1. To become calm, quieten, and relaxed; to stop being so noisy and boisterous. ("Quieten" is a chiefly British English variant spelling of the verb "to quiet.") Primarily heard in UK. A small amount of television after dinner is actually a good way of helping kids quieten down before bed time. It takes the engine a minute to quieten down after you turn it off.

2. To cause, compel, or instruct someone or something to become less noisy and boisterous. In this usage, a noun or pronoun is used between "quieten" and "down." Primarily heard in UK. A last-minute goal by the visiting team quietened the crowd down almost instantly. It can be a real chore unto itself trying to quieten the students down first thing in the morning.