Didgeridoo Sound Perception in the Ear

As sound approaches the ear, the pinna, the visible part of the outer ear, amplifies and directs it through the auditory canal. As sound moves through the auditory canal, it eventually hits the tympanic membrane, which is a thin membrane at the end of the auditory canal. The sound causes the tympanic membrane to vibrate at a frequency that is directly proportional to the frequency of the sound waves. The kinetic energy of the vibrating tympanic membrane is then transferred by the ossicles, three connected bones known as the malleus, incus, and stapes, through an area called the middle ear. At the base of the stapes, the last in the series of ossicles, there is a footplate that presses against the oval window, which is a small membrane on the surface of the cochlea. The large difference in surface area of the tympanic membrane and the smaller oval window results in an amplification of the sound signal at a ratio of 14:1. As the ossicles vibrate, the footplate vibrates the oval window, which causes wavelike motion in the cochlear fluid. The wave motion of the cochlear fluid bends the stereocilia of the hair cells. The bending of these stereocilia is what accounts for the conversion of the mechanical signal of the pressure waves in the cochlear fluid to the electrical signal in the form of action potentials in the auditory nerve.



The stereocilia are positioned in a series of lines within the Organ of Corti. The stereocilia are connected to their neighbors by tip links, which are protein structures resembling the links of a metal chain. The ends of the tip links are connected to potassium ion channels embedded in the membranes of the stereocilia of the hair cells. When the stereocilia are bent, the tip links cause the potassium ion channels to open resulting in an influx of ions into the stereocilia. Subsequently, the influx of ions results in a change in the electrical potential across the cell membrane and eventually the depolarization of the cell, which leads to an action potential in the auditory nerve.

The transfer of information from sound waves in the air to perception in the brain is an extraordinary process. Information conversion takes place throughout this process from the airwaves that make up sound to the mechanical waves of the tympanic membrane and ossicles to fluid waves of the cochlear fluid to electrical signals in the hair cells and the auditory nerve. The auditory nerve then takes this signal to the auditory cortex in the brain, which processes these sounds into cognitive signals of what we hear and perceive.