Sound engineers work to capture sound in a way that either imitates, enhances or manipulates the way the human ear perceives it. A common theory among sound engineers is that in order to create a recording that sounds full and natural, multiple microphones should be used and placed at certain distances to represent the human ears.
The mixing process involves creating the right balance between recorded or live sounds, adding effects and manipulating audio. A good mix creates an illusion of a certain environment. For example, according to sound theory, reverb is a natural characteristic of any enclosed space. Therefore, artificially increasing the amount of reverb by adding reflected (indirect) sound waves will create an illusion of a larger space. When applied correctly on instruments or vocals they will sound larger, powerful and will stand out in the mix.
Mastering is the process in which the sound engineer enhances audio quality, volume and clarity in the final mix to the highest possible level before the product is reproduced. The levels of certain audio frequencies are often adjusted by using equalizers, and knowledge of sound theory is extremely important at this step as this process can either bring out the most important elements of a song or take away from them.