Acoustic vibrations propagate as mechanical waves of pressure in a transmission medium such as gas, liquid or solid. The speed of sound in air at 20 ºC is about 343 m/s (1,235 km/h) and complexly depends on density and pressure/stiffness of the medium. Audio range falls between infrasonic (<20 Hz) and ultrasonic (>20 kHz) frequencies. Sound is amplified and transformed into nerve signals by mechanically activated hair cells emitting glutamate neurotransmitter in a basilar membrane in the cochlea of the human inner ear. It happens in a spiral organ with 2.5 coils of tonotopically organized bone tissue resonating with different frequencies in its different locations.
Human auditory system represents information about different characteristics of the sound. Slowest vibration of the sound gives us a sense of a particular pitch, while spread and intensity of overtones over time build up the feeling of timbre and overall sound pressure and complexity give us a sense of its loudness. Cognitive separation of auditory objects builds sonic texture and differences between signals from two ears help recognize spatial location of the sound in an environmental context.