quieten down

verb3

1. •
   (UK) To become quieter.

   • As the lights dimmed the audience quietened down.
2. •
   (UK) To reduce intensity of an activity.

   • Diplomacy can only begin when the violence quietens down.
3. •
   (UK) To make someone or something become quieter.

   • The teacher did her best to quieten the children down.