Aliright, in this example, pull up a song you have recorded with drums. Put a reverb plugin on the drum bus. In other words, run all your drum tracks through this one reverb plugin. Make sure it's a reveb plugin what has controls for size of the room. The Waves Rverb is a good one for this test.

For this test, let's keep the reverb set to 100%. I don't to hear any dry signal yet. Set the room to have maybe a 2 second reverb time. Set the room size to be as small it can possibly be. Make sure that early reflections are all the way up. Now listen. You may need to pull the reverb time down a little bit.

With the room the size of a telephone booth (which there really isn't a way to fit a drumset into a phone booth that I know of) you'll find that the drums have this familiar boxiness. Slowly increase the size of the room. You'll find that the drum kit gets less and less dull sounding and has a great presence to it. It's almost more “clear” sounding. Push the size of the room all the way up.

Alright, now keep the room size all the way up and play with the reverb mix until it sounds pretty good on the drums. I'm guessing 30%, but who knows. So know you should be hearing your real drums with quite a bit of reverb on them. Start pulling the size of the room down slowly and listen. There is a point where the boxiness will begin to kick in again.

Why?

Well, the boxiness comes from a signal being out of phase with it's reflection. When the reflections is only a few milliseconds behind the original signal,comb filtering is caused. The effect of comb filtering is diminished or completely destroyed when this reflected signal is delayed further. There is a point where you will begin to hear a slapback.

Of course, most rooms do not have a slapback. While a concrete wall could easily do this, most studios are build with lots of wood and are angled and diffused in a such a way that you usually wont' hear a clear and obvious slapback.

Conclusion

If you are trying to record drums in a very small room, you have your work cut out for you. It's extremely difficult to get drums to sound great in a small room. There are rooms that do work well for drums that are very small, but they have been treated by an acoustical engineers who knows how to get rid of the boxiness. Just putting up Rockwool, blankets, foam, or pink insulation isn't the answer. While Rockwool / rigid fiberglass is usually the best solution (when setup properly) simply slapping up a blanket, foam, or pink insulation will only absorb the high end. This means that that the low mids will still be slapping off the wall and really clouding up your recordings....essentially making the problem worse in a lot of ways.

While treatment can help, there is no substitute for size. You have never heard a drumset until you have heard a drumset in a big room. I've noticed that drums almost always sound great (acoustically) when setting up for a live show because the rooms tend to be big enough to acoustically handle the drums without boxiness. The tone is even better in a real studio that has been acoustically treated and uses a big room.

One last thing to remember is that the room is part of the instrument with drums. If you listen to just about any recording, you will hear ambience in the drums. The room is a big part of the drum sound we all know today. As you may know, I always say that the source is 99% of the audio engineering game. So recording great drums in a great room is fairly easy. You just need to setup a few mics to capture that. However, it's much harder to use fancy gear to fix a shitty room. Good luck.