1. To become calm, quiet, and relaxed; to stop being so noisy and boisterous. A small amount of television after dinner is actually a good way of helping kids quiet down before bed time. It takes the engine a minute to quiet 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 "quiet" and "down." A last-minute goal by the visiting team quieted the crowd down almost instantly. It can be a real chore unto itself trying to quiet the students down first thing in the morning.