Auditory phonetics is concerned with the study of the reception and perception of sounds. The physiology of the ear and how acoustic sensations are sent to the brain comprise the two areas of auditory phonetics. Due to the different physical characteristics of sounds (received as sound waves), different sounds can be perceived by the listener. It is still unclear which processes are involved in the perception of sounds and no further comments will be made here. Interested readers should consult J.D. O'Connor's book Phonetics (1973 | 1991) and the references given therein.
References: O'Connor, J.D. 1973. Phonetics. Harmondsworth.