Advice about sound treatment - small home studio?

[Stratman68]

So I have a bunch of Auralex 2" Studiofoam wedges. Also 2 Auralex 2'x4' 3" thick panels on mic stands. Never used the Auralex-Duh!
Studio Monitors MAckie HR624 mkii's are 3' from the wall. I have 2 DXR10's behind me in case I want to blow my eardrums out. My small guest bedroom\home studio is only 11X12' but it's only for me so OK. BTW just bought a Rode Boom and it is so good for the 2.1lb Large Dia Tube Condenser Mic. I can record sitting or standing easily. Just swing it into place.
Anyway, is there any point to putting this stuff (Auralex) around the room on the walls and corners? I have no idea WHERE?
Any advice welcomed. I've been spending $$$ on my DAW, bough the Scarlett 18i20, etc, etc. So I figure I may as well ask you folks.
I normally record with my Beyer 770 pros 250ohms . But I do mix without them also using a variety of playback speakers.

Thanks

​

 

[RevDrucifer]

Yeah, there’s a BIG point to it. I learned this lesson big time when I moved out of my house and got my apartment. My studio in the house used to be so crammed full of gear that there wasn’t a flat surface in the room, my new apartment has a bigger room I’m using for the studio and nothing but flat surfaces. The first night I moved in and set up my studio it sounded like straight ass and remained that way until I put some foam up. Still need a lot more and I need at least an area rug in there as well because it’s a tile floor.

The sound ends up bouncing off the walls/flat surfaces and you just get dominated by the sound of the room, more so than the sound of the monitors. If you can clap and hear an echo, you’ve got room reflection. The treble and mids were just all over the place and there was zero definition in anything.

I first went with the corners, using that angled foam for all 4 corners, floor to ceiling. Then I got those octagon shaped panels to put on each side of the monitors. Just the traps in the corners cleaned things up considerably, with the octagon panels doing a little extra. I still haven’t mixed in there yet, but I know I still have work to do.

I keep planning on making my own traps, using 2x4’s, insulation and some towels/cloth on the outside to make ‘em look nice. I’m assuming 4 of those about 6’ tall, 2’ wide will lear away most of the remaining issues, along with a rug.

 

[LiamH]

Hi Stratman68,

Got to agree entirely with RevDrucifer, this is life changing stuff, especially in a small room. I spent about 3 years using a spare room similar size to yours with no acoustic treatment. The low frequency reverberation build up was a major limitation to listening pleasure, and made mixing anything with much low end impossible.

I was lucky enough to be able to have a home studio built this year, and the control room is a little bigger than your room (and my old one), but the acoustic treatment has worked out amazingly well. Mine is all done with rock-wool insulation of varying density, hidden by fabric wall coverings.

Bass traps in the corners made the biggest difference. They are pretty wide (maybe 18"), and filled with a high density - low density - high density sandwich of insulation in all 4 corners of the room over the full room height. Most of the rest of the treatment is just 4" thick low density insulation, covering most of the back wall, most of the front wall, and then at potential reflection points on the side walls. We played around a tiny bit with moving the insulation once I had playback gear in there. There's also a big slab of insulation in the ceiling directly above my listening position, mainly because I have relatively reflective laminate floor.

What really helped with the experimentation was making measurements, particularly of reverberation time against frequency. If you have even basic recording facilities in your PC, and preferably a responsive condenser mic, ideally a purpose made measurement mic, then Room EQ Wizard (REW) is amazingly useful:

https://www.roomeqwizard.com/

If you use this you can often identify inadequacies in build up of particular frequencies because of their long reverberation times. My baseline was the old spare room, because I knew it was pretty hopeless, and early measurements with REW and an old C1000s confirmed this beyond doubt. I'd strongly recommend making measurements without treatment at the listening position, and then moving treatment around to see what happens. The ideal is to get ever decreasing reverberation times as frequency goes up. If you want to know which frequencies you need to kill, the amroc calculator is very easy to use, and definitely picks the right frequencies for the few rooms that I have modelled, designed and measured.

https://amcoustics.com/tools/amroc

As to whether there is any point? Jeez, where do I start, it's just incredible. I started out using exactly the same gear (Scarlett 6i6 into Adam A5X), on the same stands as I had in the old room. The frequency and stereo separation is absolutely amazing. If I'm listening to albums I know well I can hear really obviously the completely different feel of the tracks that were mastered in different suites, or recorded in different studios. I am finding subtleties in low and high end registers that I never even knew were there. I haven't really got started on recording much (see my other thread... ), but I don't care so much, because everything I listen to, and everything I play in there is like a new experience.

Could you achieve a similar epiphany with foam wedges and some panels? I reckon you will get much of the way there. Wedges in the vertical corners, back wall to start with, front wall as well if you can. Put them at speaker height if you can, so they are trapping the bass at its maximum intensity. Then for the flat panels, try to find the reflection points on the side walls where sound will arrive at your ears as first reflection. This way you should only hear what's coming out of the speakers once (initially at least, the reverberations never go away...) Importantly, make measurements of how the room and the sound system interact, using REW, as you make any changes. I tend to put the measurement mic right next to my right ear when checking the right channel, and vice versa for the left.

Pretty good description of placement in this article: https://www.soundonsound.com/techniques/studio-sos-guide-monitoring-acoustic-treatment

Make sure you let us know how you get on. You might find you want to get some more panels and wedges.

My experience with the control room has been jaw-dropping. I haven't yet decided if the live room has worked out perfectly, which surprised me as it's the room I spent most time in detail designing the acoustics. Full band with acoustic drums was always going to be an issue in a relatively small space (14' x 19').

I thought the control room was too small to sound good, but I'm most likely going to get a sub speaker in there soon, which I had thought was going to be pointless given the potential difficulties of controlling bass reverberation. Turned out to be a bit easier than I had hoped.

Apologies for the long reply, first time I thought about rationalizing what I've been doing in my spare time for the last 14 months!

Liam

 

[marsonic]

Is there a point to treating a room? Yes. Absolutely. It's mind-blowing the difference it makes.

First priority is bass traps in the corners and first reflection points, then (in most rooms) multiple subwoofers or AVAAs to finish "treating" the bottom couple octaves, then second reflection points. Each step gets diminishing returns. Or, given the budget and freedom, you can just have someone design it from the ground up....but that's orders of magnitude more expensive.

Is there a point to treating a room with the super-thin Auralex egg-crate looking stuff? Not really. All it does is reduce flutter echo similarly to putting basically anything in the room other than bare walls and right angles.

The problems in small rooms (acoustically, your room is small) are mostly with the lows, and those things don't do anything to address anything below a few kHz. It's a complete waste of money and time. It'll make it sound superficially "better" by reducing that flutter echo that gets annoying, but it won't make it any more accurate. If you actually measure the room, within the limits of the measurement mic (which usually won't even hear the bottom few octaves), you'll probably find 20+ dB swings below ~200Hz. If you do it yourself and just listen to a sine wave stepping through semi-tones (ideally with a pause between each note to let the sound die out) from 20Hz to 20k, it'll be painfully apparent how badly you're hearing details in the low end.

And because they're also significantly related to decay times rather than just a static frequency response, things like applying a corrective EQ does not actually solve the problem either. That's a last couple percent thing, not a first step.

It's generally a lot easier to use HRTF Crossfeed to get a great sound out of headphones or IEMs than it is to get an acoustically small room to sound right.

And, yes, I know that the multiple-subs thing came from the Hi-Fi & Home Theater world. But, it's just physics. And, they're not wrong.

There's a reason top-end studios have freaking huge control rooms (compared to basically all DIY studios). It's so they can shift the Schroeder Frequncy of the room (below which sound "works" by changing the air pressure in the room and resonating rather than bouncing around inside it and reflecting) down to where it's less of a problem.

After hearing it myself, I will never set up a critical listening system without at least 2 subs unless the room is huge.

 

[LiamH]

Is there a point to treating a room? Yes. Absolutely. It's mind-blowing the difference it makes.

First priority is bass traps in the corners and first reflection points, then (in most rooms) multiple subwoofers or AVAAs to finish "treating" the bottom couple octaves, then second reflection points. Each step gets diminishing returns. Or, given the budget and freedom, you can just have someone design it from the ground up....but that's orders of magnitude more expensive.

<snip>

After hearing it myself, I will never set up a critical listening system without at least 2 subs unless the room is huge.

@marsonic, really interested by your post. My control room is a little bigger than the OP's, and is well treated with 4 big bass traps as well as other treatment at reflection points behind fabric walls and in fabric covered ceiling. Aside from my mind being blown by how good it sounds, the bottom end of my monitors is rolled off below 100 Hz, but as they are only have 5.5" drivers they don't do so much at the very bottom end anyway. It did occur to me that because it is an unusually good sounding small space I ought to try a sub in there. Having read your post it might mean I try one, but if it does what I hope it might, will actually get another one. That room is a win-win space, it sounds amazing now, so if I make it worse, I can always revert. Would love to hear 80Hz down to 50Hz and below, even if the reverberation time is long enough that I keep its volume low. You have inspired me!

More intriguing in your post was my first knowledge of AVAA's. My live room was designed with the only surfaces parallel to one another being very absorbent acoustically to try to avoid the first longitudinal mode. (2 smallest area end walls). One of them is fully treated, with a 100mm air gap behind the treatment, as well as big bass traps in all 4 corners of the room. I foolishly thought with that kind of overkill I'd defeat room resonances, but I have a serious issue around 137 Hz, so probably similar at 70 Hz and 35 Hz. All of these are unfortunately well below the Schroeder frequency for the room. I never wanted to kill the "room feel" in that one, so although the big walls have been set at an angle, and I pitched the ceiling so it doesn't resonate with the floor with standing waves, I didn't think too hard about the entire volume of the room resonating. We didn't treat the pitched ceiling, because I don't want my live room to sound dead, so I'm still avoiding that.

I'm never going to consider subs for that room, but I do intend to use it for recording guitar and bass amps, and I had hoped at reasonably high SPLs. So it's great(ish), but I know it could be better.

So, about these AVAAs? Read a few reviews, and I gather they actually work. Not cheap, but a small investment compared with building my little studio complex. I think I know where I need to put them in the live room, and they will probably get some use in the control room if I get a pair. In your experience, stark enough in changing room "feel" to be worth the money?

Liam

 

[marsonic]

@marsonic, really interested by your post. My control room is a little bigger than the OP's, and is well treated with 4 big bass traps as well as other treatment at reflection points behind fabric walls and in fabric covered ceiling. Aside from my mind being blown by how good it sounds, the bottom end of my monitors is rolled off below 100 Hz, but as they are only have 5.5" drivers they don't do so much at the very bottom end anyway. It did occur to me that because it is an unusually good sounding small space I ought to try a sub in there. Having read your post it might mean I try one, but if it does what I hope it might, will actually get another one. That room is a win-win space, it sounds amazing now, so if I make it worse, I can always revert. Would love to hear 80Hz down to 50Hz and below, even if the reverberation time is long enough that I keep its volume low. You have inspired me!

More intriguing in your post was my first knowledge of AVAA's. My live room was designed with the only surfaces parallel to one another being very absorbent acoustically to try to avoid the first longitudinal mode. (2 smallest area end walls). One of them is fully treated, with a 100mm air gap behind the treatment, as well as big bass traps in all 4 corners of the room. I foolishly thought with that kind of overkill I'd defeat room resonances, but I have a serious issue around 137 Hz, so probably similar at 70 Hz and 35 Hz. All of these are unfortunately well below the Schroeder frequency for the room. I never wanted to kill the "room feel" in that one, so although the big walls have been set at an angle, and I pitched the ceiling so it doesn't resonate with the floor with standing waves, I didn't think too hard about the entire volume of the room resonating. We didn't treat the pitched ceiling, because I don't want my live room to sound dead, so I'm still avoiding that.

I'm never going to consider subs for that room, but I do intend to use it for recording guitar and bass amps, and I had hoped at reasonably high SPLs. So it's great(ish), but I know it could be better.

So, about these AVAAs? Read a few reviews, and I gather they actually work. Not cheap, but a small investment compared with building my little studio complex. I think I know where I need to put them in the live room, and they will probably get some use in the control room if I get a pair. In your experience, stark enough in changing room "feel" to be worth the money?

Liam

The AVAAs are a little expensive for what they are. They kind of boil down to being a pair of subwoofers (and my limited experience is that you "need" then in pairs) that "listen" to the room and actively cancel out what the problems are. Their advantage is that you just put 2 of them spaced out where they work best for you (often on the back wall) and they take care of at least the biggest remaining problems on their own.

I know Justin Perkins (Mystery Room Mastering) has written about his and loves them. We talked about them a bit when I was setting up my room (through the Reaper forums, where he's been pretty active). They're expensive because they're simple. He's also running a pair of subwoofers with their built-in crossovers, placed near his mains about as simply as you could possibly set them up (though PSI does a better job than most with that setup).

I ended up going with a pair of more normal subwoofers. IMO, the JBL LSR-whatever subs are the cheapest & most basic anyone should be considered because they're "fine" combined with being able to turn off their internal crossovers (most studio subs won't let you bypass their crossovers). In addition to generally just being fixed at 80Hz, all minimum phase filters cause a significant null at their crossover frequency, and that poses a serious problem for accurate sound unless the designer jumped through a lot of hoops. You can absolutely get better....most of the servo subs are impressive, so long as you can bypass their crossovers.

IME, That's actually the reason a lot of people avoid subwoofers in control rooms. The solution, as much of a PITA as it is, is to use linear phase crossover filters and deal with the "ridiculous" latency they incur. There are a handful of mastering engineers (most notably Bob Katz) who are completely bypassing all of the internal crossovers of their speakers and using linear phase filters in software and separate feeds from a multi-channel DAC for each speaker (Bob Katz uses Acourate Convolver, last I heard). E.g., a pair of outputs for tweeters, another pair for midrange drivers, another pair for woofers, another pair for subwoofers....with all of the analog crossovers either eletronically bypassed or physically removed from the speakers. And Bob Katz is doing that with very high end (5-figure) Dynaudio towers, IIRC. Going whole-hog on that active multi-way setup also gives other advantages, mostly being able to phase align individual drivers in a more precise way than the physical construction of the towers and anything you can do simply with analog filters can accomplish.

This kind of setup is also, more or less, why I think the Kii Audio and Dutch & Dutch speakers are so interesting....that's actually how they work internally, plus having their own DACs so you just feed them with digital audio.

Personally....I don't actually need to use my speakers in a zero/low-latency context. I'll use IEMs with GoodHz Can Opener when I need to hear accurately in a low-latency kind of way (Reaper reports 0 samples latency for CanOpener, FWIW). So, I'll pay the price of almost a second of latency for the much better XOs ("better" software can reduce that, but it's more complex to set up). So, literally all of the audio my computer can play goes through a Reaper session that exists mostly as a monitor controller with several instances of FabFilter Pro-Q 3 on Linear Phase - Maximum that acts as the crossovers and was tuned to my speakers and my room. The downside of that setup is that it isn't nearly as simple as pushing a button on a monitor controller to bypass the subs and only listen to the towers. But....now that I've heard it, I can't think of a reason I'd want to do that.

I tried a handful of different locations based more or less on this article: https://www.gikacoustics.com/room-setup-speaker-placement-201-part-two-subwoofers/

They're glossing over a lot of the details....which is fine. You just try the different positions, listen to the stepped sine waves, and pick the one that's flattest as low as you can get. If you actually read up on JBL/Harmon's research into it, the "ideal" number in most rooms is usually either 2 or 3, but the placement of the third one gets complicated. Most measurement mics don't respond accurately in the 20s or 30s. Actually, I'm not aware of any that do....you pretty much have to listen to it or model the whole room in a computer (which is expensive and complex).

 

[marsonic]

....continued....

For my room, the best sound came from 2 subs in the 1/4-1/4 placement. It was a PITA to drag the subs around and try different versions of the XOs, but now....it's plenty "flat enough" down into the 20s (at least as low as my ears can actually hear) and when I turn it up, it thumps me in the chest like a dance club at a level that won't damage my ears. Good mixes/masters sound phenomenal, and bad ones expose their flaws quickly. And it was the first setup I had that actually passed LEDR. The sound stage is a little wider than the room is physically, and it extends about 8-10 feet into the front wall....which is actually one of the complaints a lot of people have with Class-D amps (that the sound stage extends infinitely into the front wall, somehow). I'm not sure how they're supposed to do that, but mine don't have that problem (and all of my amps are relatively affordable Class-D monoblocks).

My wife won't let me put multiple subs and bass traps in the living room....so that system is a much more normal hifi setup (B&W & Marantz, so still not shabby) and the difference is not at all subtle.

As far as making the room sound dead....IMO, that's more of a problem in live rooms. You do want the room to play a part in what you're recording to get a bit of natural room reverb in all the mics (not just room mics). But for a control room....you figure out the speaker placement so that you're not getting comb filtering between the mains and the front wall or chuffing from rear ports and the front wall causing turbulence (the easy way is front ported or sealed box mains, or very precise placement that happens to work with what you need for a correct stereo image). Then keep the front wall and the floor live....and everything else can be "dead" enough to get rid of the problems, and it just works out.

If you look at things like the Northward Acoustics Front-to-Back rooms, that's a part of what they're doing. They tend to use diffusers on the ceiling and back wall (because those rooms are large enough that diffusers actually do something, unlike in most DIY/project studios...diffusers actually take quite a bit of distance before they do anything productive) but in practice....the front wall and floor are live and every other surface just kills the sound.

Unfortunately, it isn't possible to have a single room "work" acoustically as a live room and as a control room. They just require completely different things. That, IMHO, is the big failing of home/project studios....people don't have the space to devote to each need, so they both wind up severely compromised....hence the proliferation of amp modeling and sampled drums, pianos, horns, etc.. They take away the need for a live room very, very well.

If your focus is a live room....there's no reason not to have a decent setup to play for other people....but for actually mixing, you'll get much better results out of IEMs and HRTF crossfeed. And that's always easier and cheaper than setting up a good control room.....it just requires a bit of adjustment to trade off all of the "oomph" and "moving air" for increased "intimacy" of the sound.

 

[LiamH]

Thanks for the in-depth replies @marsonic. I possibly wasn't totally clear, but I have the luxury of 2 separate rooms in my home studio (and a good size air-lock between them), but of course along with that comes the curse of 2 different sets of problems. They are incredibly well isolated from one another acoustically, which I hope will matter more in the future, but 2 totally separate spaces aside from multicore and digital tie-lines. And, of course, 2 outrageously thick chunks of glass so I can see from one to the other.

The control room has turned out to be the easy one. It's pretty dead acoustically, it's smaller than I had really wanted it to be, but it's a happy space for sounding amazing. A revelation in fact, so much better than I had hoped for, my favourite listening room ever. Your advice on subs for that one all taken on board, and I might well just split them off with digital filters in the desk and see if that's better for phase issues. I'll start with one JBL sub I guess, and see how I go on. I figured out pretty early on that diffusers weren't going to do much more than the furniture already in the room. Fortunately the spend on the build and acoustic treatment has left me feeling like I need to spend less on speakers, electronics and software fixes. Most things sound good in a good sounding space, but it's a heavy investment in time and money to get it built from scratch (and isolated, ventilated/heated/cooled, and EMI immune).

The live room is much bigger (contains a 5-piece band with acoustic drums and PA speakers in relative comfort). The rest of the band think it sounds amazing, but I have the misfortune of knowing what's wrong with it. The room ratios are all good according to theory, but no amount of wall absorption will get past the fundamental geometry of the room, and the acoustic behaviour of guitar and bass amplifiers played at high SPLs. If I have any cash to throw at it next year, AVAAs are on the list to see if low frequency resonances can be contained better. The builder of the current studio really wants to turn the 29" x 25" workshop next door to the current studio into a proper size live room - it has some really good ceiling height too. Unfortunately I need that space for other interests, and the cost of acoustic isolation, HVAC, etc. would be pretty high. For now, I have a 19" x 12" space that I need to make work for me.

Liam

 

[marsonic]

Thanks for the in-depth replies @marsonic. I possibly wasn't totally clear, but I have the luxury of 2 separate rooms in my home studio (and a good size air-lock between them), but of course along with that comes the curse of 2 different sets of problems. They are incredibly well isolated from one another acoustically, which I hope will matter more in the future, but 2 totally separate spaces aside from multicore and digital tie-lines. And, of course, 2 outrageously thick chunks of glass so I can see from one to the other.

The control room has turned out to be the easy one. It's pretty dead acoustically, it's smaller than I had really wanted it to be, but it's a happy space for sounding amazing. A revelation in fact, so much better than I had hoped for, my favourite listening room ever. Your advice on subs for that one all taken on board, and I might well just split them off with digital filters in the desk and see if that's better for phase issues. I'll start with one JBL sub I guess, and see how I go on. I figured out pretty early on that diffusers weren't going to do much more than the furniture already in the room. Fortunately the spend on the build and acoustic treatment has left me feeling like I need to spend less on speakers, electronics and software fixes. Most things sound good in a good sounding space, but it's a heavy investment in time and money to get it built from scratch (and isolated, ventilated/heated/cooled, and EMI immune).

The live room is much bigger (contains a 5-piece band with acoustic drums and PA speakers in relative comfort). The rest of the band think it sounds amazing, but I have the misfortune of knowing what's wrong with it. The room ratios are all good according to theory, but no amount of wall absorption will get past the fundamental geometry of the room, and the acoustic behaviour of guitar and bass amplifiers played at high SPLs. If I have any cash to throw at it next year, AVAAs are on the list to see if low frequency resonances can be contained better. The builder of the current studio really wants to turn the 29" x 25" workshop next door to the current studio into a proper size live room - it has some really good ceiling height too. Unfortunately I need that space for other interests, and the cost of acoustic isolation, HVAC, etc. would be pretty high. For now, I have a 19" x 12" space that I need to make work for me.

Liam

Awesome. Sorry...I was confused.

I know much less about setting up a live room. I've used a couple very good ones, but they were absolutely huge (smallest dimensions were the > 20-foot ceilings). I never "tested" them in terms of bass response.

Without needing to place speakers for proper stereo image...you might be able to go a long way toward controlling that low end just with where you put the amps...even if it seems weird to look at. And, even in those huge rooms we had a lot of bass traps on stands that we moved around as GoBos. It was a lot of experimentation and, at that point, just doing what the lead engineer told me without really being told why. I only set up one really big session in the big room, and we typically had cabs in isolated rooms in the smaller (but still relatively huge) room.

 

[LiamH]

<snip>

Without needing to place speakers for proper stereo image...you might be able to go a long way toward controlling that low end just with where you put the amps...even if it seems weird to look at. And, even in those huge rooms we had a lot of bass traps on stands that we moved around as GoBos. It was a lot of experimentation and, at that point, just doing what the lead engineer told me without really being told why. I only set up one really big session in the big room, and we typically had cabs in isolated rooms in the smaller (but still relatively huge) room.

Unfortunately open back guitar combos don't really care where they are placed in a relatively small room. They just tend to excite every mode available to them. However I haven't tried facing one with its back to the "dead end" we created with an air gap behind the acoustic treatment yet. You've inspired yet another good thought for experimentation.

I think the lesson you learnt from the lead engineer was a really good one - when there are too many variables the best anyone can do is "suck it and see". The more of it you do, the more you have a knowledge bank of things that worked and didn't work. Your thoughts and experience have made me even keener on the idea of a pair of AVAAs to play with. Movable active bass traps - what could possibly be wrong about that! I need to buy some more microphones, and a sub or 2 before I figure out what's next, but I think they might be it.

Liam

 

[Stratman68]

Great reading all thse posts....Good advice and I am working on it!

 

[marsonic]

Unfortunately open back guitar combos don't really care where they are placed in a relatively small room. They just tend to excite every mode available to them.

But, they will care, because not all the modes are available everywhere. They're going to throw sound all around the room, but the lows that are probably a problem only get excited from specific places. That's why a multi-sub setup works to cancel out modes. Sub-bass and higher-frequency reflections work completely differently below the Schroeder frequency of the room. Every small room is really 2 rooms...one where higher sounds bounce around and one where lows resonate like you're inside a huge subwoofer...because you are literally inside a huge relatively sealed chamber with rapidly changing air pressure....which is exactly what a subwoofer is.

If you're trying to keep the deep bass from a guitar cab (or a bass cab, for that matter) from interfering with other mics, I'd seriously try approaching it the way you'd approach a sub crawl setting up a home theater system....just with the opposite goal.

For a sub crawl (the "easy" way to set up a single subwoofer), you put the sub at the listening position and then crawl around the edges of the room looking for the spot where the bass is the loudest. Then, you put the sub in that spot, and it'll wind up loud (and efficient) at the listening position. You want to do that with the opposite goal.

This is all based on theory and re-applying what I learned setting up my monitoring rig, but....this is what I'd do...

Put the guitar cab where one of your other mics goes...something where you can't just filter out everything below your schroeder frequency. Wherever your kick drum mic is now would be a good place to start.

Play filtered white noise or a repeating sine sweep through it. You probably only need to go up to ~200-300Hz ish, but you should go down to 20 even if you can't hear it.

Walk/crawl around the room listening for anti-nodes (where the bass is substantially louder) and nodes (where it just kind of disappears). Mark the nodes on the floor with tape. These locations will change with specific frequencies, so you're looking for areas where a lot of it kind of just disappears. It shouldn't be a subtle thing.

Then move the amp to the next spot you need to put a mic and do the same thing.

If you keep doing that, you might get lucky and find a spot that has a lot of tape markers. So, put the guitar amp there and set everything back up...and the low end you get from the other mics is very likely to be cleaner.

If you can't find one on the floor, try listening at different heights too (though IDK how you'd mark them...maybe write the height of the node on a tape marker straight down from it). The floor-to-ceiling distance makes a difference as well. If you're putting a guitar amp on the floor in a room with an 8-foot ceiling, you'd expect a bump in the response at 140Hz (and each octave of it). If you somehow float the same guitar amp 2-feet in the air, that bump in the response will be reduced and many of the bumps at the octaves will be gone, since it's at a node for 560Hz and will simply not excite some of the lower partials. The amp will also get quieter because it's closer to un-loaded as opposed to half-loaded, but that's rarely a problem in a studio setting.

If that happens to work out, then you just point the amp where the front & back beams aren't really hitting anything important or block them with gobos.

I don't know why I think this, but I have a feeling that a good place to try would be at the golden ratio of each room dimension off the relevant wall/floor. So, if your room is 11' wide x 12' long x 8' high, I might try putting the center of the guitar amp 4'2" off a long wall, 4'7" off a short wall, and 3' off the floor. There should be 8 of those (the other 4 are 5' up from the floor, or 3' off the ceiling) and hopefully one of them isn't too inconvenient. I know that will do something special....but I'm confusing myself and can't figure out if that's potentially a great spot or the absolute worst place you could put the amp. I think I'm confident in saying it'll be one of the two.