Since the advent of custom-made hearing AIDS, it has given hearing impaired people a life. Communication brings great convenience. External acoustic feedback is a common phenomenon in the wearing and use of hearing AIDS. But it is these annoying "scream" wear is not successful, can cause hearing AIDS or even make customers lose confidence to wear hearing AIDS, according to foreign, almost 24% of customer complaints related to hearing AIDS the acoustic feedback, so how to eliminate the external acoustic feedback is our development, production and inspection personnel need to explore and research the problem.
In order to understand the cause of external acoustic feedback, we need to understand its working process. The basic composition of hearing aid is microphone, amplifier and headset. Shell size appropriate, when hearing AIDS to wear under the condition of normal, general won't produce sound external feedback, the gain is too high, but when the hearing aid hole is too large, or ear mould is too small, when there is a gap between the ear and ear hearing AIDS the voice of the output from the pores or ear and ear the gaps between the leaked, hearing aid in the ear canal sound generated by the output will be over by the molds/auricular provided by the attenuation, the hearing the sound of the output will become unstable, hearing aid headphones sound returns to the microphone, external acoustic feedback arises, as soon as the voice of the feedback through the amplifier amplification again, In a fraction of a second, an amplification loop is formed, and the feedback signals gradually increase and reach saturation output.
Assuming that the input signal is S1, the gain of the amplifier is Z, and the output signal is S0, the relation between the ear module and the ear canal (i.e. silent feedback) is
S0 = Z + S1 (formula 1)
When the amplified sound leaked out of the ear canal, it returned to the microphone of the hearing aid and was amplified again, forming the acoustic feedback loop. Part of Sf of Z+S1 was picked up by the microphone together with S1 through the feedback loop.
Sf = F + S0
Then S0 = Z + (S1 +F +S0) (formula 2)
It can be seen from formula 2: obviously, the gain of the whole circuit is determined by the gain of the hearing aid, so appropriately lowering the gain can eliminate some small feedback, so the initial method we solve is.
So our initial solution is to adjust the gain Settings to make sure the customer is listening. In this situation, the sound system of the hearing aid becomes very stable and does not generate sound feedback, so we use the method of plugging the air hole or filling the shell to make F as close as possible to 0. Suppose S0=&, its physics significance is that, although there is no additional input signal, there is also output signal, we call this state self-excited oscillation, at this time the sound output no longer changes with the input signal.
Based on the above analysis, we conclude that the production of external acoustic feedback mainly depends on the gain Z of the hearing aid and the size of the acoustic feedback quantity F.
There are many ways to control external acoustic feedback. First, in the initial stage when we customize hearing AIDS for customers, customers should get the most possible hearing compensation:
1. Ear impression to achieve as full as possible, and customer ear canal matching;
2. Correct horn position, should be in the middle, avoid skew;
3. No wax in the mouth and ear canal;
4. No crack in horn tube;
5. Choose the correct stoma size according to the degree of hearing loss.
Followed by acoustic method, reduce the feedback loop, such as decreasing porosity, increases the size of the molds and auricular the first corner, and so on, is to reduce the gain again, most of the hearing AIDS high frequency amplifier is greater than the low frequency amplifier, therefore, produce acoustic feedback signals are often high frequency signal, then, to drop pitch controller and high frequency cut-off device of high frequency gain. The fourth method is to locate the frequency of acoustic feedback and attenuate it. This method is mainly digital hearing aid. When the acoustic feedback is found, it will automatically attenuate the gain of the frequency at which it is located and generate a corresponding notch signal. Fifth, automatic tracking elimination of feedback, which refers to the new generation of full-digital hearing AIDS, real-time tracking of acoustic characteristic changes, instantaneous elimination of feedback occurrence, compared with the static mode, to obtain greater gain, elimination of feedback does not need to sacrifice gain as a premise.