When you talk loudly, only the vowel sounds get louder. You can’t yell consonants, such as S or SHSHSHSHSSH.
You’re better off giving the person with hearing loss (PWHL) some context (such as, we’re talking about our lunch plans tomorrow) and then slowing down a bit.
And if that doesn’t work…create the space for the PWHL to ask for clarification. If you are imparting information in a professional capacity, invite the person to ask you to repeat or clarify. A beautiful way to end the invitation is to say, “I don’t mind at all.”
Related post: How can you tell hearing loss from selective hearing?
