Or you can just buy acoustic panels that are now inexpensive. If you know what standing waves you want cancelled and/or know what amount of silence in the audio spectrum, you can order to suit your needs.
Before treating my studio I looked at all the options. Building my own traps was a lot cheaper than buying them. Also, you'll see foam traps being hawked all over the place and yes, they are cheap. But they do not work to attentuate anything under 200Hz, where most of the problems are. Panels that are dense do the job, foam does not. High density Rockwool is the best. The biggest problem in any room, especially square or rectangular ones, is in bass frequencies. Tackle those and you're on your way to a good sounding room. Then, using the "mirror technique," you finish up by throwing up some foam panels on the walls. Rugs or ceiling treatment also helps tighten things up. There's a great, free program; Room EQ Wizard, that's of great help dialing things in.