Inside Out Walls - Treatment Options

[flatfive]

Hi All - I have some questions about practical 'finishing' of inside out walls and logistics. I'm posting here vs the design board, however here is my recent design post https://digistar.cl/Forum/viewtopic.php?f=5&t=1143

I have an inside out ceiling and two long inside out walls to finish. I plan on finishing the ceiling as demonstrated in this post https://digistar.cl/Forum/viewtopic.php?t=50. My joists are 2x6, and I plan on filling with Rockwool Safe and sound w/ strapping and fabric covering.

Originally I had planned on doing the same for the walls (with open questions), however in discussion with an engineer friend of mine, he was concerned the room would be 'too dead' (Note I still have the hard surface of the floor and two standard sheetrock walls)

So my first question is really about the catch 22 situation. With regular walls, you can just remove / alter panels as need to 'get it right'. But with inside out walls to be finished, you have to commit ahead of time. Any advice on how to approach this?

My hope is that I can commit to a wall covering strategy, then measure the room after. And if needed I can hang panels on top, or add wood slats, etc. (also note this room is not going to be a critical mixing room, but practice space / office)

As a baseline treatment, does using a similar approach to the ceiling make sense? Despite all my research on this project I've not found a good resource on this concept (walls specifically) - so looking for some advice before I start winging it.

Thanks!

Joe

[gullfo]

it's the same for inside-out walls - you can add panels to increase the reflectivity. so, commit to the cloth, then add panels to provide scattering and reflectivity. here's some ideas on panels. https://3dwarehouse.sketchup.com/manage ... lter=panel

[Soundman2020]

Source of the post however in discussion with an engineer friend of mine, he was concerned the room would be 'too dead'

Yes, that's correct, and that's part of the plan!

"Tuning" a room to get the desired repose isn't easy. Yes, you can plan the exact right tuning theoretically from the beginning, but reality has this nasty habit of not cooperating entirely with theory... and it's also not easy at all to predict everything. So it's better to do the tuning after the basic room is finished.
Now, there's two ways you can do that:
1) Start with a very reverberant room that is way too reflective and "bright" (mostly hard surfaces), then tone it down gradually by adding absorption and diffusion bit by bit until you get the desired result. Or...
2) Start with a very dead room, that is way too dull (mostly absorptive surfaces), then "tone it up" by adding reflective and diffusive devices, until you get it right.

Personally, I have found #2 to be easier. In fact, its pretty hard to have a room that is "too dead" across the entire spectrum. Usually what happens is that you have way too much absorption in the high end of the spectrum, about tight in the mids, too much in the low mids, and way not enough in the deep lows. That will likely be your situation too. At a rough guess, you'll probably end up with too much somewhere in the range 150 to 400 Hz, and above maybe 20 kHz. And you'll need even more below 150 Hz.

So, as Glenn said, you'll need to add reflective/diffusive surfaces at key points around the room to return some of the missing "liveness", while probably also needing to add extra absorption of some type in the low end, perhaps for a few specific stubborn frequencies.

If you look at photos of completed studios here on the forum, and also on Glenn's website, you'll see how this usually works out: Bass traps in some or all of the corners, and wood slats over the absorption where needed, possible also diffusers.

There are methods for figuring out what your room will need once it is built, and simple equations for estimating the dimensions of the devices you will need on your walls. My usual method for tuning rooms, is by using REW ( See here: How to calibrate and use REW to test and tune your room acoustics ) extensively, to test the real response of your room at key points in the process. REW will show you exactly what is happening, where the problems are, and then you can add treatment devices to deal with those problems. That's not the only way to tune a room, of course, but that's the way I have found to be most successful.

If you aren't sure how you want your live room to sound, then one good option is to tune your room sort of "generically", then use variable acoustic devices to change the response any time you need it. Variable acoustics: adjustable devices to change room response as needed

- Stuart -

[flatfive]

Thanks Both!

When filling the stud bays, beyond just filling everything with Rockwool - am I missing an opportunity to leverage the available space to get ahead of bass treatment? Say for example filling the lower third of the stud bays with Corning 703 as is commonly used in bass traps. One of reasons I went with the inside-out approach is because the room is on the narrow side, so ideally I would want to maximize this idea. For cost and simplicity of course I have no issues with the Rockwool.

Thanks for the treatment overview, I'll be diving down that rabbit hole shortly...

Joe

[Soundman2020]

There's only a small difference between the various types of acoustic insulation, in relation to low frequencies and small thicknesses. The single largest effect you can get is by making it thicker, and the second largest change is by putting it in the most effective places: corners.

Yes, some products do indeed have better coefficients of absorption for low frequencies, as compared to other products, but thin insulation won't do a lot for low frequencies, even if you use the very best stuff.

There's a basic principle here, that waves need to "see" a substantial portion of their wavelength in order to experience useful absorption. You might sometimes see claims that the insulation has to be 1/4 wavelength thick to be effective, but that isn't true. In fact, it can still be effective at 1/8 thickness of the wavelength for normally incident sound (sound that hits the insulation roughly head-on), and 1/16 thickness for sound that arrives at more or less random directions (you can get away with less depth for "random incidence", since the sound wave has to travel through a greater length of insulation if it arrives at an angle to the front face).

So, take a look on your REW data to see what the frequencies are for the lowest issues in your room, figure out the wavelength for that frequency, and multiply by 1/16. That gives you the thinnest possible insulation (regardless of type) that will have a significant effect on damping that wavelength, and it's better to go with 1/8 rather than 1/16 in most cases. So, for example, let's say you have a modal issue at 100 Hz. The wavelength for 100 Hz is about 135 inches. 1/16 of that is 8.5 inches, or 1/8 is about 17 inches. Thus, you would need insulation at least 9 inches thick, bare minimum, preferably at least 17 inches thick, to deal with that. Note that these are the MINIMUM thicknesses. More is better

That's why bass traps are deep. The lower the frequency, the thicker it has to be.

However, the smaller the room, the larger the problem! Small rooms usually have problems in the region below about 200 Hz, while for larger rooms, their problems start lower down, often outside of the audible spectrum for very large rooms. If your modal problems only occur below 20 Hz, then you don't really have a modal problem! But if your modal problems start below 300 Hz, then you have a big problem.

That's why you'll see the general recommendation that bass traps in small rooms should be at least 2 feet thick, preferably at least 3 feet. Some famous control rooms have bass traps 6 feet thick, or more.

But now for the good news: the most effective place to put a deep bass trap, is in the part of the room where it takes up the least amount of usable space: the corners. The reason is simple: If you put a 2 foot thick absorption panel that is 4 feet wide and 4 feet high, in the middle of the back wall, then it will indeed provide significant damping for low frequency issues... but only the problems related to that specific wall! In other words, it will damp modal issues along that axis of the room: front-to-back. In fact, it will have a usable effect on any modal problem that involves the back wall. But it will have no effect at all on modal issues running across the room (from side to side), or vertically (between floor and ceiling). Since the sound waves in those modes never even reach the back wall, that thick insulation pad does not affect them. You would have to put thick pads in the middle of this side walls and the middle of the ceiling and floor to deal with those. That isn't really an option.

HOWEVER! Here's the good part: If you take that same 2 foot thick, 4 foot wide, 4 foot high panel, split it in two vertically (so you now have two panels that are 2 feet wide and 4 feet high) , and move half of it over into the left rear corner and the other half into the right rear corner, then you have not changed the effect on the front-back modes: that "split" trap is still doing the same job it did before, for modes that touch the back wall.... but since the absorption is now also against the side walls, it is now ALSO dealing with modal issues that go across the room, from side to side! So the same trap is now doing double-duty: dealing with modes in both directions! Free absorption!

And if you split those two panels again, horizontally (so you have four cubes measuring 2 feet on each side) and move two parts up to the ceiling corners and the other two down to the floor corners, then you are now dealing with modes in all three axes of the room! So your same original 2 x 4 x 4 foot panel has now been split into four panels, each measuring 2 x 2 x 2, and they are sitting on the four rear "tri-corners" of the room: two up top in the ceiling-wall corners, and two down below in the floor-wall corners. They are still just as effective in absorbing modes in the original front-back direction, but now they are also dealing with modes in the left-right direction, AND the up-down direction. So they are doing triple duty.

In other words, for the same total amount of insulation that was originally in the middle of the back wall, only dealing with front-back axial modes, simply by splitting it up and moving it to the corners, you are now getting triple the amount of acoustic damping, because you are dealing with all three axes in the room.

In fact, those cubes are actually MORE effective since the act of cutting them in half, then half again, exposes more surface area to the room, which is a good thing. It turns out that you can actually slice off the protruding corners, leaving just triangular wedge shapes, without losing much effectiveness.

This is the reason why you see the general recommendation that bass traps should go in the room corners. They still do work up against the walls, yes, but the same amount of insulation is far more effective when placed in the corners. The final benefit here is that they are out of the way in corners, and don't take up the more valuable space along the walls.

In fact, you can put bass traps in the wall-ceiling corners, where they won't be in the way at all, and get a very useful effect. If you treat the rear wall corners, and the floor-ceiling corners, with triangular shaped traps that are go least 2 feet along the walls (3 feet is better) then you get pretty decent bass control in most rooms, without taking up too much wall or floor area.

- Stuart -