Here are some communication tips for when you are addressing someone whom you know to be hearing-impaired or is simply hard of hearing.
1.
If it's a family member or relative, be sure to check to see if their hearing had has fresh new batteries.
Once the batteries wear out you should be able to note markedly decreased perceptions in their hearing senses.
2.
Sometimes a person is not equally hearing-impaired on both ears.
In which case, you can feel the person out verbally and figure out which ear tends to be better at hearing.
Normally a giveaway of this is if they lean in a certain way, suggesting they hear better with a certain ear subconsciously without thinking about it.
3.
Communications is a lot more than verbal.
Make sure the environment at which you are speaking to the person has adequate lighting and that your arms and hands as well as lips are easily viewable.
The mind's subconscious is able to process and fill in the gaps in sentences in ideas if there are other communication venues present such as body motion and lip movement (so they can read your lips as you are speaking).
These basic guidelines should serve as a baseline from which you can adapt you own communications style.