Although not specifically an "assistive device", headphones can be used by hard of hearing people to improve their hearing in some situations.
Headphones can be plugged into many different sound sources, including the TV, stereos, CD players, computers and phones.
You may have your own headphones that you find helpful, and now you may find that many of the ALDs provided by movie theaters in response to the ADA are Infrared or FM headphones.
Even people with serious hearing losses may be able to hear well with headphones, but that depends on their hearing and their hearing aids.
People whose hearing loss is relatively flat across different frequencies may do quite well with hearing aids, since what they need is volume.
People with steeply sloping hearing losses (see ski slope, , reverse ski slope, and cookie bite) may have difficulty with headphones, unless they can get their hearing aids to work with them. There are several problems and solutions:
One problem is that headphones can increase the chance of feedback. If the amplified sound from the hearing aid's speakers leaks out of the ear canal and manages to get into the hearing aid's microphone, then the sound just goes round and round, getting louder and louder until it squeals. Headphones can act as a deflector that helps the sound bounce into the hearing aid's microphone.
One solution to the feedback problem is to choose headphones with a cloth or foam earpiece that doesn't act as a good reflector. You'll just have to try until you find one that works for you.
Another problem with headphones is that people who use BTE hearing aids may find that the sound from a headphone doesn't get into their hearing aid's microphone, since it's located over the ear.
A possible solution for people with BTE aids is to use a headphone that have large earpieces with have comfortable cushions that completely surround the ear.
There can be many problems (not just with BTEs) related to getting the signal into the hearing aid. These problems can sometimes be solved by using telecoil if your hearing aid has one. Many headphones are telecoil compatible ... that is, they send out a magnetic signal representing the sound that can be "heard" by the telecoil in your hearing aid. Many of the headphone based ALDs provided by movie theaters are telecoil compatible, so be sure to flip on your telecoil when using those headphones. BTE users may have to offset the earpiece a little so it's closer to their heairng aid's telecoil.
Finally, a new development in headphones may help some people with hearing loss. You can buy headphones that are "noise cancelling" headphones. These headphones sample the sound around you and generate a sound from their electonics which cancels out the background noise and lets you hear just the sound coming into the headphone's inputs. These types of headphones are ideal for noisy situations, if you can solve the other headphone problems such as feedback.