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To add on to this, research has proved that two things which have shown to also improve hearing, especially in noisy environments, more than anything else, is actually wearing hearing aids in both ears and also using hearing aids that have directional microphones. Many digital hearing aids consist of directional microphones which work well in noisy situations, as they amplify sounds that come from in front of you more than sounds to the side or behind you. Therefore a hearing aid user can focus on what they want to hear.
Can digital hearing aids help reduce whistling? Yes. By having feedback management systems, digital hearing aids can automatically detect any feedback and attempt to reduce the whistling that tends to bother many hearing aid users.