I don't see a legal situation here. What damages were sustained by the roommate? I would calmly explain what happened to the administration and hopefully they will chalk it up to a misunderstanding.
Well, first of all, I'm not sure what the repercussions of such an accusation might be (for example, are they banning you from the nursing home)--so, you might want to consider hiring an attorney. But, to answer your initial question, I would think that, in such a case, the words that come out of your mouth, rather than the decibel level of your voice, is what is important in determining if such behavior is considered abusive. Thus, assuming that you were just trying to have a normal conversation with your mother-in-law (like, asking her what she wants for dinner), as opposed to spewing nasty words at her, then, based on the limited facts that you have provided, it would be safe to say that talking loudly to a ninety-four-year-old "deaf" person should not be considered abusive.
The decibel level of your talking alone is not abuse. Especially given the circumstances you described.
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