Sound on Sound is one of my favorite sites to find gear reviews and studio tips. In 2006, Paul White penned an article about basic room treatment, with monitoring specifically in mind. It is a good second to what I wrote here and here in Tech Beginnings.

Some key takeaways are:

  • Make sure to treat every point of first reflection, i.e. everywhere the sound bounces once, then hits your head. These are located directly behind and in front of the monitors (front and back walls), and halfway between you and the monitors on each side wall and the ceiling (see the second graphic in the article linked to at the end of this post).
  • 4″ foam placed directly on a wall only absorbs down to 200-300Hz; 2″ foam goes down to 400-600Hz. None of those frequencies are very hard to deal with. If for some reason, you’re only concerned with flutter echo, it’s fine; but that’s never going to be the case. The biggest advantage to foam is if you get a cool-looking pattern/color combination, it’ll make any YouTube videos aesthetically pleasing. Carpet and packing blankets aren’t any better than foam.
  • An air space behind an absorber lowers the center frequency that is absorbed. Generally, the larger the space, the better.
  • A mix of diffusion and absorption can give a better, more lively sound than just deadening the entire room. However, if a space is very small and practically untreatable as a live space, then deadening it is the only option; you’ll have to add artificial reverb after the fact.
  • Since the lowest frequencies take a lot of space to be absorbed, you can use corner traps, fireplaces, closets, or large holes in the ceiling for the sound to enter; these spots are filled with rockwool, then covered with acoustically transparent fabric.
  • Another way to deal with low frequencies is to use a barrier mat/dead sheet. Since it isn’t rigid and has a lot of mass, it absorbs energy instead of simply reflecting it back into the room—like bouncing a ball off sand instead of concrete, as the author puts it. The article shows one implementation of this material.

If you have a room that simply can’t be turned into a good monitoring space, or it’s financially impractical, Waves sells an attachment for studio headphones that does a great job of mimicking the acoustics of a real mastering suite. It even tracks your movement, so you can walk around the virtual room.

Here’s the full article: Link