Image courtesy of NASA
The structure of the ear can be divided into three main parts: the outer ear or pinna, the middle ear and the inner ear. The outer structure of the ear is responsible, in part, for helping us to place the original location of a sound, be it ahead or behind, above or below us. It also helps to funnel and focus sound waves on their way to the middle ear and auditory canal. The middle ear contains the auditory canal, which terminates in the eardrum, or tympanic membrane. Attached to the other side of the eardrum, in a small space of air, are three tiny bones or ossicles, the malleus, incus and stapes (or hammer, anvil and stirrup) which then attach to a fluid-filled structure called the cochlea of the inner ear at a point called the oval window. It is in the cochlea that the vibrations transmitted from the eardrum through the tiny bones are converted into electrical impulses sent along the auditory nerve to the brain. The inner ear, which is surrounded by bone, also contains semicircular canals, which function more for purposes of equilibrium than hearing.
One of the difficulties above is determining a universal threshold of pain is that the chain of ossicles can be stiffened or muted by a contraction of the stapedius muscle. This provides a form of protection against loud sustained sounds, but not of sharp, sudden ones, such as a gunshot. This reflex is far less efficient in older people, which along with differing tastes, may explain their lower tolerance to louder music as well as an increased risk level for hearing loss.
The most fascinating aspect of perception takes place in an area of the cochlea called the basilar membrane. The cochlea is a tapered tube, which circles around itself like the scroll on a violin. The basilar membrane divides the tube lengthwise into two fluid-filled canals, which are joined at the tapered end. The ossicles transmit the vibration to the cochlea where they attach at the oval window. The resultant waves travel down the basilar membrane where they are sensed by the approximately 16-20,000 hair cells (cilia) attached to it which poke up from a third canal called the organ of Corti. It is the organ of Corti that transforms the stimulated hair cells into nerve impulses. Because of the tapered design of the cochlea, waveforms traveling down the basilar membrane peak in amplitude at differing spots along the way according to their frequency. Higher frequencies peak out at a shorter distance down the tube than lower frequencies. The hair cells at that peak point give us a sense of that particular frequencyit is thought that a single musical pitch is perceived by 10-12 hair cells. Due the tapered shape of the cochlea, the distance between pitches follows the same logarithmic distance as our perception of pitch i.e. the placement of octaves are equidistant. This arrangement is responsible for the fact that a higher frequency can mask or hide a lower one, but a lower one can mask a higher on.