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post #1441 of 1854 Old 07-10-2015, 08:03 AM
 
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Originally Posted by doublewing11 View Post
Rope and pully a tree? <chuckling>

That picture is an old growth Douglas Fir in McKensie National Forest...........probably 200+ years old. We had/cut a stand/patch that survived the Tillamook fires back in Depression era.............those trees were even bigger than pictured! Helped my brother-in-law fall a few of those......and yeah, I almost pee-ed my pants!

And no.....I'm not a logger by trade.......but I do participate in family business of trees.
We used one of those come-along pulley things, but the constant one that uses centrifugal force to bit the rope and tension it. Hooked it to another big tree and cranked it good to make it fall in the direction we wanted. You can't see it in the picture but my house is right off to the left, it would have easily crushed it if it went wrong.

There is a practical limit to what you can do with a rope though... I was about near it.
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post #1442 of 1854 Old 07-10-2015, 08:10 AM
 
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On the MDF topic,

This 1.5" is really great stuff. It's much better damped and stronger than normal 3/4" so you can totally relax on the bracing scheme. A basic cross brace is more than sufficient. Also- with a full 1.5" of material to glue at a glue joint it makes for a significantly stronger joint- especially if you use a strong construction adhesive like PL Premium.

The side effect of less cabinet resonance, less wall flex, and less movement of the sub box is increased output. Not that Tim will actually need it though...

It's the same reason why a single layer of drywall on basic stud wall is acoustically superior and more desirable for bass nodes and room nulls and standing wave problems as compared to our traditional 2 or 3 layer damped GG/DW combos we use. The thicker/stronger wall causes increased standing wave problems in bass response by being a bunker- where as a normal weaker wall would flex a little and diminish it. That's why when you sound proof it's almost a necessity to consider bass traps and bass performance, or hire a pro to help if you don't understand enough.
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post #1443 of 1854 Old 07-10-2015, 08:13 AM
 
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That is a big tree ^

Cutting that with a hand powered saw ???? Seems impossible to me, but I know I'm wrong.
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post #1444 of 1854 Old 07-10-2015, 08:16 AM
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How long doesn't take to grow a tree that size? Down this way we usually harvest pi e every 15 years, I think.

And yes, my first thought was, "I wonder if that's DW?"
Depends on region of state...........Valsetz, Oregon has "Valley of Giants" which are close to 400-500+ year old Fir and are even larger than tree shown in picture!

I can show you stands which started after Tillamook fires which are technically considered "old growth" and are just as large as tree shown..........Trees are close to 90 years old................. 212 inches of rain and great soil sure helps!

Right now we harvest every 40-45 years and replant immediately............Yup, Stewarts of the land. Butts on those harvestable trees are close to three feet diameter.

I
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post #1445 of 1854 Old 07-10-2015, 08:17 AM
 
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Awesome. If u remember my doors core was suppose to be 1.5 inch mdf but we decided to go with two layer of 3/4 to get more green glue and a bit more mass but this was dave's first suggestion before he knew we were doing green glue too. He said the ultra fine mdf like urs is awesome!
The ultralight is supposed to be healthier, and it's lighter weight (20%) which matters when you start getting 1.5" thick sheet goods. Also- it's easier to cut and easier on blades.

I told Tim I'd chop it all up for him if he bought be a blade My Diablo is getting tired finally, I have a stack of TS blades I should consider sharpening. I never do, I always just buy another one.

My TS has a 42" rip, so it would work well for his box size. The only dimension I would not be able to cut is the 48" dimension, which all the sheets are already stock 48" from the factory anyways so no need to cut it. I also have a Festool REQ55 track saw that's awesome, but the blades for that are like $110 each
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post #1446 of 1854 Old 07-10-2015, 08:21 AM
 
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Originally Posted by doublewing11 View Post
Depends on region of state...........Valsetz, Oregon has "Valley of Giants" which are close to 400-500+ year old Fir and are even larger than tree shown in picture!

I can show you stands which started after Tillamook fires which are technically considered "old growth" and are just as large as tree shown..........Trees are close to 90 years old................. 212 inches of rain and great soil sure helps!

Right now we harvest every 40-45 years and replant immediately............Yup, Stewarts of the land. Butts on those harvestable trees are close to three feet diameter.

I
Tree huggers never made any sense to me. Lumber and trees is a renewable resource. Easily renewable actually. Next time you fly, look out the window and pay attention to how many trees there are. They are literally everywhere. With today's intelligence and emphasis on replanting, there is plenty of land to harvest and have new growth by the time it's needed. Perhaps on a small area scale there could be a problem, but in the big picture probably not. Replacing wood with another material in all the purposes it's used would likely be a lot worse for the environment, and require a non renewable resource to be consumed.
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post #1447 of 1854 Old 07-10-2015, 10:36 AM
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Originally Posted by Mfusick View Post
Tree huggers never made any sense to me. Lumber and trees is a renewable resource. Easily renewable actually. Next time you fly, look out the window and pay attention to how many trees there are. They are literally everywhere. With today's intelligence and emphasis on replanting, there is plenty of land to harvest and have new growth by the time it's needed. Perhaps on a small area scale there could be a problem, but in the big picture probably not. Replacing wood with another material in all the purposes it's used would likely be a lot worse for the environment, and require a non renewable resource to be consumed.

And you forgot about Spotted owl.......and other land owners.

For example, 10 years ago or more a Spotted Owl nested not far from a 64+ acre unit we wanted to log..........a Fish and Game Official said we couldn't due to moritorium stating no logging with in 1 mile of Spotted Owl nest. The following year Owl left and had to wait another 6-7 years for permission to log. If we had to wait much longer, trees would have been too large for most local Mills to handle.......would have had to ship long distance to Washington State which still has Mills accepting larger logs.

Another bone of contention........we have a stip of land bordering a county road which has 200 + feet on one side of road with marketable timber. A guy who built a large beautiful home bordering strip ( think Sunset Magazine home) was really upset with our plans logging due to not wanting a clearcut in his backyard........he got a lawyer, raised questions about erosion to his property and county road which County put a stop to our plans! Absolutely total BS! Three hundred yards up the road we logged right up to County road, replanted with no erosion issues whatsoever!

Thing that pisses partnership off........we sold the land guy built home on on the cheap........and comes back and does this?

Treehuggers are the least of our problems!
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post #1448 of 1854 Old 07-10-2015, 10:54 AM - Thread Starter
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Wow, this turned into a National Geographic blog real quick...didn't see that coming.

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It's the same reason why a single layer of drywall on basic stud wall is acoustically superior and more desirable for bass nodes and room nulls and standing wave problems as compared to our traditional 2 or 3 layer damped GG/DW combos we use. The thicker/stronger wall causes increased standing wave problems in bass response by being a bunker- where as a normal weaker wall would flex a little and diminish it. That's why when you sound proof it's almost a necessity to consider bass traps and bass performance, or hire a pro to help if you don't understand enough.
Exactly the reason why I decided to go with 2 damped layers instead of 3 and a full hanging ceiling isolation system vs. rigid clips and channel. The allowance for a slight amount of flex makes a considerable difference.

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post #1449 of 1854 Old 07-10-2015, 11:00 AM
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Wow, this turned into a National Geographic blog real quick...didn't see that coming.



Exactly the reason why I decided to go with 2 damped layers instead of 3 and a full hanging ceiling isolation system vs. rigid clips and channel. The allowance for a slight amount of flex makes a considerable difference.

And by the way...kick butt theater progress pictures will be posted on Sunday night. I just wanted to give myself the weekend to finish a few things up I've been tinkering with during my few minutes here and there throughout the week.

LOL.......

Mikey got me started........back to regulary scheduled programming!
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post #1450 of 1854 Old 07-10-2015, 01:41 PM
 
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I got plenty of logging to do in my own yard. I should have paid someone the $3500 they wanted and quoted.

"My back hurts"

And- it's silly HOT here. Humidity is about a million percent.
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post #1451 of 1854 Old 07-10-2015, 02:27 PM
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Thanks for the suggestions. I had called that company as well. HERE are all their foam cutting tools, including that planer. They said they sell exclusively through distribution and have no visibility to buyers or end users. I called every single tool rental place in the area and all current spray foam companies. Sadly, I will have to go this alone.

Electric chain saw would probably work to some extent. The caution that was given on various websites is that tools not designed for cutting foam may collect the foam dust in areas where it shouldn't be within the tool and do damage. If this curry brush thing doesn't work out, I'm going to open up the arsenal to any tool. Finesse will be set aside.
I have this memory of using an "offset" flush cutting sawzall blade that would work perfectly in your case. It looked something like this: http://www.grainger.com/product/PAWS...-Adapter-1PMG4

It wasn't an adapter, though -- it was a blade that was designed to be offset.

But now I can't find any reference to such a blade online and am now wondering if I dreamt it all. Yes, I do have dreams of tools.
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post #1452 of 1854 Old 07-10-2015, 05:48 PM
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Exactly the reason why I decided to go with 2 damped layers instead of 3 and a full hanging ceiling isolation system vs. rigid clips and channel. The allowance for a slight amount of flex makes a considerable difference.
Has anyone seen numbers on this? I understand the argument - but here's where I think there might be a problem. This is all based on my lay-person's understanding from reading here and around the internet... so there's your caveat. I ask for the purpose of discussion, not contradiction. Also, caveat number 2: all of the wave behaviors we talk about, especially those around absorption, reflection, and transmission exist on a spectrum: there will always be some amount of energy through each possible pathway.

In general, I think of a wall as acoustically opaque for frequencies above the wall's resonant frequency. Increasing the wall's mass drives down the resonant frequency, and thereby broadens the range over which it is opaque. When the wall is opaque - it is reflective. So the questions become, 1) At what frequency does your wall become reflective and 2) What frequency range includes the modal resonances you hope to allow the flexing wall to absorb and transmit instead of reflect?

My hunch is that the difference in frequency for two layers vs three layers is below the onset of room gain for most rooms. So, largely, by forgoing the third layer a builder loses some room gain and loses sound containment at very low frequencies. Meanwhile, in the range where modal control is a concern, the wall performs the same.

So, who has some numbers?
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post #1453 of 1854 Old 07-10-2015, 08:41 PM - Thread Starter
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Has anyone seen numbers on this? I understand the argument - but here's where I think there might be a problem. This is all based on my lay-person's understanding from reading here and around the internet... so there's your caveat. I ask for the purpose of discussion, not contradiction. Also, caveat number 2: all of the wave behaviors we talk about, especially those around absorption, reflection, and transmission exist on a spectrum: there will always be some amount of energy through each possible pathway.

In general, I think of a wall as acoustically opaque for frequencies above the wall's resonant frequency. Increasing the wall's mass drives down the resonant frequency, and thereby broadens the range over which it is opaque. When the wall is opaque - it is reflective. So the questions become, 1) At what frequency does your wall become reflective and 2) What frequency range includes the modal resonances you hope to allow the flexing wall to absorb and transmit instead of reflect?

My hunch is that the difference in frequency for two layers vs three layers is below the onset of room gain for most rooms. So, largely, by forgoing the third layer a builder loses some room gain and loses sound containment at very low frequencies. Meanwhile, in the range where modal control is a concern, the wall performs the same.

So, who has some numbers?
Insightful post, as always. It took me a while to find it, but I recalled @Nyal Mellor had mentioned something about this recently. Here is the post, but perhaps he can offer additional comment.

I'll give a crack at trying to explain in my own inept way.... When the wall has less mass and is less rigid, the acoustic energy has a greater effect in moving (flexing / vibrating) the wall, converting this energy into movement. The resonant frequency of this wall becomes much lower than we can hear. When you have more mass and a more rigid structure, the amount of energy translated into flexing the wall is far less and more of the energy is therefore reflected back into the room where it continues to bounce around until absorbed. Each 'bounce' reduces in amplitude and eventually diminishes, but the speed at which this happens is largely based on your room's acoustic treatments. But until this acoustic energy diminishes, it is interacting with the other audio energy in the room. That's why it's imperative that this energy be absorbed as quickly as possible.

Hopefully one of the illuminati can comment further.
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post #1454 of 1854 Old 07-10-2015, 09:31 PM
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Your remarks and Nyal's both point to something I'm not sure how to deal with: flexure vs stiffness. My general thoughts about flexure is that it functions to delay the reflection (and thereby lower the frequency of the resonance of the room mode) and probably allow greater absorption when the wall is internally damped. How much? And how does that change transmission?
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post #1455 of 1854 Old 07-11-2015, 05:29 AM - Thread Starter
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Your remarks and Nyal's both point to something I'm not sure how to deal with: flexure vs stiffness. My general thoughts about flexure is that it functions to delay the reflection (and thereby lower the frequency of the resonance of the room mode) and probably allow greater absorption when the wall is internally damped. How much? And how does that change transmission?
Make sure your morning coffee is in-hand. Some very good reading ahead....

I worked the Google on the interwebs this morning and found this excellent introductory article:
Practical Soundproofing

The equation you are looking for is calculating the Sound Reduction Index (SRI). There's a ton of very useful information where mass, stiffness and damping are all correlated as part of the equation, along with any air gaps as part of the 'system' to reduce noise.
Fundamentals of Noise and Vibration

Here is another book with GREAT information on the topic, including the effect of adding mass. Start at page 161 of this link and continue through page 167:
Constructing the Future: ND Modeling

A quote from this book:
Quote:
In practice increasing the thickness h can lead to some undesirable effects on the sound reduction index due to coincidence, and careful considerations are needed.
In the end it looks like managing and optimizing the wall's impedance through a careful balance of mass, damping and stiffness is what we are trying to optimize with these different wall assemblies.

Something else to investigate further is coincident frequency which is directly related to resonant part of noise radiation. Look at page 367 of this book:
Vehicle Noise and Vibration Refinement

For my theater, having two layers of 5/8" damped mass with a minimum 8" deep air cavity (two 2x4 walls separated by a 1" air gap, each stuffed with pink fluffy insulation) and followed by two 1/2" layers of damped mass is what I am using to reduce sound transmission. I never calculated the wall's impedance, I only knew that I went through this deconstruction phase to eliminate the triple leaf effect and to rebuild without making the wall too stiff through the increased mass of 3 internal damped layers.
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post #1456 of 1854 Old 07-11-2015, 06:29 AM
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I'm disappointed you haven't calculated the impedance of your wall. All this work and you skipped the #1 rule of home theater building 101.

Heartbreaking

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post #1457 of 1854 Old 07-11-2015, 07:58 AM
 
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Strong Google Tim. So this means you won't be using 1.5" MDF for your walls too? Think of all the awesome sawdust....
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post #1458 of 1854 Old 07-11-2015, 09:36 AM
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There's some good stuff there Tim. Thanks for the links.

In particular, I'd say that this figure brings us back to my initial question.


Modal control is most problematic, IMO, in the lowest region where resonant frequencies are farthest spaced. The first (lowest) few resonances are the primary resonances (for most common room proportions) so the resonant waves will be well-formed plane waves, with incident angle at or very near zero. The incident angle will be far from zero when a tangential or oblique mode in in question, but those modes generally overlap and pose much less of a concern to playback fidelity (again, IMO). If this is all true (I'm pretty confident, but I've been wrong before), stiffness is hardly relevant to sound reduction index or transmission loss at very low frequency.

I'm still interested in data that gets to whether sticking with two layers only allows the very lowest frequency energy out of the room while still containing all the primary modal resonances. My understanding is that proper room gain exists only (approximately) below the frequency of the first modal resonance. So, in my room, (21x12x9) the primary longitudinal resonance is predicted around 26Hz. The tangential modes kick in just above the third (vertical) axial resonance - somewhere in the mid-50Hz range. The only way that my wall construction influences this range (25-60Hz, give or take) is if it is made effectively transparent through that frequency range. First, I find that unacceptable from a performance standpoint: I require some degree of isolation in that range from passing trucks and to avoid having my neighbors call the cops. Second, If I'm to have any room gain, I need the walls to be as reflective as possible to as low a frequency as possible.

I'm probably splitting a lot of hairs here, but I don't have the experience or access to data to fill in this conceptual understanding to come to a practical recommendation. (Of course, it's moot for me as my walls are already built)
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post #1459 of 1854 Old 07-11-2015, 11:00 AM - Thread Starter
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I'm disappointed you haven't calculated the impedance of your wall.
I was hoping you would validate my calculations on Wednesday when you are here, but since that cancelled I'm just going to wing it.

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Strong Google Tim. So this means you won't be using 1.5" MDF for your walls too?
I can't afford the Fusick Signature design package!

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There's some good stuff there Tim. Thanks for the links.

That's the exact image I honed in on as well. Seems like a topic that's easy to understand but takes a lifetime to master. I find it all very interesting, but in the end it's nice knowing that there's a well-established good/better/best for construction techniques and materials that we all can follow.

In other news, Satan's back scratcher....otherwise known as a curry comb....showed up from Amazon today. I hope it works!! If it does, I'll have a quiet project I can work on while our boy is sleeping, which is nice. I hate the construction timeouts from pounding, sawing, etc.

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post #1460 of 1854 Old 07-11-2015, 11:37 AM - Thread Starter
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Looking at this drawing again when it suddenly 'clicked'....
  • Wall stiffness is the most important when controlling audio energy at its sharpest angles
  • Damping is most important when controlling audio energy within a variable cone of oblique angles and where the damping factor is a function of the material(s) used.
  • Mass is most important when controlling acoustic energy from a narrow head-on range

This is the careful balance between mass, damping and stiffness and their respective relationship. It's easier to move a wall if you are exerting energy straight on vs. coming at it from a steep angle where the amount of pressure you can exert is limited because of the steep angle. As you move further away from perpendicular, the amount of mass becomes less and less meaningful. Energy dissipation therefore relies on the stiffness of the material and deflection at steep angles.

I'm assuming this probably ties into a boundary gain conversation as well when you place a sub parallel to a side wall and get additional dB benefit in overall output, not a reduction. But then again, downward firing subwoofers fire straight at mass. Hmrph.... OK, maybe not everything 'clicks'...

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post #1461 of 1854 Old 07-11-2015, 11:44 AM
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Those are excellent links! I was researching this very topic some months ago with respect to soundproofing LFE. That is, how can we exploit the various properties of a wall to maximize attenuation of frequencies below 80Hz or so (and especially around 30-20Hz).

One of the first things I learned is that my simplistic view of the Mass Law (doubling the mass increases transmission loss by 6dB) was only applicable in a specific range of frequencies. In particular, it is the dominant law only in the frequencies between the resonant frequency and the coincidence frequency. This graph shows that:



Essentially, in the Mass controlled area, just adding mass of any type is all you need to do to increase transmission loss (increase reflection) -- the more the better.

Above the coincidence frequency, how stiff the material is starts to matter quite a bit. For the same density, the stiffer the material is, the more of a coincidence dip you're going to see. So you can either make the material so stiff that the coincidence frequency is above human hearing (steel) or you can use materials that are a bit more "floppy" and reduce the dip. Drywall is better than plywood for this region, then. You still need mass (and more is better) but the composition of the mass matters.

A lot of soundproofing efforts, like decoupling and adding Green Glue for damping and the like, are essentially focused on reducing the resonant frequency of the wall assembly and thus increasing the range of the Mass controlled zone.

But eventually you lower than resonant frequency as much as you can and it's going to be in the upper LFE range. Practically speaking, you aren't going to lower it under 30Hz. So now what?

Well, the reading I did indicated that stiffness starts to matter again. I got conflicting reports on this and so I asked about this on Gearslutz, where quite a few world class acoustics experts hang out. I got some fantastic responses.

Full thread is __HERE__

The TL;DRs I got from that thread were like so. First, material composition does matter a lot again in the LFE range, but when they stay "stiffness", they really mean "floppiness" (lack of stiffness). The property that starts really mattering is how "loose" the mass is. That's why if you compare the same amount of mass of sand and concrete, the sand will do notably better at containing LFE.

The other -- and more practical -- lesson I learned was that there really isn't any way to completely contain significant amounts of LFE in a typical stick framed house! The floppy mass that would work so well tends to be either extremely expensive (e.g., lead) or very difficult to work with in any practical measure (e.g., sand). That leaves something like concrete and then you need huge amounts of it.
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I saw your emails. I didn't forget you. When I'm on the PC I'll organize that info for your subwoofer enclosure and post it.
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post #1463 of 1854 Old 07-11-2015, 12:43 PM
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So bummed my trip got cancelled. Sorry man...sounds like you have plenty of help over there on the East coast tho.
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post #1464 of 1854 Old 07-11-2015, 01:12 PM
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Originally Posted by TMcG View Post
Looking at this drawing again when it suddenly 'clicked'....
  • Wall stiffness is the most important when controlling audio energy at its sharpest angles
  • Damping is most important when controlling audio energy within a variable cone of oblique angles and where the damping factor is a function of the material(s) used.
  • Mass is most important when controlling acoustic energy from a narrow head-on range
Yes, and this is where I was heading with my points about axial modes vs tangential and oblique.
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Originally Posted by granroth View Post


But eventually you lower than resonant frequency as much as you can and it's going to be in the upper LFE range. Practically speaking, you aren't going to lower it under 30Hz. So now what?
That's a very cool chart.

Did you get a good feeling for what range of masses (/m^2 I assume) would get the resonance into particular frequencies? I'll go read the thread...
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post #1465 of 1854 Old 07-11-2015, 05:44 PM
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Did you get a good feeling for what range of masses (/m^2 I assume) would get the resonance into particular frequencies? I'll go read the thread...
This is the page where that chart came from: http://personal.inet.fi/koti/juhladu...dproofing.html It has a few of the equations for calculating such things.

The given equation for calculating the resonant frequency (and subsequent harmonics) is: Fr = 0.45*vl*t*[(r/w)^2+(r/h)^2]

vl = longitudinal velocity of sound in the partition
r = harmonic number (1 = the fundamental frequency)
t = partition thickness in meters
w, h = width and height of the partition in meters

You can immediately see that the tricky part of the equation is getting the velocity of sound for any given material or wall. In fact, getting that value for any reasonable wall construction might be hard enough to make this equation almost useless for real world cases.

I also wonder if this only applies to very small surfaces, where the way the material is attached doesn't matter. I tried a quick sample of a sheet of drywall and got unusably small numbers as a result. Hrm.
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post #1466 of 1854 Old 07-11-2015, 06:23 PM
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The ultralight is supposed to be healthier, and it's lighter weight (20%) which matters when you start getting 1.5" thick sheet goods. Also- it's easier to cut and easier on blades.

I told Tim I'd chop it all up for him if he bought be a blade My Diablo is getting tired finally, I have a stack of TS blades I should consider sharpening. I never do, I always just buy another one.

My TS has a 42" rip, so it would work well for his box size. The only dimension I would not be able to cut is the 48" dimension, which all the sheets are already stock 48" from the factory anyways so no need to cut it. I also have a Festool REQ55 track saw that's awesome, but the blades for that are like $110 each

So when done how much are each of these going to weight?
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post #1467 of 1854 Old 07-11-2015, 07:51 PM
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I found two more equations that claim to give the resonant frequency of a closed partition. These might be more accurate since they acknowledge that any wall is going to be an air spring assembly (mass+air+mass) and to discount the spring gives a seemingly meaningless number.

The first is from the book Architectural Acoustics and it uses this: Fn = (1 / 2pi) * SQRT( (141820 * S) / (m * d) ) where S = surface area of the "piston" (m^2); m = mass of the "piston" (kg); d = depth of air space (m)

Assuming 5/8" drywall is 2.5 lb/ft^2, that's 12.2 kg/m^2. My back wall of 13'x8' is 9.66 m^2, giving us a "piston" mass of 117.852 kg. A typical wall has 3.5" of trapped air space or 0.09 m. Plug it all together and you get 57 Hz. I experimented with different wall/panel sizes and it doesn't seem to matter since the S/m gives some constant ratio (the SQRT part could be rewritten as SQRT( (141820/d) * (S/m) ))

If this equation is accurate, then having two layers of drywall would lower the resonant frequency to 40 Hz and three layers to 33 Hz.

Here's the problem, though, in the book they give the example of 5/8" drywall with the numbers I used above and claim that the equation gives 18 Hz as the resonant frequency. Eh? I believe 57 Hz a lot more than I do 18 Hz!!

Okay, if we switch to a much better known book like the Master Handbook of Acoustics (F. Alton Everest) then I see a resonant frequency equation used when describing how to create a panel absorber out of plywood. That's another air spring assembly. This time, though, the equation is: F = 170 / SQRT( m * d ) where m = surface density of the panel (lb / ft^2) and d = depth of the airspace (in). That's a very different equation!

Switching back to imperial measurements, we have 2.5 for the mass and 3.5 for the airspace. Much simpler. Plug in the numbers and you get 57 Hz. Oh! That's the same as the other equation. What if we add a layer (40 Hz) or two layers (33 Hz). Hah! Yep, the numbers track.

According to this, you'd need 8 layers of drywall (five inches thick) to lower the resonant frequency of the wall down to 20 Hz.

That's where some of the other soundproofing techniques come into play, since they naturally add more airspace (a double wall case has 8" of airspace which lowers the resonance to 38 Hz -- you only need 4 sheets to hit 20 Hz). Adding fiberglass to the airspace has the same effect as expanding the air space, but I'm not sure by what amount and if it's frequency dependent. Likewise, decoupling the drywall with clips only adds a little more airspace, but the frequency drop is (I believe) notably lower than that, indicating another spring effect that I don't know. And finally, a damping material like Green Glue also lowers the resonant frequency, but I don't know the math for that, either.
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post #1468 of 1854 Old 07-11-2015, 08:02 PM - Thread Starter
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Originally Posted by jimim View Post
So when done how much are each of these going to weight?
Rough calculations will put each box right around 315 pounds empty, with bracing. Add in 10-15 pounds of polyfill and the 115 pound driver and I think the whole thing is going to come close to 450 pounds per box.
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post #1469 of 1854 Old 07-11-2015, 08:22 PM - Thread Starter
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Originally Posted by granroth View Post
Switching back to imperial measurements, we have 2.5 for the mass and 3.5 for the airspace. Much simpler. Plug in the numbers and you get 57 Hz. Oh! That's the same as the other equation. What if we add a layer (40 Hz) or two layers (33 Hz). Hah! Yep, the numbers track.
Nice post! It would be interesting to know the resonant frequency of the assembly I am putting together with the big airspace, insulation and the double layers with Green Glue on either side. It has to be very low.

Last edited by TMcG; 07-12-2015 at 04:45 AM.
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post #1470 of 1854 Old 07-11-2015, 08:57 PM
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Thanks granroth.

How does this compare with the rattle test results you collected when you tested your sand-filled stage? I didn't see where you ever posted frequency information to go along with your observations of rattles around your house. That is to say, do you think your walls do a reasonable job of containing subwoofer rumble above the wall's resonant frequency?

Obviously, that's a more complicated issue - especially given the potential flanking paths present. I'm just really hoping that I can keep my house from rattling too much. If the wall construction is the limiting factor in a well-considered installation (with minimal/zero flanking paths) do you think it's reasonable to expect reference or near-reference level containment above the resonant frequency of the wall?
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