It is ironic that the best acoustic treatment I know of is made from ordinary stuff. Books. Albums.
We go to great expense and long lengths to acoustically treat our rooms, yet when it comes right down to it, the best sounding rooms are typically filled with ordinary stuff. And lots of it.
How do you know when your room is acoustically correct? Just listen to your voice when inside the room. If it sounds natural you're 90% the way there.
More than a few times I have recommended to people interested in damping or diffusing the point of first reflection to simply purchase a pair of tall bookshelves and fill them with either books or albums. My preference, by the way, is books. Books are uneven and that randomness helps diffuse sound in a very natural way.
Nothing I know of works better.
And, you don't even have to have read the books. :)
