Soundman2020 - Studio Design Forum

Hi,
I'm currently building a space for rehearsals with recording and mixing capabilities.
My top priority is good isolation, neighbor's house is 3 meters away, and there's a gas station 10m further
this is the plan I came up with:




the walls are hollow clay bricks, both sides covered with mortar,
cement floor
airgap without insulation
two Acustica Integral RS3/2 doors https://www.acusticaintegral.com/en/rs3-51-db/
electric and AC pipes will be entering the inner room from the ground

The aim is 50-55 db TL, we will see how this goes,
I'll be sharing the progress and results later on,

Gab

not using insulation in the air gap would be a mistake. you'll lose at least 6db of TL without it. pretty sure in PT, there are reasonably priced insulation products like rockwool which would significantly improve your isolation results. if the air lock room is only used as an air lock, then you don't really need double doors on it. also, separating the isolation booth from the main room you should add the second wall.

Hi Gab! Welcome to the forum!

Glenn beat me to it by a couple of minutes! I was about to say pretty much the same thing, about the missing insulation and the additional air gaps, but he said it all...

- Stuart -


(PS. If you'd like to understanding the technical details behind these recommendations, here's a couple of articles that go into that: What is "room-in-a-room" construction?
What is MSM? How does it work?)

Hi guys,
thank you so much for your inputs,

Glenn, such an amazing design, I hadn't thought of an airlock space for protecting the weaker isolation spot (the doors), and it's very elegant as well,

I'm gonna present this new sketch to the project design and the construction company.

A few clarifications,
- the "airlock room" is meant to be a machine room - I have some gear with fans, they're 20-30 db loud in mid high frequencies (thought a single wall with some rockwool could deal with this)
- the "iso booth" I was thinking of using it as a room with different acoustic sound, and with some attenuation from the bleed of the main room (again 20-30db) if necessary.

Your design still accomplishes these goals and has the advantages of proper isolation, thanks once again

Stuart, congratulations on the best forum for studio acoustics

I didn't address the insulation in the airgap issue, more on that later on
Gab

Concerning the airgap:

I was going to compare the TL of the system with and without insulation, but in the following formula
f0 = C [ (m1 + m2) / (m1 x m2 x d)]^0.5
the m1 & m2, mass of each leaf indicates kg/m^2. Isn't it a density unit instead of mass?
I might have the rest of my post all wrong but I'm leaving it to show what I'm trying to do, I'll edit it later

EDIT: THESE CALCULATIONS WERE WRONG, see next post

Insulation in the cavity does several things at once.
- It damps resonance of the cavity itself, specifically standing waves. Standing waves can rob you of a considerable amount of isolation.
- It damps the resonance of the MSM system itself: Without that damping, you lose considerable isolation.
(Simple demonstration of the effect of damping on resonance: find a nice floor tom, and hit it hard with a heavy drumstick. Now fill the interior with pillows / blankets / parkas / etc. Hit it just as hard with the same stick. Notice the difference! First hit = "BOOOOOOOooooommm": Second hit= "thuk").
- It lowers the frequency at which resonance occurs. This is probably the largest and most advantageous effect. The higher the resonant frequency is, the worse the isolation is. You can see this effect in your own calculations, where you show F0=155 Hz for the wall without insulation, and 111Hz for the wall with insulation. There's something wrong with your math, though, as those frequencies are way too high or solid brick walls. About ten times too high.
- It reduces the level of sound passing through the wall directly.
- It changes the properties of the air in the cavity, from adiabatic to isothermal. This refers to the way heat is dissipated by the contents of the cavity. Isothermal is a much more efficient mechanism for heat transfer.
- As a result of this change to isothermal, the apparent "stiffness" of the cavity changes. The cavity appears to be about 0.7 times less stiff, with insulation. In other words, the "spring" in the MSM equation appears to be "softer" or "springier" if you prefer, with insulation than it is with just air.
- It lowers the speed of sound inside the cavity, which in itself has several beneficial effects, but mostly increased apparent depth. An easy way of thinking of this is that the cavity "appears" deeper to the sound waves passing through it: they take longer than they would have to cross the gap (because they are going slower than they would through empty air), thus the gap appears larger from their point of view.

Overall, without insulation you could be losing as much as 20 dB of isolation, and at least 6 dB. (Some studies show a max loss of 16 dB, others suggest higher. 20 dB is a reasonable estimate).

Here's a graph that shows the difference in isolation for a wall similar to what you are proposing, with and without cavity insulation:

The curve marked "E" is for the wall with an empty cavity, as you are proposing. Rating is STC44. The curve marked "D" is for the same wall where the cavity is completely filled with fiberglass insulation. Rating is STC52. That's an 8 point improvement in isolation, and that's using the STC rating system, which doesn't even consider low frequencies, and isn't a useful indicator of studio isolation anyway, but even so shows a large improvement.

Here's another graph for a different type of wall, showing various degrees of cavity fill and the isolation the achieve, all other factors being equal.

In this case, Rw isolation increases from 45 dB with no insulation in the cavity, to 62 dB with the cavity filled with fiberglass insulation. That's a difference of 17 dB, with the only change to the wall being filled with insulation, or being empty.

the m1 & m2, mass of each leaf indicates kg/m^2. Isn't it a density unit instead of mass?

Surface density, yes. That's correct. Surface density is what you need for calculating the resonant frequency. Surface density is the amount of mass in one square unit of your wall. In other words, it takes into account the thickness of your specific wall, not just the cubic density of the material. So if you were to take an angle grinder and cut out a section of your wall measuring 1m by 1m, "surface density" is how much that section would weigh. Sometimes also called "surface mass".

Density for brick is around 2100 kg/m2 (rule of thumb: different bricks could be a bit higher or lower. Clay brick is probably a bit lower, maybe 1800 or so, but I don't have the exact figure on hand). Thus, your surface density (assuming your bricks are laid running-bond, one layer thick), where you said your two types of bricks are 0.146 and 0.220 m thick, would be 2100 x 0.146 = 307 kg/m2, and 2100 * 0.22 = 462 kg/m2, respectively, for normal brick. Or assuming a figure of 1850 for your clay bricks: 1850 x 0.146 = 270 kg/m2, and 1850 * 0.22 = 407 kg/m2,

Not sure why you are using 94.5 in your calculations. I don't see where that came from.

I didn't go through all your math, but something seems to be off somewhere. According to my MSM calculator, the resonant frequency without insulation would be around 11 Hz, and with insulation it would be around 8 Hz. Total isolation (with insulation) should be about 62 dB. (theoretically)

- Stuart -

Inner wall
Surface Density(M) = 270 kg/m2
TL = 14.5 log (M * 0.205) + 23 dB = 14.5 log (270*0.205) + 23 dB = 14.5 log (55.35) + 23 dB = 14.5 x 1.743 + 23 dB = 48 dB

Outer wall
Surface Density(M) = 407 kg/m2
TL = 14.5 log (M * 0.205) + 23 dB = 14.5 log (407*0.205) + 23 dB = 14.5 log (83,435) + 23 dB = 14.5 x 1.92 + 23 dB = 51 dB

Resonant Frequency of the MSM System
without insulation:
f0 = C [ (m1 + m2) / (m1 x m2 x d)]^0.5 = 2650 ((270+407) / (270X407x0.2))^0.5 = 2650 x ((677) / (21978))^0.5 = 2650 x (0.03080)^0.5 = 81.62^0.5 = 9Hz
with insulation:
f0 = C [ (m1 + m2) / (m1 x m2 x d)]^0.5 = 1900 ((270+407) / (270X407x0.2))^0.5 = 7.6Hz

f1 is 55/d Hz = 55/0.2m = 275 Hz

Stuart,
thanks for the explanation, I was going to ask about the other benefits of an insulated cavity beside sound transmission

Also, does it impact the properties of the acoustic inside the room? (modes, resonances, decays)

tripalot wrote:Source of the post I was going to ask about the other benefits of an insulated cavity beside sound transmission

Well, it does also help to keep you warm in winter! (And cool in summer).
Jokes aside, that's another big benefit to completely filling a large cavity with insulation: thermal insulation. So you are better isolated from outside temperatures. And since both of your leaves have to also be hermetically sealed (totally air tight: not even a single crack anywhere) for proper acoustic isolation, you also have zero air infiltration through the wall itself. Unlike typical house walls, which have numerous tiny air leaks, your studio walls have none at all, so you get that benefit too: no air moving through the wall.

Also, does it impact the properties of the acoustic inside the room? (modes, resonances, decays)

Very, very slight. I am NOT a follower of the Philip Newell theory on how this works, because research shows that it doesn't actually work they way he imagined it did, all those decades ago. His theory was that you could deal with room modes by having an outer shell that was asymmetric and non-parallel with a thick, massive outer leaf, then a thinner, less massive inner-leaf to "let the sound through" to the outer leaf, where it would be influenced in some unexplained way, by the non-parallel surfaces that also had major differences in dimensions, somehow "destroying" the modes. It doesn't work like that in the real world. Everest has shown, convincingly, that making a room non-parallel and/or asymmetric does not destroy the modal issues in the way that Newell imagined. Rather, it just moves the modes to different frequencies and makes them harder to predict. The famous Wyle report from way back in 1973 already explained the theory behind MSM isolation, and the conclusion is clear: varying cavity depth does not improve isolation, but rather makes it worse, and greatly varying the mass of the leaves also does not improve isolation. Their conclusion was simple: for a 2-leaf MSM wall, you get maximum isolation when the total mass of the entire wall is evenly split between the two leaves, and the cavity depth is as large as possible. In other words, if you follow Newell's ideas and make the outer leaf non-parallel and asymmetric, then you are making the cavity smaller in some places than it could be. Wyle shows that the best goal is to make it as large as possible everywhere (and fill it with insulation!). This is the same reason why tilting the panes of glass in a window between the control room and live room is also a bad idea, acoustically: It reduces isolation, because the gap is narrower on one edge of the window than it would have been if the glass panes were parallel. (There might be visual reasons for wanting to tilt the glass a bit, such as light glare or reflections, but acoustically it's a bad idea.) The simple equations for calculating MSM isolation are based on these assumptions too: constant cavity depth, and rough equal split of the mass between the two leaves (in other words, m1 ~= m2). In your case, you have about 60% of the mass on the outer leaf, 40% on the inner leaf: that's fine. Close enough. Your actual isolation will be a bit less than theoretical prediction, but no biggie. If you had a Newell-style wall, with more like 80% - 20%, that would be a problem. You also have high mass leaves anyway, which is good any way you look at it!

For all real purposes, you can consider the interior surface of the room (the surface you would see when you stand inside the completed room, before treatment or furniture), as the acoustic boundary of the room. That's the dimensions you use for modal calculations, ignoring the outer leaf completely. The effect the outer leaf has is insignificant. So use the inner leaf surface for all your calculations, and all your acoustic treatment design.

By the way, your math looks fine now, but your graph seems to be off. You should be getting much more than 70 dB for those mids and highs! For most real-world acosutic barriers, isolation rises steadily with frequency, roughly following 2-leaf mass law above the cavity resonance region (R1 + R2 + 6), then again above the coincidence dip. (I would expect to see a coincidence dip in your graph somewhere, too). Yes I realize that you are using a very simplified equation to produce that graph, but if you wanted to be more accurate, you could add in additional parameters to get it closer to the real world. (Yes, I know it's not easy to do! Ask me: I've been trying to create a spreadsheet that produces an accurate isolation graph, for about 20 years now... )


- Stuart -

Hello,

I have been away for some time and meanwhile the construction is almost finished,
I was unable to change the original design due to the added costs (the budget was negotiated 3 years ago and now everything is much more expensive), but managed to fill the cavity with 5 kPa.s/m2 mineral wool,

Doors aren't installed yet but the next step is floor and inner ceiling, and that's what I would like to ask you about:

COVERING THE ENTIRE CEILING WITH 40CM 5 KPA.S/M2 MINERAL WOOL

that was my idea but I've just read Stuart's posts on another Topic:

Clouds are good. Deep clouds are better. Angled clouds are better still (higher above your head, lower towards the speakers), and hard-backed clouds are even better yet!

eightamrock wrote:
Source of the post When you mention hard back, do you mean something like plywood or MDF as the backing material of the cloud? so from the "top" down I would have my ceiling -> an air gap -> plywood/mdf -> rockwool -> fabric wrap.
Right! That's the idea. And after you have angled it, there will probably be space on top of the hard-back where you could put in some more insulation, just resting there. Helps a little with a few minor issues up there.

Also, any time you have insulation hanging overhead, it's a good idea to have some thin plastic below that (between the insulation and the fabric), for two reasons: 1) In case the insulation starts shedding fibers over time, so they don't get into your gear (or you!), and 2) It helps to avoid over-absorbing the high end, since the plastic will reflect back some of the highs, at frequencies above a certain point that is governed by the thickness of the plastic.

The second reason above also applies to other absorption you have in the room (rear wall bass traps, for example). You can sort of "tune" the absorption range a little like that, by using the right thickness of plastic in the right locations. If you figure that the plastic would have to be crazy thick to target the frequencies you want, then use wood slats instead, across the front of the fabric. The width of the slats is also a factor in what frequency range they reflect best.

I need to figure out the ceiling so I can install the lights and start taking measurements of the room's frequency response

cloud versus simply mounting absorption on the ceiling will be more a function of ceiling height and any significant modal issues for your listening spot. regardless you will to use ICW light boxes or equiv when you have lights in contact with insulation - even LED lights can run hot.