What is the best way to communicate with a resident that has a hearing impairment?

How about trying a dry eraser board and markers. I use this with my mother. Easy and fast.

Wait to make sure that you have the person's full attention.
Position yourself so that the person can see your full face.
Speak clearly and deliberately - not so slow or so loud that it distorts the words, but enunciating carefully.
If the person asks "what?" or obviously didn't hear you, first of all repeat exactly what you said, word for word.
If the person perhaps couldn't understand what you said, rather than didn't hear it, then try phrasing it more simply. For example "it's lunchtime, would you like to come and join everyone at the table?" could become "let's go to lunch."
Finally, I agree with MsMadge that this is an art form. If you're a caregiver in a facility, you will probably get extremely good at it over time :)

My mom is very hard of hearing but seems to hear low tones better

Speaking loudly without yelling is an art