Everything You Wanted to Know About Ceiling Finishes
As best I can tell, the reason it’s called a “knockdown ceiling” is because calling it a “knock-off ceiling” would have been too confusing.
What gets knocked off ” er, knocked down ” is the joint compound that’s first applied to the ceiling, and allowed to partially dry.
According to Wikipedia, “the joint compound will begin to form stalactites as it dries. A trowel is then run over the surface of the drywall, knocking off the stalactites and leaving the mottled finish.”
It’s Not Popcorn . . . It’s Acoustical!”
Knockdown ceilings are distinguished from “flat” and “popcorn” ceilings, the two other most popular ceiling finishes.
At least in some circles, popcorn ceilings have a slight stigma, because they’re cheaper and easier to do than flat ceilings.
Some people (the same ones??) also consider them less aesthetic.