i'm after some advice about how to make a large iso guitar booth sound great. I'm particularly interested in the role of airflow. Could this be part of the key to making it work?

The background.....I'm about to build a large guitar iso box inside a closet that can house a few speaker cabs inside. I intend to fit a 4x12, 1x12 and a 1x15 cab. Won't be much space left over. My intention is to attach two layers of drywall with green glue to the existing closet walls effectively making a box inside the existing closet and sealing it. I'm also going to use extremely thick quality absorption inside where possible (1 foot in the corners for example) to bass trap as much as I can. I guess you could call this a very large iso booth or a very tiny amp room.

I currently have the 4x12 cab sitting in a smaller non sound proofed closet that sounds utterly incredible. (better than than in the large room) The catch is that this small closet is not soundproofed properly and can't be used for this purpose. In this smaller closet however, there is excellent airflow due to an aircon return vent inside. I have always wondered if this available air is what makes the cab sound great in there against all expectations? Could it be? Delusion? Coincidence?

I hear that iso cabs suffer from two problems a) bass build up with standing wave, modal issues etc and b) the lack of ability for speakers to properly flex due to airtight pressure restricting their movement. As a solution to problem b, could it make sense to introduce a return aircon vent inside the booth that is pulling the air out giving it a clear path? I could alternatively make a tunnel out of the closet into the roof space to allow the air to vent. Perhaps use a sucking fan at the end even. Obviously this introduces issues with sound proofing that I don't know how to solve yet but I wanted to get people's thoughts on the importance of airflow in this application. I've read everything I can on the topic but never heard of putting vents in a small booth that actually suck the air out like an air con return vent. I would never have thought of this but for my tiny closet sounding so good.

Does anyone think there could be any truth to this?

There could be.

I'd try a different approach, which would be to not seal it at all.

I've built a "room within a room" out of 12 inch thick absorbent material for mixing. The material is against the front, side walls and ceiling, but for the back wall I pulled it far forward to have an 8-10 foot air gap and absorb more low end at the listening position.

I wasn't making any effort at sound proofing the room, however when I go into the next room over with barefoot MM 27s all the way up it's actually pretty difficult to hear because the volume is so low.

Generally people approach sound proofing by using rigid materials and making things air tight. What I think has happened in my case is that I have so much full range absorption that it ends up having a sound proofing result.

Maybe someone else will chime in to confirm that this is a viable approach, but I'd buy a bunch of Knauf R-38 in 12x16x48 and turn it sideways so that it's 16 inches deep, or even make a double layer everywhere and make it 24 inches deep.

It might pack the room extremely tight, but it would absorb so much that I bet you wouldn't cause any problems with what leaked out. Even better is that you'd be absorbing full range and you wouldn't end up with any resonances from a sealed box so it would sound better.

Also because it's not truly air tight, you won't have the issues with the air pressure from the speaker movements.

I'm not sure why this isn't a more common approach - sound proofing through absorption - since it's also beneficial for acoustics. My guess is that people don't want to give up the space.

I don't think this would be truly "sound proof" but I think it would probably reduce the volume to acceptable levels unless you're cranking things to the point where the mics probably can't record as well.

Thanks I actually Had this exact idea and was told that it would not sound proof. I think I might try it myself though and learn the hard way!

For isolation you need mass, decoupling, and airtight construction. This contains sound within the structure. By containing all the sound energy you often create acoustic issues, and uneven responses. This is where the acoustic treatment comes into play.

The basic rule of thumb is the more isolation you have, the worse the internal acoustics are going to be.

You need to define how much isolation you need, and then you can plan how much treatment you need or can fit practically.

The reason people don't use insulation for isolation is because it doesn't work well for that purpose. There is a difference between using it in gobos around a drum kit to aid bleed into mics, vs the isolation requirements for say keeping things quiet enough for your neighbors ect. Sound can be inaudible to mics while being clearly audible to the ear.

It won't "sound proof" because that's about isolating the sound by something that prevents it from passing through the walls.

This is about absorbing sound which means whatever passes through is far lower in level. My point is that it may work well enough in terms of reducing volume and it will be better in terms of recording results.

Also, if it reduces the volume to equal to or less than the ambient noise, it's had the same net effect as "sound proofing" even if it's technically "volume reduction".

Thanks everyone for the great input. Excellent discussion and I'm very appreciative. I realised that what I needed was volume reduction, not soundproofing. I never made that distinction which is my fault. I actually decided to chuck the cabs in the closet and used extremely thick quality fibreglass absorption inside. I didn't bother with anything else. With an attenuator on the amps to knock the volume down a little bit as well it gives me just what I need. It's not airtight inside but the edge is absolutely taken off the volume and mainly what is leaking out now is low frequencies. It sounds every bit as good as it sounded in the live room when recorded. Indistinguishable in fact.

Clearly what I have achieved is nothing remotely like sound proofing but it's enough for my purposes. Its a dramatic difference from what it was before and a decent solution. The low frequencies leaking out rather than being stuck inside probably makes it sound better as well as people have noted. I think I was confused as others probably are about what reduces the volume in the studio and what eliminates it for the neighbours. People saying that absorption does nothing for sound proofing always left me scratching my head. I hope I never need to actually sound proof anything. It sounds like a nightmare to trap bass frequencies. Eric Valentine has decided to use tube traps to treat his new studio to absorb way down low. He's the man and I'm sure that is an effective path.

Which explains why people who build “a really good isolation booth” almost always end up hating the damn thing.

Yeah which is another reason to get only reduce the volume u absolutely need. In my case, most of the lows are escaping right out the closet which is probably why I’m thrilled with how open it sounds.

What material are you using, how thick, how much and where do you have it placed?

Can you able to post a photo?

Martini poly max XHD and HD is what I’m using. It’s not fibreglass but has excellent absorption stats. I live in Australia. I use the stuff in my studio control room and measuring before and after. It’s totally fixed my 60-90hz range. It even effects stuff at 30hz albeit in a for more minor way. I love that stuff. I have it very thick in the closet in chunks in corners. About 3 feet high and 1 feet deep. I have another big chunk in the middle and then 2-4 inches in the walls depending. I can get you a pic but left the state for a couple weeks. It’s pretty full of absorption. I will take a pic for u though. Might even post some audio eventually.

The isolation your getting is from the closet. The insulation on its own does next to nothing for that. Insulation up against the wall does do something because it dampens the resonance of the wall panel.

Insulation on its own doesn't do much when talking about reducing spl of a source to the ears.

Its the difference between the insulation acting on its own, or as part of a system.

Glad you got the result you needed. Erics studio is looking like it's going to be a nice place.

Lol that's cuz they didn't build a really good booth. Isolation is just part of the equation as im sure your aware.

Ah yes that makes sense. The closet is providing the isolation. The thick insulation is removing the terrible reflections and helping bass buildup. The poor soundproof is allowing nasty bass to escape. End result... it sounds beautiful.

Without any measurement or proof, but as a common sense thing and from many years in studios (and “studios” in name only) I agree with the OP’s thought that effective trapping in the room does lower transmission to adjacent areas to some degree. It isn’t isolation, but if you trap reflections and random non-direct garbage and bounce in the room, you should lower the SPL in the room, which should lower the level of sound leaving the room.
But I’ve been wrong before in assuming what is happening in a room without measurement, so...?

I was able to find photos of that on line. It's different from what I would have used.

I was curious to see how much you'd stuffed the room, and whether it looked like there was room for more and that might reduce the bit of low end that's coming through.

Since it's different from what I would have used, I can't really rely on it as an indication of what fluffy insulation would have done if you'd stuffed the room with it, but I'm glad to hear that the absorption approach got you results you're happy with.

If absorption lowers levels of reflections, it would lower levels of transmission too.

No. I you use f.e. a tuned aborber in front of a wall to trap a mode you weaken the isolation .

Could I ask what u would have used? I did lots of research on this product even though it is an unusual choice. Gas flow and density and stats all seemed very good. Ultimately I took a punt because I didn’t want to handle all the fibres. Interestingly, I had a huge null in my listening position last year between 70 and 100hz. I use fibertex 350 (Australian thing similarIsh to 703) as bass traps in corners etc and on sides. This null would not go away. I then made a massive cloud of this Martini Absorb HD over a foot thick above me. Quite huge. It totally got rid of the null and even improved things a bit right down to 30hz. I have graphs of it all measured through the Genelec software. This was a total game changer. I figured if it did so well it that application, it would work in others. I’m really sold on the stuff. Martini polymax HD for super thick and HDX for under 6 inches is what I went with.

I’ll try and send u a pic. My aircon has stopped working in studio and nobody can fix for ages. That’s all the air I have so disaster and no studio for at least 2 weeks. Lost income tragic!!!!!!!! There is lots of absorption though in the room yeah

What? How would an additional trap INSIDE the isolating perimeter weaken the level of isolation?
I admit that the science of acoustics can confound what seem like logical expectations, but I’d love to hear why your statement is true.

I'm curious about this as well.

AARGH! The additional leaf reduces isolation at the MAM resonance frequency. For music that is significant. Look at the TL graphs in IR586 comparing various additional leafs to 190 mm brick walls. The low frequency is reduced.

Enjoy!

I am reading what is basically a foreign language here.
I see information about insulation in the wall cavities, but that’s not what we’re asking about.
For example, in a room already constructed with very effective isolation, if I put a thick, tall, freestanding trap angled across a wall/wall junction, it will make some change in the acoustics IN the room. Are you saying that trap lessens the isolation between the room and the outside world?

At the MAM frequency the isolation will be reduced. You are reading it right. It is not intuitive.

Thank you. I have learned to trust you on the room acoustics stuff that is beyond my understanding.

If you put a resonating structure, a mass spring system, in front of a wall you alter the sound isolation of the wall.
If the resonance freq fs of the structure is very low (with floating floors and heayvy walls you aim at 5-8 Hz) you get extra isolation from SQRT(2)*fs.
If you want to eliminate a mode you have to tune the construction at the freq of that mode, so the isolation will only start at 1.41*that fs.
As you want per definition to isolate under the fs of the mode isolation is per definition degraded.

OK...now you smart guys are just showing off!

This is like reading about string theory. I trust these guys but have no idea