Dude literally anything that is soft and/or angled will work. Basically, if it absorbs mid and high frequencies and/or diffuses them (bounces them off at odd angles) and keeps them from bouncing between walls over and again, it’s doing its job. Preferably a combination of both is probably best. But in any case, you don’t have to buy the expensive stuff from guitar center or Sweetwater. Cover the walls in thick blankets and you’ll get a dead sound just as well for a fraction of the price. Hell, I’ve surrounded a guitar amp with blankets with the mics inside. Worked just fine, no reflections made it back into the mics. The test is to clap. If it’s dead in the room you’ll hear no reflections. And believe it or not, a lot of studios have just a little bit of reflections in the tracking rooms on purpose. They don’t make it completely dead, in other words. And then they may have the vocal booth dead as dead can be. It depends what’s being recorded and how lively they want it. Tracking drums in a room with a little bit of reflections actually sounds better to my ear than a completely dead space usually. All in all, if you can pull off real live reverb and make it sound right, it beats the shit out of digital emulations pretty much every time. Just last week, I got this wild idea to put a couple of mics in the corners of my mix room to create “room mic” tracks. I played the drums track through monitors, and those mics recorded what came through the monitors. Sounds crazy, but it did what I intended…which was to give me a room mic stereo track to liven up the drums a bit. Muting it on and off in the mix, it was a no brainer to keep it. It sounded way better with the mic’ed tracks than without. All that to say, a little room reflection isn’t bad if you can control it, are intentional with it, and you know what you’re doing. I ended up doing a little experiment where I recreated more “room mic” tracks for the other different instrument groups in the mix. Granted, the room mics in solo sound like shit, but that’s not what matters. What matters is how they sounded in the full mix.. and they made the full mix come alive! To be clear, all the tracks were already mixed really well. EQ and compression, all of that stuff. The “room mic” tracks were the finishing touch, and they probably sounded so good because the tracks I used to create them were already mixed pretty far along. Point being, if I would have created those room mic tracks with untreated audio, it probably would have sounded like shit. The room mic tracks themselves were only slightly EQ’ed and compressed. I didn’t have to do much to them except set them at the appropriate volume relative to the other tracks in the mix. This is something I’m going to start doing more since it worked so well. I guess I’ve got off on a tangent at this point, but I’m just saying that there are rules, but then there also aren’t at the same time. Try stuff and see what works. The most expensive solutions are not ever necessarily the best ones for any given problem.