In the purest sense, Audio could be defined as periodic fluctuations of air pressure or electrical energy at frequencies and amplitudes within the human range of hearing. Audio is, in other words, sound, or electrical energy that represents sound. Sound is a class of kinetic energy called acoustical energy which consists of fluctuating waves of air pressure. In recording or sound reinforcement, audio is also commonly referred to as sound, electrically represented in the form of fluctuating voltage or current. Thus, audio is acoustic, mechanical, or electrical frequencies corresponding to normally audible sound waves.