Straight Plywood as a Resonator? :?:

[bmcilvaine]

Hello all,

First post here. I have a small bedroom that I use as a design and light mixing suite for Television Post. Dimensions are 9.66 Ft x 10.5 ft x 8 Ft (Ceiling Height). Floor is carpet w/pad over hardwood, 1 thermapane window on the short side to my left, and a 60" wide closet to my rear right. Everything else is gypsum on standard home spaced 2x4 studs.

My desk is against the long wall, with a pair of B&W Bookshelf (forget the model #) speakers at ear hight, approx 5" off the wall. I also have a medium sized Yamaha Sub under the desk (YST SW 150) that I like for it's even tone. (On a budget here). I have my chair, and a floor lamp in the back left corner. No sofas or anything else that might act as an absorber, nor any thinking of one in the near future.

Through my years of employment, I have amassed a small collection of commercial foam products (2x703 boards w/Guilford fabric & Sonex/ Auralex squares) that I've used to shore up the imaging in the room. Still have some lobing and combing in the bottom end, though. Found some additional 703, $100/box, but the home budget is being shot on other things that need to be done around the house.

The Mrs. probably won't be into me doing too much to the space in terms of hangers and boxes, not that there's a lot of room anyway. However, as I was reading through the SAE doc, I noticed that different thicknesses of plywood act as resonators at different frequencies. So could I rip some 1/4" and 3/16" plywood into interesting shaped panels, apply a light finish, mount on battens on the back wall, and use them as resonators.

I did a search on this and found nothing really absolute about this idea.

My thinking, which could be terribly misguided from the info overload on this site (Great Site, BTW!), but I was thinking that if these shapes were mirrorerd left and right and mounted at angles that reflected the highs away from the sweet spot, I might have a solution that meets my budgetary and decor requirements, with minimal "Hole in the Wall Permanence" to get the Boss riled up. (If we decide to have a second child, this room will need to be dismantled and I will wind up in the basement... which by then I will be confident that it's dry, or it will have been made dry. We've only been in the house 4 weeks).

As I do the math on my space (and listen as well), it appears as if 50-70hz, 100-140ish hz, 175ish hz, and 210-280hz seem to be the real problem areas. I then noticed that plywood seems to have increase absorbtion from 500hz down. I am imagining taking dent sized rectangles of plywoood and applying fanciful edges with a jig saw, as larger is undoubtedly better?

Add to this, we are also interviewing designers for some new construction here at the day job (John, interested?) and a "World Renowned" NYC Acoustician/Designer told us that one of our refits of a linear video suite into a mixing room would be pretty easy, and added that replacing some of the ceiling tiles in the grid with plywood is an excellent way to bring the room under control. (This is in addition to a wee bit of construction, and there is a decent bit of acustical material in the suite already.

Anyway, as I have knowledge through practice, but none through practical application, I am all ears. As I am busy painting my soon to be first child's (daughter) bed room and readying this new-to-us 1966 built house for the rigors or child rearing, I can't spend too much time "perfecting" the suite. It exists primarily as a design suite, with some mixing happening there. But more so that I can bring the overtime home and do it away from the office. Any drastically crucial mixing will be done in the studio at the office.

OK, time to be quiet and get some opinions... just please remember that I am looking for a super cost effective, minimal impact solution to make things better, not perfect. For perfect, I'll drive into the office.

Thanks!

Brad.

[Ethan Winer]

Brad,

Yes, plywood panels resonate and can absorb. But that's not a good way to treat a room for a lot of reasons. Even if you can get some amount of absorption it will be very narrow. But all rooms need absorption at all low frequencies, not just the few you calculate (or even measure) as being modal.

--Ethan

[bmcilvaine]

Brad,

Yes, plywood panels resonate and can absorb. But that's not a good way to treat a room for a lot of reasons. Even if you can get some amount of absorption it will be very narrow. But all rooms need absorption at all low frequencies, not just the few you calculate (or even measure) as being modal.

--Ethan

Without a doubt.... and the plywood would not be the only acoustical system in the room. I came upon this hair brained idea while reviewing the chart that appeared in the SAE Handbooked linked on John's home page. In that chart, plywood showed a steady curve of increasing absorbtion from the high frequencies increasing down to the low frequencies, similar to the 1" panel solution with an airspace that is represented so well in these forums.

Below are the numbers I found. Use this legend to make sense of the numbers I've pasted further down:

-=-=-=-=-=-=-=-=-=-=-
Subtance being evaluated
125Hz coefficient
250Hz coefficient
500Hz etc...
1KHz etc...
2KHz etc...
4KHz etc...
-=-=-=-=-=-=-=-=-=-=-

These are the numbers from the SAE Chart:

Plywood (5mm(3/16") paneling over 50mm(2") airspace)
0.38
0.24
0.17
0.1
0.08
0.05

Plywood (5mm(3/16") panel, 25mm(1") fiberglass in 50mm(2") airspace)
0.42
0.36
0.19
0.1
0.08
0.05

Plywood (6mm(1/4") paneling, airspace, light bracing)
0.3
0.25
0.15
0.1
0.1
0.1

Plywood (10mm(3/8") paneling, airspace, light bracing)
0.28
0.22
0.17
0.09
0.1
0.11

Plywood (19mm(3/4") paneling, airspace, light bracing)
0.2
0.18
0.15
0.12
0.1
0.1

It would appear as if Plywood (5mm(3/16") paneling over 50mm(2") airspace) gives similar numbers (less increase on the bottom end) to the Plywood (5mm(3/16") panel, 25mm(1") fiberglass in 50mm(2") airspace)

Is this true?

I realize that A) this is a very broad representation of the reaction of plywood at various frequencies, and there could be alot of activity between these frequencies that is both plus and minus to the goal, as well as there is nothing over 4K represented in the chart, and B) Building a room inside a room out of plywood is not the solution.

There are other solutions involved with the room as well.

You mention that there are various reasons why implementing plywood as a resonator in not a good way to treat a room. Can you elaborate? I'd love a little more info on the behavior of plywood as a resonator. Don't be afraid to get a little geek with me. I follow along quite well! And if I miss something you mention, I'll do some research on it before I bug you for an explanation. It's all just a quest for greater understanding.

OK, back to my BBC Research reading!

Thanks for the prompt reply!




Brad.

[AVare]

What Ethan wrote was entirely correct.

You mention that there are various reasons why implementing plywood as a resonator in not a good way to treat a room. Can you elaborate? I'd love a little more info on the behavior of plywood as a resonator. Don't be afraid to get a little geek with me. I follow along quite well!

Adding to what Ethan wrote:

In rooms with very high decay rates, as in small sufficiently acoustically dampened rooms, the effect of the moving panel has a significant negative. The sound hits the panel, the panel vibrates absorbing energy, which is good. The sound stops and the panel continues to vibrate, introducing sound into the room. As you add damping to the system the ringing reduces, but so does the maximum absorption.

The equation for determining the peak frequency of absorption with no absorption in the airspace is

f= 170/(md^0.5)

where:

f=peak frequency
m=mass in lb^3
d=cavity depth in inches.

If there is absorbent filling the cavity then the numerator becomes around 120.

If the cavity is between two panels, then another equation is used.

Note: you already have panel absorption in your room by virtue of the the walls!

Andre

[bmcilvaine]

What Ethan wrote was entirely correct.

{snip}

Note: you already have panel absorption in your room by virtue of the the walls!

Andre

Ah, yes... forgot about that... damn.

And all good points, BTW. But to further the discussion, unless this is getting annoying, the effective volume of the ringing in db might possibly be so far down that it would be more of a mathematical problem than a significant practical one, no?

I understand that the entire room is a mechanism, and that all of the parts of it interact, making this a very difficult science. So you are saying that my walls, being panel resonators, might be ringing, contributing to my issues. Therefore, specific or broadband absorbtion is the better answer to removing the energy in the room that is problematic, as this will decrease the amount of energy present from both the monitors and the sympathetic resonance from the walls?

Wow, this is starting to sound like a good conversation over pints with bar napkin diagrams to clear things up!

Boy, I would love to build things, shove them in the room, mess around a bit, take them out, build more things, shove them in the room, etc.... just to get a better idea of the result of each. Kind of like sampling the buffet at the chinese restaurant before going back and filling your plate with just the things that will quench your palate.

But alas.....

OK, back to the drawing boards. Or this board... gonna keep reading.

Thanks for the input.

[Ethan Winer]

Brad,

Plywood (5mm(3/16") paneling over 50mm(2") airspace)
0.38
0.24
0.17
0.1
0.08
0.05

That data is basically useless because it completely ignores the frequencies at which a resonant panel's absorption peaks. Imagine a parametric EQ with one band set to very narrow and boosted all the way. That is more or less how a resonant panel absorbs - a whole lot at one frequency and much less at all other frequencies. So unless you happen to measure at the exact peak center frequency, you'll completely miss seeing what the panel is really doing.

Andre's point about the panel adding new tones into the room is also a good reason to abandon this idea.

--Ethan

[AVare]

unless this is getting annoying, the effective volume of the ringing in db might possibly be so far down that it would be more of a mathematical problem than a significant practical one, no?

No.

Read my previous post. I wrote:

In rooms with very high decay rates, as in small sufficiently acoustically dampened rooms, the effect of the moving panel has a significant negative

In large rooms the ringing from panels is low compared to reverberation in the room. Small damped rooms do not have a real reverberation, more of a decay rate. The ringing is audible as a smearing of those frequencies.

So you are saying that my walls, being panel resonators, might be ringing, contributing to my issues

I said nothing of the kind. You asked why using panel absorbers is not good, and I added to Ethan's post.

Your issues evolve around using absobent mateerial that you already have, and how to balance them.

Indirectly, your initial post implied that you think you have no absorption at the low end. I pointed that you already have panel absorbers at the low end.

If you weren't so close minded in your posts I would:

direct you to Bob's excellent absorption database;

tell you that any of the products that have low good low end absorption will do; and

to buy on the basis of best price within the performance you want.

I would also suggest to maximize the eveness of absorption of the absorptive surfaces to double or triple up the current material to even out the absorption frequency wise and thus reduce the amount of absorbent you have to buy.

Andre

[bmcilvaine]

If you weren't so close minded in your posts I would:

direct you to Bob's excellent absorption database;

Wow Andre...

Close minded? I'm just doing some reading, evaluating input from various sources (including yourself, this site, other sites, the BBC Research Reports, etc... so thanks for the info, if not the attitude...), and also trying to do some creative thinking! You'll notice in my first post that I had been involved in a discussion with an acoustician regarding the potential benefits of Plywood as absorbers in limited use, along with other acoustical systems in a much larger mix suite than the room in question here. That is where my questioning/discussion began.

You'll also notice in my last post that I was "going back to the drawing board" for a better solution for my scenario, due, in part, to the information provided by yourself and others.

Close minded? Jeeze.

Thanks for the link, however.

Brad.