Telephone networks and audio noise reduction systems make use of this fact by nonlinearly compressing data samples before transmission, and then expanding them for playback. In addition, the ear has a nonlinear response to sounds of different intensity levels this nonlinear response is called loudness. Data compression techniques, such as MP3, make use of this fact.
The inner ear, for example, does significant signal processing in converting sound waveforms into neural stimuli, so certain differences between waveforms may be imperceptible. Hence, in many problems in acoustics, such as for audio processing, it is advantageous to take into account not just the mechanics of the environment, but also the fact that both the ear and the brain are involved in a person's listening experience. These nerve pulses then travel to the brain where they are perceived. The outer hair cells (OHC) of a mammalian cochlea give rise to enhanced sensitivity and better frequency resolution of the mechanical response of the cochlear partition. Hearing is not a purely mechanical phenomenon of wave propagation, but is also a sensory and perceptual event in other words, when a person hears something, that something arrives at the ear as a mechanical sound wave traveling through the air, but within the ear it is transformed into neural action potentials.
