|This was taken from www.opening-the-door.com.|
Introduction : Otoacoustic Emissions
Otoacoustic emissions are sounds which can be recorded in the ear canals of functionally normal ears. They are the result of compressions and rarefactions of the air in the ear canal due to movements of the ear drum. Prior to 1977 no one suspected that ear drum vibration by the cochlea was a part of the normal hearing process. We now know however that in order to achieve its exquisite sensitivity to sound the cochlea must actively manage the sound energy it receives and direct the individual frequency components to the appropriate sensory cells which lie along the Organ of Corti.
Recent measurements in healthy live ears have shown that the travelling wave within the human cochlea can build up as it travels to exceed the vibration entering the cochlea at the oval window by hundreds of times. This rapid build up in travelling wave energy is only found in healthy cochleas where the outer hair cells are in perfect condition. Any insult to the cochlea or disease tends to rapidly depress the travelling wave size. This leads to a decrease in sensitivity and elevation of hearing threshold.
It appears that the cochlea cannot completely contain all the energy from the enhanced travelling wave. A fraction of this wave scatters back to act upon the middle ear rather than the sensory cells. As the scattered back travelling wave impinges on the oval window of the cochlea which moves the middle ear ossicles and consequently the ear drum. The oscillations of the ear drum move the air in the ear canal. This on its own would generate very little sound because the air can move freely in and out of the ear canal. When the ear canal is closed by the otoacoustic emission probe a substantial sound pressure is developed as the small trapped volume of air is periodically compressed and rarefied by the ear drum motion.
|What can we learn from OAEs?|
Because otoacoustic emissions are a natural by-product of a strong cochlear travelling wave, their appearance in the ear canal helps us to confirm normal function of a large part of the peripheral auditory system.
Because we know otoacoustic emissions are presented to the ear canal through ear drum motion they confirm normal mobility of the entire middle ossicular chain. They are a measure of the level of activity inside the cochlea related to the strengthening of the travelling wave. The presence of otoacoustic emissions therefore confirms that the general anatomical and physiological environment of the inner ear is able to function normally.
Otoacoustic emissions can therefore provide highly frequency specific information about the cochlear response. An individual's otoacoustic emission response is highly individual and very specific to the intensity and spectrum of the stimulus sound. If the latter two are kept constant then changes in the acoustic emission with time are a sure indicator of changes in the physiological status of the peripheral auditory system. This property has been used as a sensitive indicator cf changes caused by noise or therapy on a patient's ear.