The core concept is very simple, and has, in fact, been thought of and implemented by various companies at least since 2009: Smartphones have built in microphones and they can play audio through headphones. By creating an app that simply takes the audio feed from the microphone, amplifies it, and plays it in the headphones, you can create a rudimentary hearing aid.
From there you can add additional features that bring the app closer to what a real hearing aid does -- algorithms that filter out certain noises or selectively amplify certain frequencies, for instance, or recording features that let you play back something you might have missed. Of course, how well a hearing aid app works depends not just on the app itself, but also on hardware: When the New York Times wrote about apps as a hearing aid substitute last May , they profiled a man who used a $5 iPhone app -- but with a $100 external microphone and a $150 pair of headphones.
BioAid is novel in that its developers are offering it for free in order to solicit feedback from users to continue to develop the algorithm that mimics the normal noise filtering of the human brain. Also, the app both amplifies quiet sounds and de-amplifies loud ones. The team hopes to change the way hearing care is delivered. The app also allows the user to save different amplification profiles, something other apps on the market don't offer -- for instance, one for watching television at home and another for meeting friends at a crowded bar.
"The mobile phone is a great platform for rapidly transferring hearing aid technology from the laboratory to the hands of the public," Nick Clark, one of the UK-based developers, said in a statement. "Standard hearing aids, which can cost thousands of pounds, are only dispensed by a professional after a hearing test. BioAid offers a simple alternative accessible to anyone with an iPhone or iPod. The hearing test is replaced by an exploratory process, allowing users to find which setting works best for them. In the short term, people unsure about visiting a hearing care professional might be swayed to do so by BioAid, which can only be a good thing."
Here's a list of iPhone apps available through the US app store that use the built-in microphone and headphone jack to help people with hearing loss. Some are regularly updated, others appear to be "zombie apps." Some are free, while others cost up to $35. Some claim to be hearing aids, others shun that label. At least one is made by a hearing aid company.
soundAMP from Ginger Labs
Although hearing aid apps seem to be an open field, a few apps look like they've made their mark. Ginger Labs, whose soundAMP app got a nod in the New York Times article mentioned above, also claims to be the hearing aid app that Apple includes on demo products in its Apple Stores. The $5 app has a complex mixing control, recording capability, with a bookmarking feature, and multitasking. On the other hand, the app hasn't been updated since 2010, so it's unclear whether it works on the iPhone 5.
Hearing Aid from TiAu Engineering UG
This iPhone and iPad app, from a German company, has noise canceling capabilities and a basic "low-medium-high" frequency sound equalizer. It can work with both plugged in headphones or Bluetooth devices. The company offers a free version and a $1.99 version, but it's unclear what the difference is between the two.