In an interesting study done a few years ago (and I believe it rings true today) the two biggest challenges to hearing aid design and usage is feedback and occlusion. Unfortunately and interestingly, the solution to one often causes the occurance of the other in hearing aids.
There are three primary types of feedback that occur within hearing aids – acoustic, mechanical and electromagnetic). Many builders have moved to laser shell manufacturing technologies to overcome the feedback issues but it is not 100% doing the trick.
As mentioned, the challenge exists that even if new technologies do lend themselves to aiding the feedback dilema, they often feed the occlusion issue. In all honesty, the technology behind these hearing aid issues go a bit beyond us hear at hearing aids 1000. If the topic grabs you and you want to learn more about the two biggest challenges facing hearing aids today, please visit this URL and read the article they present on the issues challenging hearing aids: http://tia.sagepub.com/cgi/content/abstract/8/4/125
