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The ears receive sounds and send them to the auditory cortex for processing. The eardrum transmits sound vibrations to the ossicles, which shunt it to the cochlea. The organ of Corti contains specialized sensory cells that detect sounds at various frequencies and transform them into nerve signals. Humans can hear sounds between 20Hz and 20kHz and hearing tends to degrade later in life. Hearing aids and cochlear implants can be used to deal with this problem.
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 collecting sound in the ear is the eardrum, or tympanic membrane, which separates the outer ear from the middle ear. The eardrum transmits sound vibrations to the ossicles, the three smallest bones in the human body, which then shunt it to the fluid-filled, labyrinth-like structure in the inner ear called the cochlea, where the real organs of hearing reside. Collectively, the set 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 small hair capable of picking up tiny vibrations in the cochlear fluid. If destroyed by loud sounds, these hairs never grow back. Different hairs specialize in detecting sounds at various frequencies and transforming them into nerve signals to be sent to the brain. Also in the cochlea is a triad of fluid-filled rings 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 20Hz and 20kHz and louder than 5 – 15 decibels. People hear best in the frequency range between 1 kHz and 5 kHz. Some very low frequency sounds cannot be heard consciously, but people tend to have a vague feeling of unease when in their presence. This may be partly responsible for the alleged ghost phenomenon.
The upper and lower rate limits are based in part on the physical size of the cells and organ and in part on the sensitivity of the cells. With the help of the right scientific tools, sounds of almost any frequency and volume can be recorded.
People’s sense of hearing tends to degrade later in life. Hearing aids are the most common way to deal with this problem, although cochlear implants may be used in extreme cases.
