Pipe down
Definition 1 of 1
Pro Tip 1/3Informal Use
Used mainly in casual situations among friends, family, or in informal groups. 
At the sleepover, the host's dad told everyone to pipe down.
Pro Tip 2/3Not Rude
'Pipe down' can be direct but isn't usually seen as rude, although tone matters. 
Hey, could you guys pipe down? I'm trying to concentrate here.
Pro Tip 3/3Urgency Implication
Sometimes implies an immediate need for quiet, often due to a sudden reason. 
Pipe down! The baby just fell asleep.