Acoustic feedback occurs when amplified sound from the hearing aid is able to escape the ear canal and the microphone picks up the sounds and they are re-amplified, this starts a cycle of sound leakage and re-amplification which will result in the whistle that is defined as acoustic feedback.
There are many ways to cure acoustic feedback, if feedback occurs the first thing to check is the fit of the hearing aid in the ear it is important where possible not to rely on the feedback management systems that the HA software provides as this can alter the amplification thus not giving the desired amplification for speech intelligibility. Has the correct hearing aid been prescribed by the HAD, if the HA uses a mould is that mould correctly situated in the ear? Does it create an acoustic seal? Is the event in the HA the correct size? Should the hearing aid have a vent? Does the mould/tip go deep enough into the canal? If the patient has a severe or profound hearing loss because of the high amounts of amplification needed there is more chance of acoustic feedback and therefore it may be necessary to use a feedback management that can be found in the software.
Feedback can cause speech discrimination and intelligibility to be affected when relying on the hearing aids automatic feedback cancellation systems. The first electronic feedback suppression systems is called notch filtering this work by the hearing aid detecting at what particular frequency the feedback is being produced by analysing the feedback to see what frequency the feedback was being produced a. The hearing aid will then automatically reduce the amplification at those frequencies, this does stop the problem of acoustic feedback but the patient’s speech discrimination could be jeopardised due to the lack of amplification at the specific frequency.
The latest form of feedback-reduction system is phase cancelling,most manufactures are using this and it has advantages to notch filtering. The software will then analyse the frequency peaks and identify feedback; the software generates a reverse phase signal (an identical sound at 180 degrees) and adds it to the amplification, this will eliminate the feedback. The function is to reduce or eliminate these peaks without having a detrimental effect on the speech cues that are within the frequency range, all this is done very quickly within a second. Phase cancelling generally allows on average an additional 10dB of gain before feedback will occur than on a hearing aid that doesn’t have phase cancelling.Each individual manufacturers phase cancelling algorithm will have different response times, it may be important to counsel the patient to say if feed back occurs then after a second it may disappear.
Although we can see that electronic feedback cancelling systems are improving I still feel as a Hearing Aid Audiologist that it is very important to get the basic acoustic cues correct, firstly make sure that the clinical aspects of the fitting are correct.