How We Hear
We hear when sound waves (vibrations) reach our auditory system (hearing organs).
Humans usually can hear from 20 Hz to 20,000 Hz, and sounds need to be strong enough to generate a neural signal in the auditory system.
The ear consists of three parts; the outer ear, the middle ear, and the inner ear.
Outer Ear - consists of "the auricle" (the parts we see) and the ear canal. The groove called the concha is acoustically the most important.
Sound waves (acoustic energy) travels to the ear and is delivered to the eardrum (or tympanic membrane, TM) through a narrow tunnel that is called the external ear canal (EEC). Your ear lobe or auricle is shaped to collect sound more efficiently, and also EEC is naturally emphasizing high frequency to enhance part of speech frequencies.
Middle Ear - eardrum ("tympanic membrane" or TM) and three tiny bones ("ossicular chain"). The three bones are called malleus, incus, and stapes. There are also two muscles that are attached to the ossicles, the eustachian tube, and middle ear cavities.
When sound reaches an ear drum, it vibrates and causes a motion of malleus that is attached to TM, then incus, and states in order. The acoustic sound is transformed to mechanical sound at the ear drum then as it moves form one bone to the next it is magnified.
Inner Ear - the ear can be served as a sensory organ because of sensory cells that are located in the cochlear (a snail-shaped structure). Two important functions of the inner ear are hearing (cochlear) and balance (semicircular canals and vestibule). The cochlear not only converts sound into a neural impulse for the auditory nerve, but also first analyze sound to prepare for further analysis in the auditory nerve system.
The end of the third bone (stapes) is attached to the entrance (oval window) of the cochlear that is filled with fluid. The sound energy that is passed through the outer ear and middle ear is now transferred to this cochlear and vibrates the fluid inside. When fluid moves by vibration, hair cells bend and open gates of channels where electric energy can transfer. Here the sound energy changes from mechanical energy to electric energy. Cochlear is a tonotopic organ (from Greek tono = frequency and topos = place) – a certain location responds to a certain pitch of sound more sensitive than the other. So as sound energy travels inside of the cochlear, sound energy that is strong enough would move their assigned locations (certain frequency) while sound energy that is not strong enough do not move in their given structure. Auditory (hearing) nerves are also a tonotopic organ as well as the auditory cortex in a brain.
Electric sound energy now then goes to the auditory (hearing) nerve then to the brain. The auditory nervous system is known as the most complex of all sensory pathways. There are two pathways: ascending and descending.
Any sound that moves up through the auditory nerve enters into the brain where sound is recognized according to a database that is stored in one’s memory.
When we talk about the auditory system it includes all of these systems from outer ear to the auditory cortex of the brain.