Our ability to hear depends on a complex series of events.  Sound is first picked up by the ears and then "read" by the brain. 

When something makes a noise, it sends vibrations, or sound waves, through the air.  The ear is made up of three parts: the outer ear (pinna or auricle), the middle ear, which includes the eardrum (tympanic membrane), and the inner ear (cochlea), which is shaped like a snail shell and lined with tiny hair cells.  Sound waves are collected by the outer ear and strike the eardrum, causing it to vibrate. The eardrum's vibrations are amplified through the chamber of the middle ear along three tiny interconnected bones which pass on the vibrations of sound waves to the cochlea. Inside the cochlea there are thousands of sensory cells and nerve endings that detect the vibrations and produce electrical signals. The hearing centers of the brain are sent these messages through the auditory nerve where they're interpreted as sounds.  The brain sorts different sources of sound based on their unique pitch, or frequencies, and their timing.  That allows you to concentrate on some sounds and suppress less important ones.  The brain also uses the source and direction of the sound and loudness as clues to decipher messages.  The entire process occurs within a split-second timeframe.

Hearing loss is the loss of hearing ability characterized by decreased sensitivity to sound in comparison to normal hearing.  It's different for different people.

There are generally three considerations in evaluating hearing loss:

• Type
• Degree
• Configuration or "shape" of the hearing loss as shown on an audiogram

Type of hearing loss is generally categorized according to what part of the auditory system is affected. The types of hearing loss are conductive, sensorineural, mixed, and central auditory dysfunction.

• Conductive hearing loss occurs when the passage of sound through the outer and/or middle ear is blocked and usually reduces our ability to hear faint sounds.  This type of hearing loss can often be medically or surgically corrected. Examples of conditions that may cause a conductive hearing loss include fluid in the middle ear from a cold or allergies, ear infection, perforated eardrum, tumors, otosclerosis (calcium buildup on the bones), impacted earwax, presence of a foreign body or malformation of the outer ear, ear canal or middle ear.

 

• Sensorineural hearing loss occurs when there's damage to the inner ear or to the nerve pathways from the inner ear to the brain.  It not only involves a reduction in the ability to hear faint sounds, but also affects speech understanding.  It can be caused by disease, birth injury, drugs or chemicals that are toxic to the auditory system, genetic syndromes, noise exposure, viruses, head trauma, aging, and tumors.  Sensorineural hearing loss cannot be medically or surgically corrected. It's a permanent loss.  Hearing aids or cochlear implants can help.

 

• Mixed hearing loss exists when conductive hearing loss occurs in combination with a sensorineural hearing loss.

 

• Central auditory dysfunction results from damage or dysfunction at the level of auditory brain stem or cerebral cortex.

Degree of hearing loss represents the severity of loss. The numbers are representative of a patient's thresholds, or the softest intensity at which sound is perceived, and ranges from slight to profound. Hearing is measured in decibels (dB) and the severity of hearing loss is graded as:

• Normal hearing:  -10 to 15 dB
• Slight loss:  16 to 25 dB
• Mild loss:  26 to 40 dB
• Moderate loss:  41 to 55 dB
• Moderately severe loss:  56 to 70 dB
• Severe loss:  71 to 90 dB
• Profound loss:  91 to 120 dB

Configuration of hearing loss is the extent of hearing loss at each frequency and the overall picture of hearing that's created. A hearing loss that only affects the high frequencies means you have good hearing in the low frequencies and poor hearing in the high frequencies. If only the low frequencies are affected, the configuration would show inferior hearing for low tones and better hearing for high tones. Some hearing loss configurations are flat, showing the same amount of hearing loss for low and high tones.  Higher speech frequencies are responsible for helping us to understand consonants clearly, whereas lower pitches are for vowels.