Hearing begins with the ears, which receive sounds and send them to the auditory cortex, near the back of the brain, for processing. The primary instrument for sound gathering in the ear is the eardrum, or tympanic membrane, which separates the outer ear from the middle ear. The eardrum passes along sound vibrations to the ossicles, the three smallest bones in the human body, which then divert it to the fluid-filled, labyrinth-like structure in the inner ear called the cochlea, where the true hearing organs reside. Collectively, the assemblage of sound-processing organs is called the auditory system.
The main auditory sensory organ is the Organ of Corti, named after the Italian anatomist Alfonso Corti, who discovered it in 1851. The Organ of Corti is internal to the cochlea and contains about 15,000 - 20,000 specialized sensory cells, each with a little hair capable of picking up minute vibrations in the cochlear fluid. If destroyed by loud sounds, these hairs never grow back. Different hairs are specialized to detecting sounds at various frequencies, and turn them into nerve signals to be sent to the brain. Also in the cochlea is a triad of fluid-filled loops oriented in the vertical, diagonal, and horizontal directions, which help give people a sense of balance.
Most humans are capable of hearing sounds with a frequency between 20 Hz and 20 kHz, and a volume greater than 5 - 15 decibels. People hear best in the frequency range between 1 kHz and 5 kHz. Some very low frequency sounds cannot be consciously heard, but people tend to get a vague feeling of unease when in their presence. This may be partially responsible for the alleged phenomenon of ghosts.
The upper and lower frequency limits are based partially on the physical size of the cells and organ and partially on cell sensitivity. With the aid of the right scientific instruments, sounds of almost all frequencies and volumes can be recorded.
People's sense of hearing tends to degrade later in life. Hearing aids are the most frequent way of addressing this, though in extreme cases, cochlear implants may be used.