To better understand why hearing loss happens, it's important to first know how hearing works.........

1. Sounds enter the ear canal
   Sound waves move through the ear canal and strike the eardrum.
2. The ear drum and bones of hearing vibrate
   These sound waves cause the eardrum, and the three bones (ossicles) within the middle ear, to vibrate.
3. Fluid moves through the inner ear
   The vibrations move through the fluid in the spiral shaped inner ear – known as the cochlea – and cause the tiny hair cells in the cochlea to move. The hair cells detect the movement and change it into the chemical signals for the hearing nerve.
4. Hearing nerves communicate to the brain
   The hearing nerve then sends the information to the brain with electrical impulses, where they are interpreted as sound.

There are two types of hearing loss.....

SENSORINEURAL

This is the most common type of hearing loss. Sensorineural hearing loss is caused by damage to the tiny hair cells in the ear that help transmit sound to your brain. These can bend or break due to:

• Exposure to very loud noises
• Genetic disposition
• Virus infections in the inner ear
• Certain medical treatments such as chemotherapy and radiation
• Head injury
• Age

The effects are almost always the same – it becomes harder to distinguish speech from noise, certain high-pitched sounds such as birdsong disappear altogether, people seem to be mumbling and you often have to ask them to repeat themselves.

Sensorineural hearing loss is permanent and can’t be corrected medically, but a hearing aid can almost always help.

CONDUCTIVE

This is caused by any blockage that prevents sound reaching the inner ear. This might include:

• A build-up of earwax
• Fluid in the middle ear
• Unusual bone growth in the middle ear
• Middle ear infections
• Small holes in the eardrum

Conductive hearing loss is not necessarily permanent and can often be corrected medically or surgically. 

MIXED- hearing loss is a combination of conductive and sensorineural hearing loss.

References
Arts HA. Sensorineural hearing loss in adults. In: Cummings CW, Flint PW, Haughey BH, et al, eds. Otolaryngology: Head & Neck Surgery. 5th ed. Philadelphia, Pa: Mosby Elsevier;2010:chap 149.

http://www.betterhearing.org/hearingpedia/hearing-aids/binaural-advantage
http://www.hearingaidhelp.com/hearing-loss.html
http://www.gnresound.com/your-hearing/about-hearing-loss/types-and-causes-of-hearing-loss