EarMachine

EarMachine is an app that makes it easy to listen to the sounds that you want to hear. Those sounds can be picked up by the microphone in your phone or earbuds, and can be played from your music library! EarMachine is helpful in situations ranging from talking in noisy restaurants to watching television at home.
This work is funded by the National Institutes of Health (of the United States of America).
Features:
Simple Interface, Complex Processing
With a simple interface, EarMachine, allows you to adjust more that 150 signal processing parameters with a simple swipe of your finger on a wheel. The mapping from wheel position to parameter values is the result of a large-scale analysis of the hearing status of more than 10,000 people.
EarShare: An intelligent recommendation engine
Ear Machine also contains a recommendation engine called EarShare. This engine will listen to the sounds around you and use that information to recommend wheel positions. The recommendations are based on both large-scale analyses of hearing abilities as well as your usage of the app. EarShare will give better recommendations the more you use the app.
Listen to whatever sounds you want.
Ear Machine allows you to choose whether you want to listen to the microphone in your iOS device, or the one in your earbuds (iOS 7 only). You can also use Ear Machine to listen to songs/podcasts in your music library.
State of the Art Signal Processing
The signal processing in Ear Machine is similar to what is used in most hearing instruments. A wide-dynamic range multiband compressor ensures that quiet sounds are amplified, while protecting you from sounds that are already loud. A multiband limiter provides further protection from loud sounds. An expander makes sure that very quiet sounds (e.g., refrigerator hum) are not annoying. You can even use Ear Machine as a sound level meter (works best with phone microphone).