Once sound enters the ears, it is processed by the physical components of the middle and inner ear and by the auditory centers of the brain. Pitch perception is affected by several factors, including the nature of how sound is processed in the body. Neural coding in the brain has an effect on how humans hear sound. The presence of multiple sounds, changes in volume, or frequency of sound pulses can affect how pitch is perceived. Theories that have attempted to define pitch perception include place theory and temporal theory.
Sound enters the ear and is first processed by the ear drum, or tympanic membrane. It then translates to movements in three small bones before passing into the cochlea, a spiral organ in the inner ear with hair cells that move with sound waves. The motion of the hairs translates to signals that get passed into the auditory nerve. Different frequencies are picked up by different parts of the basilar membrane inside the cochlea, as explained by place theory. This theory also describes how high harmonics close together are not perceived to be separate.
Theorists have also speculated that sound is perceived in relation to time, known as temporal theory. Nerve cells fire at periodic time intervals, but also serve to analyze frequency. A combination of the two theories helps in researching the nature of sound perception in humans. Neural activity in the auditory cortex has a profound effect on pitch perception, with pitch changes stimulating areas in the right hemisphere. Problems in the right temporal areas of the brain sometimes affect how someone is able to detect pitch changes.
The inability to understand pitch change is part of being tone deaf. When melodies are transposed to different keys, the relative pitch of each note is generally perceived. Sound intensity also has an effect on pitch perception. Increases in intensity for high frequency sounds will make them sound like the pitch is rising. Raising the intensity of low frequency sounds makes them sound like the pitch is getting lower.
Sounds that are pulsing or sustained also have a different pitch perception. Compared to a steady pulse, sound pulses with decreasing amplitude seem to have a higher pitch. Noises and tones that are heard over one tone can change its apparent pitch as well. Pitch perception is therefore affected by internal and external factors. A sound that remains steady in pitch sometimes seems to be changing as heard by the human ear.