Hearing loss, or hearing impairment (say: im-pare-ment), happens when there is a problem with one or more parts of the ear or ears. Someone who has hearing loss or impairment may be able to hear some sounds or nothing at all. Impairment means something is not working correctly or as well as it should. People also may use the words deaf, deafness, or hard of hearing when they ' re talking about hearing loss.

About 3 in 1,000 babies are born with hearing impairment, making it the most common birth defect. A hearing problem can also develop later in life. To understand how and why hearing loss happens, it helps to know how the ear works.

The ear is made up of three different sections: the outer ear, the middle ear, and the inner ear. These parts work together so you can hear and process sounds. The outer ear, or pinna (the part you can see), picks up sound waves and the waves then travel through the outer ear canal.

When the sound waves hit the eardrum in the middle ear, the eardrum starts to vibrate. When the eardrum vibrates, it moves three tiny bones in your ear. These bones are called the hammer (or malleus), anvil (or incus), and stirrup (or stapes). They help sound move along on its journey into the inner ear.

The vibrations then travel to the cochlea, which is filled with liquid and lined with cells that have thousands of tiny hairs on their surfaces. There are two types of hair cells: the outer and inner cells. The sound vibrations make the tiny hairs move. The outer hair cells take the sound information, amplify it (make it louder), and tune it. The inner hair cells send the sound information to your hearing nerve, which then sends it to your brain, allowing you to hear.

Degree of Hearing Loss

Degree of hearing loss refers to the severity of the loss. The numbers are representative of the patient's thresholds, or the softest intensity at which sound is perceived. The following is one of the more commonly used classification systems:

Source: Clark, J. G. (1981). Uses and abuses of hearing loss classification. Asha, 23, 493–500. 

The configuration or shape of the hearing loss refers to the extent of hearing loss at each frequency and the overall picture of hearing that is created. For example, a hearing loss that only affects the high frequencies would be described as a high-frequency loss. Its configuration would show good hearing in the low frequencies and poor hearing in the high frequencies. On the other hand, if only the low frequencies are affected, the configuration would show poorer hearing for low tones and better hearing for high tones. Some hearing loss configurations are flat, indicating the same amount of hearing loss for low and high tones.

Other descriptors associated with hearing loss are:

• Bilateral versus unilateral. Bilateral hearing loss means both ears are affected. Unilateral hearing loss means only one ear is affected.
• Symmetrical versus asymmetrical. Symmetrical hearing loss means that the degree and configuration of hearing loss are the same in each ear. An asymmetrical hearing loss is one in which the degree and/or configuration of the loss is different for each ear.
• Progressive versus sudden hearing loss. Progressive hearing loss is a hearing loss that becomes increasingly worse over time. A sudden hearing loss is one that has an acute or rapid onset and therefore occurs quickly, requiring immediate medical attention to determine its cause and treatment.
• Fluctuating versus stable hearing loss. Some hearing losses change—sometimes getting better, sometimes getting worse. Fluctuating hearing loss is typically a symptom of conductive hearing loss caused by ear infection and middle ear fluid, but also presents in other conditions such as Meniere's disease.

Warning Signs of Hearing Loss

• Difficulty hearing conversations, especially in the presence of background noise
• Frequently asking people to repeat what they have said
• Misunderstanding what someone has said
• Difficulty hearing on the telephone
• Requiring the TV or radio volume to be louder than preferred by others in the room
• Feeling that people are mumbling when they are talking
• Difficulty hearing certain sounds or pitches
• Agreeing or nodding your head during conversations when you are uncertain of what has been said
• Removing yourself from conversations because it is too difficult and taxing to hear
• Reading people ' s lips in order to follow what they are saying
• Straining to hear or keep up with a conversation