Here is a question I was asked on Quora, which I thought may interest others...
How do hearing aids work? I know about the mics and speakers but I’m curious what you can do with them? Can you turn the sensitivity up to hear things further away? Can you just shut them off if you don’t want to hear someone? Stuff like that.
Hearing aids take your hearing test results (audiogram) and amplify incoming sounds to the level required by you personally at each frequency band.
They have three input levels: soft, moderate and loud. Moderate is the level of normal speech. They can recognise speech patterns and focus on those signals. They can recognise background noise and dampen it. They recognise loud impulse sounds and either reduce them or filler them out.
High tech hearing aids are made to enhance speech in any listening environment, and make it louder and sharper compared to environmental sound. They scan the environment and adjust accordingly to “lift” the speech of the nearest/loudest speaker, making it louder relative to the background noise (signal to noise ratio).
An audiologist can alter the gain (volume) of each frequency band and input level individually. So I could turn up soft sounds and leave moderate/loud alone. This would help you hear sounds from further away relative to speech. This is, incidentally, the opposite of what we are normally trying to achieve! It would make it difficult for you to focus on speech.
Additionally, we can switch on/off different features, and strengthen or weaken them. E.g. I can improve the feedback cancellation system if you are getting whistling. I can strengthen the impulse noise cancellation, or reduce the frequency compression (FC takes high pitch sound and transforms it into a lower pitch so you can hear it better if you have a high frequency loss).
You can do some of these things yourself in an app if you have modern hearing aids - e.g. you can increase the treble if you are tired and want things to be sharper for a while. You can change the directionality in some apps, and focus the mics forward/backward/sideways for a time.
The issue is, each tweak you make has a positive and a negative effect. So increasing the feedback cancellation reduces the volume of incoming high frequency sound, which makes it harder to hear certain speech sounds. Filtering out impulse sounds results in a strange cutting out which is quite distracting.
It’s all quite fascinating and the more advanced (expensive!) the hearing aids, the more we can tweak them to get the best performance.