibration
The vibrations which the human ear perceives to be sound are created by a body in contact with the air. For example, the cone of a speaker. The moving body will alternately compress and expand (or rarefy) the air particles, resulting in slight fluctuations in normal atmospheric pressure.
Sound is produced when an object vibrates and causes the air
around it to move. Certain waves are produced depending on the direction of travel of the waves to the source. The diagram below represents a longitudinal wave as the air particles move in the same dimension as the direction as
wave travel.
NB - As sound is an experience perceived by the ear, they have to be represented in a graphical form by means of an oscilloscope.
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Compression = Increase in atmospheric pressure
Rare Fraction = Decrease in atmospheric pressure
Sound must always have a medium to travel through. This could be a variety of different materials or gases, for example, air, water, wood. However, it cannot travel through a vacuum which means that nothing would be heard in space! Very rarely would you find yourself in a situation where you would encounter a vacuum to experience this trouble but it is important to understand that because everything around you in is made from slightly different materials so they would react different to the same sound. Simply put, the denser the medium, the faster the speed of sound.
The opposite to longitudinal wave motion is transverse wave motion. An example of a transverse wave motion is in vibrating string. As strings move up and down, the sound waves being produced are moving at right angles.
C - Compression
R - Rare Fraction
N - Normal