The broadband absorber

The broadband absorber is an easy to build, reasonably cheap and very rewarding basic acoustic treatment. And it doesn't need any more effort to install than hanging a painting and is almost as easy to move. Almost.

Cover with fabric

After that I streched the back fabric and stapled it onto the frame with my trusted staple gun.

Put the fiberglass in place. As you see it rests upon on the joists (the frame is as wide as the fiberglass) and gets support by the boards.

And then strech the front fabric over the fiberglass and the frame and staple it onto the back of the frame. You may need to cut off some excess fabric.

Done!

If you got the right tools it's pretty simple and doesn't take that long either. I did a bunch of them a couple of evenings in the garage.

Since the frames leaves some airgap you can use that to fasten them with a couple of hooks onto the walls. Or manufacture some stands if you like them to be freestanding.