Approximately 17 percent (36 million) of American adults report some degree of hearing loss. But how can we define hearing loss? And where does hearing loss occur, exactly? To fully understand hearing loss, we'll first introduce the normal anatomy and function of the human ear.

Anatomy of the ear
The ear consists of three different sections: the outer ear, the middle ear and the inner ear. Although hearing occurs first in the ear, it is ultimately interpreted by the brain which receives transmitted sound signals.

1. Outer ear - The outer ear consists of the part of the ear you can see (pinna) and the ear canal. The outer ear channels and filters sounds into the ear canal.
2. Middle ear - The eardrum is the start of the middle ear, which is filled with air and contains three small bones (the ossicles), which connect the eardrum to the inner ear. The bones transform sounds from the environment into mechanical energy to the inner ear. Bones of the middle ear include: the malleus (the hammer), the incus (the anvil), and the stapes (the stirrup)
3. Inner ear - Mechanical energy is transformed into neuroelectric impulses that are then transmitted to our brains in the inner ear. The cochlea, filled with two kinds of fluid, protects thousands of fibers, which enable proper transmission of the neuroelectric impulses to the brain. The receptors for balance are also located in the inner ear. That is why inflammation of the inner ear might be accompanied with vertigo.

Types of hearing loss
Hearing loss can be categorized by where or what part of the auditory system is damaged. Hearing loss is also defined by the extent of loss at each frequency of sound, or configuration. For example, a hearing loss that only affects high frequencies is described as a "high frequency loss". Some hearing loss configurations are "flat", indicating the same amount of hearing loss for low and high tones. The main types of hearing loss include:

Conductive hearing loss - sound is not conducted efficiently through the outer ear canal to the eardrum the ossicles and involves a reduction in sound level, or the ability to hear faint sounds. This type of hearing loss can often be medically or surgically corrected.

Mixed hearing loss - occurs when both the outer or middle ear and in the inner ear (cochlea) or auditory nerve are damaged.

Sensorineural hearing loss - characterized by damage to the inner ear or to the nerve pathways from the inner ear to the brain. This type of hearing loss involves reduction in sound level, or ability to hear faint sounds, but also affects speech understanding, or ability to hear clearly. Sensorineural hearing loss cannot be medically or surgically corrected and is permanent.

Unilateral hearing loss (UHL) - UHL occurs when hearing is normal in one ear while hearing loss is present in the other ear and can range from mild to very severe. UHL increases risk of academic, speech/language and social/emotional difficulties.

What is hearing loss?
Hearing loss, also known as hearing impairment, includes being born without hearing (congenital hearing loss). Hearing loss is a sudden or gradual decrease in how well you can hear. Someone who experiences hearing loss or impairment may be able to hear some sounds or nothing at all. Depending on the cause, hearing loss can range from mild to severe and can be reversible, temporary, or permanent. The words deaf, deafness, or hard of hearing can be used when talking about hearing loss.

Who is at risk for losing hearing? And what causes hearing loss in the first place?  Continue reading our next section on hearing loss to learn about associated risk factors and causes of hearing loss here.