There are dozens, if not hundreds or more, of these so-called 'golden ratios'. Their whole idea is to distribute your dimensions in such a way that your modes don't bunch up or get spaced out too far. That's why room volume comes into play - it's tied directly to dimensions. When you go shorter than 10' for your ceiling, your modes are going to have less-than-ideal distribution.
So, if you want a 20' room, make sure your ceiling is greater than 10' by the requisite amount - and don't forget the side walls - or you're going to have a 2x room dimension which will only cause you headaches later.
Just adding a few points that will support what has been said, in point form.
1. Don't forget to decouple your drywall, apparently it only helps about 1db but it mainly helps with vibrations escaping this is not as effective vice versa but it would also help with noise getting inside, a extra wall will still have some sound isolation both ways. The gap should also help with some bass absorption.
2. You can easily put your speakers inside the front wall this would negate speaker boundary interference.
3. I would use more dense rockwool within your drywall, I'm sure this has been talked about in this forum. Don't get confused with this recommendation with acoustic treatment we are only talking about sound isolation. You will see many places recommend the softer stuff between drywall and wall but if you keep reading and understand that the dense stuff absorbs more bass its a better option. For acoustic treatment inside that is for another thread.
Are you doing a room within a room or just dry-walling the room then adding acoustic treatment?( Maybe I missed this in your replies)
I only ask cause if you go the room within a room route I am just wondering if you could hit your target room ratios with bringing in the side or back walls a little bit? Just a thought the others can chime in to help with this not sure if this works...
PS. If anyone has any input about number 3 let us know, its just the conclusion I have come up with after reading about densities.
I know virtually nothing about sound isolation but, from what I understand, rigid insulation is used in industrial sound-control situations with machinery and whatnot. Basically more absorption in less space. As far as it goes with acoustic treatment, I really don't know how fluffy compares directly to it. I wish I did, though, because I want to install more treatment and I'm probably going more fluffy.
Maybe Ethan can chime in - or someone who knows the history of acoustic treatment - to talk about how rigid came into use and why fluffy isn't more widely known as 'acoustic treatment/bass traps'. Seems like the common gestalt is foam/rigid 'bass traps'. Fluffy seems to work a treat assuming you have the space - maybe that's the whole concern.
It's worthy of a whole separate thread, I suppose. If rigid's optimum diminishes after 8" then how does that compare to fluffy which seems to have no practical limit on thickness. I'm just curious how this plays into 1/4 wavelength vs GFR/density, etc. For instance, if one was an acoustics consultant, is space your only concern when deciding on a broadband absorber? Another point is that, with enough coverage, thick fluffy seems to be less attractive - depending on application - given the reduced strictures on resonances and SPL below a nominal sub-bass frequency according to so many authoritative sources and how rigid is sufficient above that cutoff assuming sufficient coverage.
After another great post, I always analyse and try to understand the concepts of your write up. Remember their is always someone who over analyses and appreciated your responses. I am one of them.
"what I understand, rigid insulation is used in industrial sound-control situations with machinery and whatnot"
Hence why I wanted to clarify that for sound insulation go with the rigid stuff
"As far as it goes with acoustic treatment, I really don't know how fluffy compares directly to it. I wish I did, though, because I want to install more treatment and I'm probably going more fluffy"
You had a great reply that I have saved somewhere in my notes in which you explain that after a certain thickness thickness doesn't play a major role and that the soft stuff technically would be more effective. You also mentioned some other unique concepts in that reply which are not around in forums.
"I'm just curious how this plays into 1/4 wavelength vs GFR/density, etc."
This is where the bucks stops for me, trying to understand this.
I think the conclusion is well at least for me, rigid at reflection zones to maximise absorption and space with soft stuff in corners going as thick as possible.
Gapping the fluffy makes a huge difference at 125Hz though we do see that nefarious 'midrange dip' side-effect an octave up. We can also see the side-effect of FRK as it really starts nixing effectiveness above 1kHz.
Not sure whether the rigid is gapped here - probably unfaced. Either way, rigid is 'significantly' better at 125Hz with 2" less. However, you need a total of 22.5" to equal 4" of rigid when you include the air gap. I didn't see anything for air gap plus paper - maybe that's where the real gains start for the low-end.
As far as 8" being the max return for rigid, it's just what I've heard; something about the density causing internal reflections. The thing is that, although the theory is that RFZ is all about mid-and-highs, the reality is that modal issues are caused by all primary boundaries at all locations - bass doesn't know where your RFZ is. Given that - if you really want to dial in your response - you may want to make your RFZ dual-purpose. This ties into your last point in that bass is not just a corner issue, as you probably know, although it is a good place to start.
Really, I just want to treat my room to such an extent that I never have to worry about absorption again. By now, I've had my panels for almost 10 years. They've proven their value and will probably continue to do so. As far as I know, the room is the weakest link in almost all recording chains so I'm just hoping to execute this beast once and for all in the next year. The question that remains is which sword will cut most truly.
Again, if you don't know, GFR is Gas Flow Resistivity and the real factor - not 'density' per se - that acts on the sonic velocity. These two metrics are related but the exact math is presently beyond me. At least, that's what I currently understand.
Regarding 125Hz and the 1/4 wavelength law, it's 27.5" at that frequency - very close to the 22.5" (~150Hz) we're seeing in the absorption coefficient lowest given value. So, if I want to go at least an octave lower, I'm probably going to need to fill that space because I'm certainly not putting 4' of absorption on either side of me because that's just not realistic. This again leads me to think that 2' of fluffy is going to be the ideal: it comes prefabricated at that size and ticks all the technical boxes as far as I can tell. If I ever hit the jackpot and get a bigger space, I can always gap these monsters and change my specialization to Infrasonic Design.
Then again, 4" rigid does mighty well at 125Hz so maybe just 8" of rigid will be all I need. I could even gap it a little and still have a little extra room left over to stretch my arms. 8" rigid panels are gonna weigh a lot though.
Actually, now that I think, 6" is also probably viable with enough coverage. It's not like I plan to record 8x10 Ampegs. Heck, even more 4" panels is probably sufficient. Gonna have to do something eventually though because it kind of sucks having exactly two points that are really usable. Not exactly complaining here - two is better than none - but I want to make my space more versatile for video and general aesthetic as well as optimizing mix position accuracy.
Let's be honest - I just want to buy a Daking mic pre but everyone says you need your room handled first. There, I said it. haha.
I think the origin of the question "rigid VS fluffy" in this thread was in reference to what kind of insulation to put in the wall cavity between the two mass components on either side (let's say each are double drywall). IIRC, in Rod Gevais' book (or online I can't remember:( he states he opts for fluffy because of the cost vs effectiveness. I don't believe he gave other reasoning but it's just my guess that it may also have to do with that the insulation INSIDE the wall has a slightly different function as does an absorber in the room. In wall construction, the two MASS layers on either side do the "heavy lifting" (attenuation) where the insulation in the "AIR" layer inside damps reflections in side the wall cavity. The size of the cavity dimension has something to do with the wall attenuation too. I don't have the book handy, maybe someone who does can clear this up.