Frequency theory attempts to explain how the brain experiences sound waves. While frequency theory is primarily a physiological theory that seeks to explain how the anatomical structure of the ear accounts for hearing, it is also a psychological theory that explores how sound is experienced by the mind.
Before frequency theory can be fully understood, a brief description of the physiological structure of the ear is necessary. Sound is picked up by the outer ear, which is comprised of the auricle and the external auditory canal. At this stage, sound is an acoustical signal. Separating the outer and middle ear is the tympanic membrane or eardrum. As the acoustical signal enters the middle ear, the acoustic signal becomes mechanical due to the rocking motion of the ossicular chain, which transmits the signal and increases the gain of the signal about 22 decibels (dB) to the inner ear, where the sound will enter a fluid filled cavity, called the cochlea.
The cochlea sits in the inner ear and consists of three fluid-filled chambers: the scala tympani, the scala vesibule, and the scala media. The scala media contains the organ of corti, known as the hearing organ. The organ of corti houses hair cells that become excited when the signal enters the cochlea, which is now a hydraulic signal, and displaces the fluid. When the fluid is displaced, it excites the hair cells, which in turn causes them to convert the hydraulic signal to a mechanical signal. This causes the auditory nerve to fire, sending an electric signal to the auditory system of the brain, which the brain experiences as sound.
Frequency theory states that the pitch is encoded by the frequency of discharge in the primary auditory fiber. The basilar membrane moves up and down due to the displacement of the perilymph and endolymph fluid in the cochlea, caused by each individual sine wave. The movement of the membrane causes the hair cells in the cochlea to become excited. Each nerve correlates with a specific frequency. Once that specific wave enters the cochlea, its frequency and intensity is sensitive to a specific nerve and it causes that nerve to fire. The nerve cannot send another message until the message has been sent and the nerve recovers. Each nerve fiber in the auditory nerve sends the information to the auditory cortex where it assembles the information and puts it together to perceive and interpret the auditory signal.
Frequency theory, in more simplistic terms, explains how the human brain basically experiences a representation system of hearing. Frequency theory in essence claims that human beings do not actually experience sound waves themselves, but rather vibrations on the auditory nerve, the frequencies of which are identical to the frequencies of the sound waves coming into the ear.