The shape and the acoustic characteristics of 1 ear of any one of the ear canal can be considered as an acoustic resonance cavity. Resonance would make some sound frequencies has been strengthening.
2.助听器外壳或耳模问题 当助听器的外壳大小合适，配戴正常的情况下，一般是不会产生外部声反馈的。但是当助听器的增益过高，气孔过大，或外壳、耳模过小，与耳道间存在缝隙时，助听器输出的声音从气孔或耳道间的缝隙泄露出来时，助听器在耳道中所产生的声输出就会超过耳模或外壳所提供的衰减，此时助听器的声输出就会变得不稳定，助听器耳机发出的声音返回到麦克风时，外部声反馈就产生。一旦反馈回来的声音经过放大器再次放大，在几分之一秒钟的时间内，形成了一个放大回路，反馈信号就会逐渐增大并达到饱和输出状态。
2 hearing aid shell or ear mold problems when the shell is the right size, wearing normal circumstances, generally will not generate external acoustic feedback. But when the hearing aid gain is too high, the stomatal is too large, or shell, ear mold is too small, and the gap existing between the ears, hearing aid output sound from the vent or leak out ear gap, acoustic output will aid in the ear canal of the ear mould or shell over the for the attenuation, the acoustic output will aid becomes unstable, hearing aid earphone voice returns to the microphone, the acoustic feedback is generated. Once the feedback amplifier amplified sound through again, in a fraction of a second, the formation of a magnifying circuit, feedback signal will gradually increases and reaches saturation output state.
3.助听器调节问题 助听器的音量过大或助听器的高频增益太大。
Regulation of hearing aid 3 hearing aids the volume too high frequency gain or hearing aid is too big.
4.助听器自身问题 由于多方面原因，任何一只授话器都不可能保证频响曲线绝对平直，肯定会有某些频率出现尖峰的情况。于是，在授话器放音时，会出现某些频率声音过强的现象，这个过强频率的声音就有可能造成啸叫。麦克风的频响曲线也不可能保证绝对平直，所以也会发生对某些频率的拾音灵敏度过高的情况在所难免，这就是说，麦克风对于各个频率的拾音灵敏度不同，这就会造成某些频率的声音输出过强，其结果就可能在这些频率出现声反馈现象。一般来说，麦克风在高频段中的某些频率灵敏度偏高，故更容易在高频产生啸叫。
4 hearing aid their own problems due to various reasons, any receiver can not guarantee absolutely flat frequency response curve, there will certainly be some frequency peaks of. So, in a speaker playback time, there will be some frequency sounds too strong phenomenon, the high frequency sound it may cause howling. The frequency response curves of the microphone may not guarantee absolutely straight, so will pick up can hardly be avoided, audio and high sensitivity of frequency that is to say, the microphone for each frequency pickup sensitivity is different, this will cause some sound frequency output is too strong, the results may occur in these frequency acoustic feedback phenomenon. In general, the microphone at high frequency in certain frequency sensitivity is high, so it is easier to cause squeaking at high frequency.
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