In this chapter the characteristics of sound as an airborne vibration and as a human sensation are tied together. Sound is simply an airborne version of vibration, which is why the two topics are inextricably linked. By definition, the sound quality of an audio system can only be assessed by human hearing. Many items of audio equipment can only be designed well with a good knowledge of the human hearing mechanism. The sense we call hearing results from acoustic, mechanical, nervous and mental processes in the ear/brain combination, leading to the term psychoacoustics. The frequency response of the ear is not at all uniform and it also changes with sound pressure level (SPL). The frequency range of human hearing is extremely wide, covering some ten octaves without interruption. An understanding of the mechanisms of direction sensing is important for the successful implementation of spatial illusions such as stereophonic sound.