Feedback And Occlusion Flashcards
What is occlusion?
It is the bunged up echo feeling in your head. It is one of the major reasons for rejection of a hearing aid. It is caused by low frequencies getting trapped in the ear canal that cannot escape and start to echo.
How to prevent occlusion
Stop this sound from entering the space between the mould and TM - fit the mould closer to TM to skip the cartilage.
Allow these sounds to dissipate to some extent through a vent
Venting allows low frequencies to escape which results in:
Reduction of occlusion affect
Clearer more natural speech
Improved clarity in background noise
Less Upward spread of masking
Types of vents?
Parallel vent: parallel to sound tube in either an ite or bte mould
Y vent: allows sound to escape and travel down the y vent. Can cause distortion as the sound can back up tubing instead of out the vent.
calculations for vent sizes?
Looking on audio gram between 250hz and 500hz if it is between:
- 0-20 db large vent 3-4mm
-20 - 40 medium vent 2mm
-40-60 small vent 1mm
What is feedback?
It is high frequency sound that is leaking and is picked up again by the microphone and re amplified the high causes a whistling sound. The signal adds in phase with the incoming signal and can be amplified over and over again- this causes the whistling.
Causes of feedback?
- wax
- loose fit of shell
- receiver tube detached
- vent too large
- mic and receiver touching eachother.
Feedback solutions
- impressions of each ear
- deep as possible
- good seal on earmould
- longer meatal projections
- larger hearing aid means more distance from mic to receiver
Electrical feedback solutions
Phase inversion: feedback sound is inverted to be made out of phase of the incoming signal
Frequency shifting; input signal is shifted slightly before receiver.therefore leakage will not match any peak in aids frequency response
- gain controls: if feedback is happening at certain frequencies we can adjust gain received on this frequency.
