# Low Frequency Isolation Challenges



## Ted White

Since we're all involved with rooms generating a great deal of sound, I though some here might find this interesting. Generally the most difficult aspect of high level isolation is controlling the low frequencies (bass). Keep in mind that STC doesn't measure bass, as it does not consider frequencies below 125Hz, and we're obviously dealing with rooms that put out a great deal of sound below that. Generally construction efforts to reduce low frequencies will naturally take care of the lower energy high frequencies.

In short, every compressible cavity (such as air cavities in walls and ceilings) will define a specific resonance point (frequency) in a decoupled system. If we have a double stud wall, or ceiling with clips and channel, then we have a decoupled system. Think of this decoupled system as a spring that oscillates. This system will have a calculable low frequency resonance point, defined by the Mass-Air (spring)-Mass parameters. Let's say this resonance point is 70Hz. 

At 70Hz, we don't stop a lot of sound, since resonance allows that frequency to pass fairly easily. At 100Hz, we're doing much better, but as we start looking at frequencies lower than 100Hz, Transmission Loss gets worse and worse until we hit 70Hz rock bottom. So at resonance (70Hz), and just above resonance (70-100Hz) things are not great for our sound isolation. Generally the math is from the resonance point up to around 1.5X the resonance point we don't do as well in sound isolation. 

If we could move that resonance point from 70Hz. to 40Hz. we would be much better off:

Scenario #1 has 70Hz resonance point, and weakness from 70Hz through 105Hz. (70 x 1.5= 105).

Scenario #2 has a 40Hz. resonance point, and a weakness from 40Hz through 60Hz. (40 x 1.5= 60).

This is why we spend time looking to incorporate methods to lower that LF resonance point as much as possible. How do we accomplish this? Keeping in mind that a decoupled system is a spring system:

We can add absorption in the form of simple (standard thermal) insulation. This will lower the resonance point (frequency) of the system a bit.

We can add mass to the system. This essentially weighs down our spring system, slowing the oscillation = lowering the resonance. The added mass is more effective than the insulation.

We can add cavity depth to the system. For the same reason that insulation helps, so does more air in the cavity. This also isn't as effective as adding the mass.

So again, if we can progressively march that low frequency point down, we minimize the frequencies that will display weakness.

Hope this helps.


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## ejbragg

Thanks, Ted.

Some years back, I was in a rock band with some buddies - all of us were in the military. To have a place to practice, we rented a house in some (nice & quiet) neighborhood. We used the living room as our practice room, assured by our landlord that this would be just fine. Instantly, we made enemies of everyone within half a block! In attempts to fix the problem, we took turns standing outside while each other played, but only the bass and kick drum were noticeable. We tried to "absorb" the sound by stuffing foam in the windows, but nothing seemed to work. We eventually got evicted!

This is the same scenario (is it not) for people attempting to isolate sound between rooms? We have all the practical theory laid out, but how do we apply it? What, for example, would you suggest a practicing band do in such a scenario, and how would it apply to a studio? (For commercial studios, I think we should assume that we hold nothing back in construction needs, since a budget approach isn't necessarily going to work for the real thing). The walls, by the way, were brick on the outside and plaster on the inside - 4" cavities, new construction (in England). The windows were double paned, but thin glass. Our practice level was about 110 dB (thanks to drums and Marshall stack, everyone else had to crank it up).


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## Ted White

That's the scenario, all right. Not all studios are going to be built with unlimited budgets.

You can build a very well isolated room at a reasonable cost if you follow the following basic, tried and true methodology.

#1 Decouple the framing. This can be done with staggered stud or double stud walls. To decouple the ceiling, consider clips&channel. Resilient Channel (RC-1) attempts to decouple, however there is no industry standard or specification for its construction, so I’d be concerned about using it.

#2 Install absorption in the cavities. This means standard fiberglass R13 in the walls, R19 in the ceiling. Know that there is no data that supports that any other insulation (including the “acoustic” labeled, and recycled cotton) works better. Also, foam (open or closed cell) is superior for thermal, but distinctly worse for acoustic. Use the cheapest fiberglass you can find.

#3 Add mass. Nothing better than standard 5/8” TypeX. Great mass at 70+ pounds a board, and cheap at $7 a sheet. Use two layers. Only mud and tape the final layer.

#4 Consider damping these drywall panels with one of several field-applied damping compounds. Some work better than others, and independent lab data shows you get what you pay for here. 

After that, you’d turn your attention to the ventilation, lights and doors. All of these are flanking paths for sound to get out of the formidable room you just built. They can be dealt with fairly easily, but you’ll want to design this in.


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## spacedout

This is really interesting, Ted.

There's something I don't quite understand, though: given that resonance occurs (by definition?) in a decoupled system, and increasing mass will reduce the resonant frequency (which I understand), why are rooms decoupled in the first place? Surely coupling to the ground would be introducing nearly infinite mass, which would then result in nearly no resonance at all?

I'm sure I'm missing something basic here, I'm just not sure what... :dontknow:

Thanks!


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## Ted White

Great question. Resonance actually occurs when any two (or more) mass layers (leaves) are on either side of a compressible layer(s). A spring system is created. So could be coupled or decoupled framing with a compressible cavity. Both framing types will have cavity resonance. The air is most commonly the spring mechanism. 

However a compressible layer is also created when you lay down a rubber underlayment. While no air cavity exists, it is compressible nonetheless, and a corresponding resonance is created. Side point there, I guess, but I always found that intresting. Sorry.

Anyway, the area of resonance is defined by those two mass leaves on either side of the spring. The spring is a spring despite its attachment to the slab or earth.


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## fractile

A question: Would wall board absorb low frequency better than particle board due to higher mass, or equal thickness particle board for its lesser resonance (if I'm thinking right)?


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## Ted White

Hi fractile,

The sound loss (called Transmission Loss) is guided partially by the mass of those outer layers (leaves). What the leaves are made of is not really relevant. If the outer leaf weighs 4 pounds per square foot, it doesn't matter if the 4 pounds comes from dense cement board or low density foam. The denser materials generally allow you to pack in more mass per inch, and that's great.

So it's not that the outer layers are actually "absorbing" anything, rather it is the complete wall system that is doing the isolating.


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## spacedout

Ted White said:


> Great question. Resonance actually occurs when any two (or more) mass layers (leaves) are on either side of a compressible layer(s). A spring system is created. So could be coupled or decoupled framing with a compressible cavity. Both framing types will have cavity resonance. The air is most commonly the spring mechanism.
> 
> However a compressible layer is also created when you lay down a rubber underlayment. While no air cavity exists, it is compressible nonetheless, and a corresponding resonance is created. Side point there, I guess, but I always found that intresting. Sorry.
> 
> Anyway, the area of resonance is defined by those two mass leaves on either side of the spring. The spring is a spring despite its attachment to the slab or earth.


Nothing to be sorry about :T

So, in effect you're saying there's no real need to decouple a room at all? Why bother then? I'm confused... :dontknow:


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## Ted White

The Decoupling is significant and desired. Decoupled walls generally have lower resonance points. This means better low frequency isolation
_Posted via Mobile Device_


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## spacedout

I see... I think! Is that because the entire mass of the room is vibrating as one so that the room is then resonating as a unit, rather than (I assume) separate parts of the room (eg walls) resonating in relative isolation?

Just trying to understand this... thanks!


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## jaddie

Ted White said:


> Hi fractile,
> 
> The sound loss (called Transmission Loss) is guided partially by the mass of those outer layers (leaves). What the leaves are made of is not really relevant. If the outer leaf weighs 4 pounds per square foot, it doesn't matter if the 4 pounds comes from dense cement board or low density foam. The denser materials generally allow you to pack in more mass per inch, and that's great.
> 
> So it's not that the outer layers are actually "absorbing" anything, rather it is the complete wall system that is doing the isolating.


How would you feel about making the two leaves different in mass so that the leaves themselves are not tuned to the same resonance? Something like Quietrock 545THX for the inner leaf, 2x 5/8" for the outer leaf? Would that not broaden the "Q" of the total wall resonance and improve LF isolation? OK, this is probably a "cost no object" wall, but for the DIY-er, the inner leaf might be 3x 5/8" instead, and assume insulation in all cases. How do you feel about that?

Jim


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## Ted White

spacedout said:


> Is that because the entire mass of the room is vibrating as one so that the room is then resonating as a unit, rather than (I assume) separate parts of the room (eg walls) resonating in relative isolation?
> 
> Just trying to understand this... thanks!


I appreciate the interest. The wall and ceiling structures each have a new resonance as defined by the mass, the cavity depth, etc.


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## Ted White

jaddie said:


> How would you feel about making the two leaves different in mass so that the leaves themselves are not tuned to the same resonance?
> 
> Would that not broaden the "Q" of the total wall resonance and improve LF isolation?
> 
> Jim


What a great question. Thanks for that.

Partitions having resonating panels is maybe best dealt with by deploying a specific damping material. This removes the need to have dissimilar thickness in builds. It used to be more common to build walls from differeing thicknesses of drywall, for example 1/2" + 5/8". Each thickness has a different coincidence point (resonance). These days we can use a damping compound that directly addresses resonance, so we can use more mass, even if the same thickness.


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## jaddie

Do you use any of the "exotic" drywall products, or can you achieve equal or better results with standard materials? The Quietrock folks promote the exotics as a cost saving measure. It increases material cost, but cuts labor because it's one sheet instead of several. I've never used their high-end 545THX, but I have a sample...pretty serious stuff. Thoughts?

As an afterthought, I've used a high performance sheet product in only one project, but the client ended up penetrating it so many times (including a non-sealing door!) that I doubt we gained much by using it.

Jim


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## Ted White

Any pre-damped drywall (there are 4 manufacturers) is simply layers of standard board that you can buy at Home Depot (drywall or cement underlayment). Between those boards they sandwich some type of damping compound. There is no mystery to the materials. So the decision to use these pre-damped boards comes down to:

Price (less is better)
Mass (more is better)
Damping (more is better)

Generally you will always be able to field assemble a more massive, more damped and significantly less expensive wall or ceiling. An additional plus is that field assembly will allow you to overlap seams between the drywall layers.

Regarding the labor savings arguement, national labor (while they definately vary) costs to install drywall is less than $0.20 a square foot. So for $6 you can hang a sheet of 5/8" drywall that cost you $7. Considering that the average dywall job has 11% waste, You might not want to fill the dumpster with $70 specialty drywall scraps.


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## ejbragg

Ted White said:


> Hi fractile,
> 
> The sound loss (called Transmission Loss) is guided partially by the mass of those outer layers (leaves). What the leaves are made of is not really relevant. If the outer leaf weighs 4 pounds per square foot, it doesn't matter if the 4 pounds comes from dense cement board or low density foam. The denser materials generally allow you to pack in more mass per inch, and that's great.
> 
> So it's not that the outer layers are actually "absorbing" anything, rather it is the complete wall system that is doing the isolating.


This is a good point. But I'd like to point out a deeper distinction for those who are still learning about soundproofing walls...

There is a greater resistance to sound penetration at a boundary where the transition in mass is greatest. At the boundary where air meets sheetrock, there is a change from low density to high density. Because sheetrock is very dense, most sound striking its surface will reflect (provided the wavelengths are not too long). Some will still penetrate and travel through to the airspace behind the drywall. Where the air exists behind the wall surface, there is another change from high density to low density. At this point, there will be another reflection of sound, most going back into the sheetrock, but some escaping into the air cavity inside the wall. This boundary effect takes place again at the opposite side of the wall. Because insulation inside the air cavity is not very dense, it's effect is not the same - there is no "boundary effect", per se, for absorptive material.

So for the boundary effect, one can see that the denser the material, the better the reflection. In fact, one could logically surmise that for the same overall weight, a thinner, more dense wall would be superior to a thicker, less dense wall, even if the two walls weighed the same.

But then comes the low frequencies, messing up the whole affair. Low frequencies tend to travel through things that are thinner than their wavelengths. This is in fact, the very reason we use 2 walls instead of one. Using two walls that are airtight from the rooms on either side, we give the wavelength the illusion that one deep wall exists in its path. Because insulation alone is not dense enough to significantly detract energy from a sound wave, when the cavity is surrounded by dense walls, what sound _does_ penetrate, once it begins to reflect back and forth within the wall, the absorbent material is able to destroy the energy quite efficiently, counteracting the "spring-like" activity Ted is referring to.

So although it is the overall wall mass that must be ultimately considered, it should also have most of its density concentrated on the outer layers - but not too little (and not too much) density in the cavities.
................

And as for using medium or heavy particle board, yes, you _can_ get better _boundary_ results. However, this material is also very stiff. Drywall is less rigid and is a little better at absorbing resonances than the harder materials. And because it's so much cheaper, it still remains the ideal overall construction material. If you really have an itch to spend the extra cash and go all out, you can consider adding a layer of "mass loaded vinyl" between the sheetrock layers. Most of this stuff is made from sand and pvc. Sheetblok is one brand, or BlockAid and Quiet Barrier, to name a couple others. These materials can be very dense (about 1 pound / sq ft), at about 1/16 inch thick, and is a limp material, which helps eliminate vibration and resonance. The cheaper versions are a bit less dense, but certainly massive, nontheless. Note that this material is best for use as part of the wall barrier, not as cavity filler!


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## Ted White

Thanks for this great contribution!



ejbragg said:


> ... one could logically surmise that for the same overall weight, a thinner, more dense wall would be superior to a thicker, less dense wall, even if the two walls weighed the same.


I would agree. Panel thickness & density partially define the panel's resonance point. The flex can contribute to the overall system's resonance point. Resonance and more resonance... We chase it everywhere.

We use damping material to subdue panel resonance. We introduce insulation to help with cavity resonance. And we build massive decoupled walls to set the low frequency resonance point as low as possible. 

All to limit the extreme outer surface from moving and re-creating sound waves...


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## ejbragg

I'd like to revisit my question pertaining to the "band in the neighborhood" scenario. Here is a question that I at one point thought I knew the answer to, but after reading further, discovered that the first glance at this problem is not necessarily enough.

Going back to that house, what you're suggesting is ... I'm assuming ... one of the two, and I'd like to go into some detail into each of the following two scenarios to comtemplate which is the better method and why:

1. Leave inside wall as is, and build a new frame inside the house, say, about 6 inches inside the existing wall, and add a couple layers of well-sealed drywall, filling the extra space with insulation. (There will be three layers of wall: Outside brick and stone, a new wall in the music room, both of which will now sandwich the original wall - both cavities with insulation, or maybe only one cavity with insulation..) 

2. Rip out the original interior wall, build the frame out to the same distance from the brick wall as in method 1, and add a couple layers of well-sealed drywall, filling - or half filling - the cavity with insulation. (There will only be two layers of wall: Outside brick and the new wall.

Assume both methods 1 and 2 are the same gross thickness. One merely has an extra layer of wall (and more mass, accordingly).

Which (in your opinions) of the two methods is superior, and why?


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## Ted White

Thanks so much for the interest. OK to clarify, Scenario #1 has two air cavities. Scenario #2 has one air cavity.

The better solution is #2, even though it is the same overall thickness but less mass. The reason is the second air cavity created with scenario #1. We can insulate walls to reduce cavity resonance, but not eliminate it. The two air cavities will therefore resonate and reinforce each other. It is entirely possible that if we test the original wall as-is we get an STC of 40 (whatever). Then proceed with option #1. Test again, and get an STC of 42. Not much improvement considering all hat work.

Now we test scenario #2 and find we have an STC of 55. Huge improvement. Even with less mass. The low frequencies (below the 125Hz STC cutoff) are even more improved. The lack of a second air cavity helps us significantly. Scenario #1 creates what is often called a Triple Leaf. More details with actual lab test data on this can be found here: http://www.soundproofingcompany.com/library/articles/triple_leaf_effect/


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## ejbragg

That, sir, is the crucial information I was looking for!

Thanks, Ted.

I hope that all of you who weren't aware of this phenomenon will take note! The magic is in the depth of the cavity, _first_! Surface density is important as well, but as long as you're at least using drywall or something along those lines, overall wall thickness is where it's at.


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## Ted White

I'm so glad we're talking about this. It's interesting (to me, anyway) and a great topic for the forum.

Thanks for the interest!


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## spacedout

It's hugely interesting to me as well 

All the reading I've done about studio acoustics so far has centred around controlling reflections within the room, which seems to get a lot more attention - presumably because it's noticed more by the people in the room. Soundproofing seems to be a different kettle of fish altogether - thanks for bringing it up!


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## Ted White

Thanks for the interest!


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