How can a dog help a hearing impaired person?

By alerting the owner to a knock at the door or a ringing telephone.

I googled your question just as it stands & all these sites popped up. Amazing, huh? what a simple search can do.

http://centurylink.net/search/index.php?q=How+can+a+dog+help+a+hearing+impaired+person%3F&context=search

I do not have any experience with dogs for the deaf, so I just googled your question so you could easily find some answers.

Service dogs for the Deaf/hearing impaired often translate noises into visual or tactile cues. Some examples,

- Someone knocks on the door and the dog fetches its handler.
- Alerting its handler to any sort of smoke or other emergency alarm that typically works through sound. (Especially in the middle of the night)
- Accompanying its handler each day to help warn about things the handler might not be aware of (e.g., a car horn telling about danger, or someone yelling down a back alleyway)

It is very similar to how a guide dogs works.
A dog for the hearing impaired will do things like let you know when someone is at the door, get the phone for you when it is ringing, things like that. You can train any smart dog to do this for you, if you have a bit of skill with training.
My friend, who was born deaf and can only hear very low base notes, has a pug. This pup lets her know when people are at the door, when alarms go off for things like the oven, and helps her fetch things. He knows modified sign language, so she will use one hand to sign to him, and he will get her the things that he has asked for.