There is a common misconception that a hearing aid acts just like a pair of glasses does but for your ears rather than your eyes. This is not quite the case, unless you want to include those reading glasses available at the drugstore, which certainly magnify the world rather than making precise changes to the way the wearer sees the world. In many ways, these drugstore glasses are the closest comparison there is to the auditory counterpart, though the technology behind the latter is quite a bit more advanced. Not to mention much more expensive. If you're wondering how these devices work to amplify sound, here is a quick overview of the technology.
The first step in the circuit of technology is the microphone, which is used to detect sound waves coming at the listener. Older, analog microphones are able to make little distinction between types of sound waves, making wearing a hearing aid in crowded, noisy areas more of a hindrance than a help. Newer digital technology has attempted to fix this to some degree with directional microphones that focus on sound coming from in front of the listener, thus fading out the background noise by default. Suffice to say, this kind of audio discrimination is far from perfect, but many users concede that it is a big improvement.
From the microphone, the sound moves into the deaf circuitry of the hearing aid and this is where the work is done. In newer digital devices, more work is done to distinguish important sounds, such as waves identifiable as speech, from those that may not be as imperative. It will take these sound waves and amplify them, cranking them up to a decibel level easier for the listener to hear. This is why these devices can be compared to magnifying glasses, because they do not change the sound in any meaningful way. It certainly makes it louder, which is sometimes enough to overcome mild to moderate auditory loss.
Once the microphone has detected the sound and the circuitry has amplified it, it is time to deliver the new sound on to the listener. The receiver is much the same as the receiver on a phone, delivering sound directly to the user's ear.
A hearing aid would not work without the battery, which gives it the power it needs to accomplish its tasks. Most devices this small have equally small batteries, meaning their lifespan may not be exceptionally long. Most users are able to get somewhere between a few days to a couple of weeks, depending on which type of battery their device uses.