Humans hear by collecting sound waves from the environment and converting them into signals that the brain can understand. Hearing would not be possible without several highly specialized anatomic structures, some of which you have probably heard of, and others that may be new to you.
A sound is a vibration in the environment that reaches the ear and moves the eardrum (tympanic membrane). The outer, visible portion of the ear that we commonly refer to as the ear (auricle) is only the first and most noticeable part of the hearing organ. The auricle, with its conical shape and many ridges, collects vibrations from the environment and funnels them into the ear canal. These vibrations strike the eardrum, causing it to vibrate.
Once the vibrations move the eardrum, soundwave energy is converted to mechanical energy. Vibrations of the eardrum are transmitted through the three small bones of the middle ear the malleus, incus, and stapes (you probably learned them as the hammer, the anvil, and the stirrup, respectively). These tiny bones (ossicles) then transmit this energy to his seashell-shaped structure called the cochlea.
The cochlea is a remarkable biological structure. It has a swirling pattern that tapers toward the center. It is filled with two different types of thick fluid (perilymph and endolymph) through which the sound energy travels. Along with the internal walls of the cochlea are specialized hair cells (cilia) that extend into the cochlear fluid. Hair cells near the opening of the cochlea respond to high frequency sounds while hair cells near the apex of the cochlea respond to low frequency sounds.
When energy from the original soundwave strikes a hair cell, it bends. This bending process stimulates an electrical signal that is sent along nerve cells to the brain. The brain (auditory cortex) then receives these electrical impulses and interprets them as sound.
The brain can decipher many bits of information from sound waves such as volume, frequency, and direction. This information is then shared with other brain regions that perform other tasks, such as interpreting the content and meaning of human speech.
Given the complexities of the system, it is not surprising that hearing loss can occur at any number of places within the system. Your audiologist can perform a number of sophisticated, noninvasive, and painless tests to determine if you have hearing loss, the source of the hearing loss, and the best treatment options for your situation.