I will share phrases that are commonly used in the US. Situation: I am in the library and there are two teenage girls talking and giggling while I am trying to study. I look at them, they look at me and see that I am not pleased. They start to whisper instead of talk and attempt to giggle in a quiet way. Since that still keeps me from focusing, I lean towards them and whisper one of the following phrases: "Would you be a little more quiet? I still can't study." "Would you mind being a bit quieter? I still can't study." "Everyone else is being quiet in the Library. Would you mind going somewhere else to have your conversation?" "Everyone else is a lot quieter than you two - which really helps the rest of us study. Is it possible for you to have your conversation somewhere else?" I used actual sentences that I would use (If I were frustrated enough!) Hope this helps!! :)

(I'm a US native speaker). Both are fine. "Quieter" is more common. They mean the same thing. As far as I know, "more X" is fine, even if "X-er" exists. A quick Google search of Project Gutenberg turns up many examples of both forms: "I really do eat better, and am more quiet than I was."--Charlotte Perkins Gilman "Sahwah was more quiet, and there was a sober look in her eyes." "He had fallen back in his chair, not looking at her now, and with his hands, from his supported elbows, clasped to keep himself more quiet."--Henry James "a barbarous aberration, from which other more quiet nations abstained..."--Otto Jespersen "Jane was as much gratified by this as her mother could be, though in a quieter way."--Jane Austen "The horse was bought; nothing could go quieter; Evelyn was not at all afraid."--Edward Bulwer Lytton "Then her own life took a quieter turn; we met again; I went for a good while often to her house."--Henry James, who also used "more quiet," above. "I think the baby will be quieter if I put him the other way."--Wilkie Collins

Hi Amber, The comparative form of "quiet" is "Quieter". The superlative form is "Quietest". It would be grammatically wrong to write "more quieter" as "quieter" already has the meaning of comparison. Hope this helps. Cheers, Lance

Quieter would be the most appropriate form.

They both are correct and quieter might be more common according to google.