Acoustic noise is a major cause of user dissatisfaction with hearing aids, and it is also a major problem encountered by fitting engineers in the debugging process of hearing aids.

At present, all hearing aid manufacturers will use "feedback suppression" technology to solve the user's acoustic noise problem.

So, are acoustic feedback and whistling the same thing?

1. The relationship between acoustic feedback and whistling

When the microphone picks up the sounds emitted by the receiver, these sounds enter the receiver and are amplified twice, thus forming a feedback loop of the sound.

Acoustic noise is an audible sound oscillation generated on the basis of acoustic feedback, and only when the sound repeatedly enters the feedback loop and the repetition is amplified, the acoustic noise will occur.

In fact, acoustic feedback is a phenomenon that often occurs during the work of hearing aids, and only when the feedback is large enough to cause the sound to oscillate, there will be a whistle.

Therefore, feedback is a mechanism, while whistling is a phenomenon, and the two are not the same concept. There is feedback not necessarily a whistle, and what we usually call acoustic feedback is not exactly a whistle.

2. What level of feedback will cause whistling

Sound leaking out of the residual ear canal will re-reach the microphone, and if this distance is not enough to attenuate the sound leaking out of the residual ear canal, the hearing aid will gradually superimpose the sound, forming a loop, which in turn produces audible sound oscillations, causing whistling.

For example, if the sound intensity of the hearing aid can reach 90dBSPL after amplification, and the sound intensity picked up by the microphone is 60dBSPL after the residual ear canal is attenuated, the critical value that causes the hearing aid to produce acoustic noise is 30dBSPL.

At this point, if the gain of the hearing aid exceeds 30dB, it may cause acoustic noise.

The blue arrow represents the propagation path of the sound amplified by the hearing aid; The red arrow represents the feedback path that results from a sound leak.

3. What factors are related to acoustic noise?

The generation of acoustic noise mainly comes from the following three aspects:

01. Gain

The acoustic noise is gain-dependent, not the maximum output.

If the gain of a high-power hearing aid and the gain of a low-power hearing aid are the same, then the likelihood that they will produce acoustic noise is the same.

The reason why we think that high-power hearing aids are more likely to produce acoustic noise, that is because the maximum output of high-power hearing aids is larger, and when acoustic noise is caused, the oscillation of the hearing aid will be more intense than that of low-power hearing aids, so we will feel that high-power hearing aids are more likely to produce acoustic noise.

02. Phase shift

Phase shift, or phase transfer, is the phase difference between the output sine wave signal and the input sine wave signal.

When the phase shift in the loop is an integer multiple of 360°, it causes a whistling noise.

03. Ear canal attenuation

The process of sound coming out of the receiver and being picked up again by the microphone is called sound leakage.

Because the ear canal attenuates the leaking sound, the intensity of the sound emitted from the receiver is actually greater than the intensity of the sound received by the microphone.

If the attenuation value is greater than the gain value after hearing aid, no acoustic noise will occur;

If the attenuation value is less than the gain value after hearing aid, it increases the likelihood of acoustic noise.

Acoustic noise is an audible sound oscillation generated on the basis of acoustic feedback, and it is also an important factor affecting the sound quality of hearing aids.