The old riddle, “If a tree falls in the forest and there’s no one to hear it…” reminds us that sound is not “sound” at all. The sound is the brain’s interpretation of the vibrations set off by sound waves. Those waves pass through our ear canal reaching the tympanic membrane causing it to vibrate like a drum. The membrane (a.k.a. “ear drum”) transmits those vibrations to the ossicles, a set of three small bones; the hammer, anvil, and stirrup. The ossicles connect the outer ear with the inner. They also concentrate sound, amplifying and clarifying it as they transmit it to the inner ear through a small “oval window.”
The inner ear begins with the cochlea – a spiral shaped set of liquid filled tubes that resemble a snail shell. Inside the cochlea are tiny hair cells, inner and outer. The inner hair cells convert the vibrations into electronic impulses. Those impulses travel through the auditory nerve to the brain where they are interpreted as sound. The outer hair cells amplify weak sounds, enabling mammals to hear lower frequencies and to differentiate between types of sound, as in voices or music.
How Hearing is Lost
Underdevelopment or blockage of or damage to any of the parts used to process sound can result in hearing loss. Specifically, there are three categories for losing the sense of hearing:
- Conductive Hearing Loss
- Sensory Hearing Loss
- Neural Hearing Loss
The American Speech-Language-Hearing Association (ASHA) says:
Conductive hearing loss occurs when sound is not conducted efficiently through the outer ear canal to the eardrum and the tiny bones, or ossicles, of the middle ear.
This can occur with a blockage, damage to or improper development of the outer or middle ear. According to the Nemours Foundation, such loss is generally mild and treatable.
Cochlear Hair Cell Damage Source of Sensory Hearing Loss
A sensory hearing loss results from damage to the outer or inner hair cells within the cochlea, either reducing the ability to discern low-frequency sounds or destroying the conversion and transmission of the electronic impulses. Nemours calls this the most common hearing loss. It may be hereditary or not, but is most often permanent.
Sign language teachers often deal with students having this type of hearing loss. Interested parties may apply jobs to help hearing-impaired students lead near-normal lives.
Acoustic Neuroma Most Common Cause of Neural Hearing Loss
Damage to, malformation of or even lack of the auditory nerve is called Neural hearing loss. It is the primary cause is acoustic neuroma – a benign tumor that grows on the vestibular nerve (that guides balance) and presses against the auditory nerve. With early detection, the tumor may be removed preventing any further loss of hearing.
One other type of hearing loss is not centered in the ear, but in the portion of the brain that processes hearing. Nemours calls it “Central hearing loss” and says it may show as difficulty “processing speech and other auditory information. This is often referred to as ’Auditory Processing Disorder’ and may be misdiagnosed as a behavioral disorder.”
The types and causes of hearing loss are several and varied. Treatments, when they are possible at all, must address the specific type and cause to bring about any real improvement in sound reception and processing. Whether or not that treatment is wanted is a whole other matter.