Wiktionary
quieten down
vb. 1 (context intransitive British English) To become quieter 2 (context intransitive British English) To reduce intensity of an activity. 3 (context transitive British English) To make someone or something become quieter.
Usage examples of "quieten down".
Oh well I expect theyll quieten down now but I must say Ive enjoyed the last few days!
Whyn't you go on out on the couch where you can quieten down nice?
If I shush loudly they will calm and quieten down - for a few minutes at least.
But the junked up thought fragments refused to quieten down, he kept seeing flashes of Launde Park, the Abbey, those bloody lakes, Denzil's data-rich tour, the students' broken shocked faces.