Low Frequency Isolation Challenges - Home Theater Forum and Systems - HomeTheaterShack.com

Old 05-25-10, 09:33 AM Thread Starter
Senior Shackster

Ted White

Join Date: May 2009
Location: Michigan
Posts: 888
Low Frequency Isolation Challenges

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.

Ted White

The Soundproofing Company
Ted White is offline

Old 05-27-10, 12:34 PM
Senior Shackster

Eric

Join Date: Dec 2009
Location: Dora, Ok
Posts: 193
Re: Low Frequency Isolation Challenges

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).
ejbragg is offline
Old 05-27-10, 12:49 PM Thread Starter
Senior Shackster

Ted White

Join Date: May 2009
Location: Michigan
Posts: 888
Re: Low Frequency Isolation Challenges

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.

Ted White

The Soundproofing Company
Ted White is offline
Old 05-27-10, 05:25 PM
Senior Shackster
spacedout

Join Date: Dec 2007
Posts: 192
Re: Low Frequency Isolation Challenges

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...

Thanks!
spacedout is offline
Old 05-27-10, 05:36 PM Thread Starter
Senior Shackster

Ted White

Join Date: May 2009
Location: Michigan
Posts: 888
Re: Low Frequency Isolation Challenges

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.

Ted White

The Soundproofing Company
Ted White is offline
Old 05-27-10, 11:59 PM
Senior Shackster
fractile

Join Date: Mar 2009
Location: San Francisco
Posts: 212
Re: Low Frequency Isolation Challenges

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)?
fractile is offline
Old 05-28-10, 07:59 AM Thread Starter
Senior Shackster

Ted White

Join Date: May 2009
Location: Michigan
Posts: 888
Re: Low Frequency Isolation Challenges

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.

Ted White

The Soundproofing Company
Ted White is offline
Old 05-29-10, 05:46 PM
Senior Shackster
spacedout

Join Date: Dec 2007
Posts: 192
Re: Low Frequency Isolation Challenges

Quote:
Ted White wrote: View Post
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.

So, in effect you're saying there's no real need to decouple a room at all? Why bother then? I'm confused...
spacedout is offline
Old 05-29-10, 07:34 PM Thread Starter
Senior Shackster

Ted White

Join Date: May 2009
Location: Michigan
Posts: 888
Re: Low Frequency Isolation Challenges

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

Ted White

The Soundproofing Company
Ted White is offline
Old 05-30-10, 06:35 AM
Senior Shackster
spacedout

Join Date: Dec 2007
Posts: 192
Re: Low Frequency Isolation Challenges

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!
spacedout is offline

 Bookmarks

 Tags challenges , frequency , isolation , low

Message:
Options

## Register Now

Random Question
Random Question #2

User Name:
OR

## Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.