Wiktionary
quiet down
vb. 1 (context intransitive US English) To become quieter 2 (context intransitive US English) To reduce intensity of an activity. 3 (context transitive US English) To make someone or something become quieter.
WordNet
Usage examples of "quiet down".
It is so quiet down in West Eighteenth Street at such an hour that you can hear yourself think, and in fact I hear myself thinking very plain that I am a big sap to be on a job like this, especially with a baby, but I keep going just the same, which shows you what a very big sap I am, indeed.
Bo waited for her bawling to quiet down to moaning and then to whimpering.
As both began to quiet down, a singular presence slipped forward out of his chair.