The physical stimuli for the sense of hearing are sound waves, which produce vibration in the eardrum. Frequency is the number of cycles per second in a wave, expressed in a unit called hertz. Frequency is the primary determinant of pitch–how high or low the tone seems to be. Amplitude is the magnitude of a wave; it largely determines the loudness of a sound. Loudness is measured in decibels. The complex pattern of overtones determines the timbre of a sound.

Hearing begins when sound waves strike the eardrum and cause it to vibrate. This vibration, in turn, makes three bones in the middle ear–the hammer, the anvil, and the stirrup–vibrate in sequence. These vibrations are magnified in their passage through the middle ear deep into the inner ear. The oval window, which is attached to the stirrup, and the round window are membranes between the middle and inner ear. In the inner ear, the vibrations cause the fluid inside the cochlea to vibrate, pushing the basilar membrane and the organ of Corti up and down.

Inside the organ of Corti are tiny hair cells that act as sensory receptors for hearing. Stimulation of these receptors produces auditory signals that are transmitted to the brain through the auditory nerve. The brain pools the information from thousands of these cells to create the perception of sounds.

There are two basic views that explain how different sound-wave patterns are coded into neural messages. Place theory states that the brain determines pitch by noting the place on the basilar membrane where the message is strongest. Frequency theory holds that the frequency of vibrations of the basilar membrane as a whole is translated into an equivalent frequency of nerve impulses. Neurons, however, cannot fire as rapidly as the frequency of the highest-pitched sound. This suggests a volley principle, whereby nerve cells fire in sequence to send a rapid series of impulses to the brain.