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Hearing testing
  By Kristina Plewes, M.Sc., Registered Audiologist
   
   
  Our sense of hearing is a product of an amazingly complex system that converts sound from air pressure to mechanical vibration to neural impulses. This article is intended to outline the basic physiology and function of our hearing system from the outer ear to the inner ear.

The
outer ear consists of the pinna (the outside part of the ear that is visible) and the ear canal. The pinna helps to amplify sound by funneling it towards the ear canal and filters sound in a way that helps us localize sound in our listening environment. The first one-third of the canal is made of cartilage while the inner two-thirds is made up of bone. The earwax glands are located along the walls of the canal, and serve to lubricate the canals and protect our eardrum from dirt, debris and insects. The ear canal also causes a resonance that amplifies higher pitch sounds travelling to the eardrum, helping us hear many softer speech sounds.

 
The middle ear begins at the tympanic membrane – more commonly known as the eardrum, which is about 8-10 mm in diameter and made up of a thin layer of skin. Sound vibrations are transferred from the eardrum to the three ossicles (bones) of the middle ear system – the malleus, incus and stapes. These are the smallest bones in the human body. Sound is amplified through the middle ear system because of the size Audiogram
difference between the eardrum and Click image to enlarge
the end of the stapes bone, and also  
  from lever-type action of the ossicles. Air pressure in the middle ear is equalized by the Eustachian tube, which is connected to the throat and opens when we yawn and swallow.

From the bones of the middle ear, the mechanical back and forth movement of the stapes creates pressure waves that travel through the fluid of the hearing portion of the inner ear – a snail shaped, bone-incased organ called the cochlea. Here, waves travel along the surface of a rigid membrane called the basilar membrane. Higher pitch waves travel shorter distances, and lower pitch waves travel longer distances. This means the cochlea is arranged tonotopically – it is tuned at all points to specific pitches. It is here that receptor cells – called hair cells – transfer the sound waves into neural impulses. Some of these hair cells send impulses to our brain that we interpret as sound, while others work to change the way the basilar membrane moves to help fine tune our ability tohear subtle differences in speech sounds such as the difference between "sun" and "fun".

From sensation to perception, sound is a complex yet fascinating journey, a journey that one should never take for granted.
   
  Science of hearing
   
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  Victoria BC, location - 250.370.2833
  Sidney BC, location - 250.656.2218
   
 
 
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