on panel vs gap - absorbent in poly

[tonio]

What is the difference of having the absorbent (rockwool/703) touching the poly panel vs a gap of say 1/4'?.

I assume :
1. touching the panel- scattering properties based on the poly size/ low level bass trap absorption due to it doesn't resonate
vs
2. gap -same scattering properties / resonating panel resulting in a tuned trap

Is there a way to calculate what frequency and Q a 2x4' vertical poly would have in scenario #2?

[SAC]

To address the second part...

Yes, there are detailed analyses of perforated and slotted resonant traps in such texts as the D'Antonnio & Cox Acoustic Absorbers and Diffusors.

If you will pardon a bit of over simplification:

The problem with simple resonant panel/membrane absorbers is that there is considerable variability in the mounting of the panel as well as the restorative (spring) component (k) of the panel itself. Unless you can accurately quantify this critical factor, it reduces the design to trial and error.

In the slotted and perforated designs, you are dealing with the k value of the compressible medium (air), which is well understood. And the additional variables are able to be reasonably ascertained.

So, a suggestion. If you are going to explore these (and I would suggest more do this!), choose the designs with more easily determined criteria and make the job a bit easier!

[PaulP]

Originally Posted by tonio

2. gap -same scattering properties / resonating panel resulting in a tuned trap

(As I understand things) for a resonating panel to absorb a frequency the
panel has to be damped. Otherwise it'll just resonate like drum and you'll hear
it. Damping can be accomplished either by having the insulation touch the
panel or by having a sealed box behind the panel in which there is usually
also insulation but it doesn't touch the panel.

Paul P

[tonio]

Sorry if I was not specific enough....

I was reffering : poly as a polycylindrical diffuser(scatterer).

Playing around with dystraster/Avare(Andre)'s design posted here:
D. I. Y. Polys

Which I have cloned-well almost done. Just need some spraytac or the 3M spray adhesive dealio. If I remember correctly dykstraster had a gap(absorbent not touching the panel), but is a sealed unit.

Also I am making a somewhat traditional poly (polycylindrical diffuser) in the same size (2x4') without the ribs and not sealed.

My understanding is that traditional poly's have a peak due to the resonating character. However wouldn't it have more of an broadband nature since it is not flat?

[SAC]

Any resonant behavior in polys should be a function of the internal volume - NOT of the vibration of the surface material which should be rigid..

[Glenn Kuras]

Originally Posted by SAC

To address the second part...

Yes, there are detailed analyses of perforated and slotted resonant traps in such texts as the D'Antonnio & Cox Acoustic Absorbers and Diffusors.

If you will pardon a bit of over simplification:

The problem with simple resonant panel/membrane absorbers is that there is considerable variability in the mounting of the panel as well as the restorative (spring) component (k) of the panel itself. Unless you can accurately quantify this critical factor, it reduces the design to trial and error.

In the slotted and perforated designs, you are dealing with the k value of the compressible medium (air), which is well understood. And the additional variables are able to be reasonably ascertained.

So, a suggestion. If you are going to explore these (and I would suggest more do this!), choose the designs with more easily determined criteria and make the job a bit easier!

Thus the reason I would not recommend it without years of background.

[DanDan]

Hi tonio, I reckon there is no versus. A poly is a reflecting scattering device due to the material and shape. As such is should not vibrate at all. Stone might be a good material. However in the real world they are wooden panels. The vibrate very well at at Bass frequencies. Undamped they will re-radiate this resonant boom into the room. Fully damped, stuffed with fibre they will almost stop vibrating. But partially damped, they become pretty good Bass traps. Very similar to panel bass traps as in Ethan's original RealTraps.
DD

[SAC]

Originally Posted by Glenn Kuras

Thus the reason I would not recommend it without years of background.

I agree...and I remain slightly surprised with the seeming fascination of many in pursuing a resonant membrane/panel absorbers.

Speaking generally...

Of all the variants of resonant absorbers, I do think one must be a bit of a masochist to pursue a resonant membrane panel trap - if for no other reason than that even if they can build one that works perfectly, duplicating the process over a longer run necessary to have a sufficient supply will present problems with variability in construction and performance, as the compliance of the suspension of the rigid membrane will likely vary - and thus the tuning!!

They remind me of projects one would be assigned to complete if one were consigned to hell.

But what surprises me is that more, if one is interested in pursuing the topic of resonant absorbers, do not gravitate toward the more predictable and better 'behaved' perforated (especially multi-layer variants) or slotted variants!

Here they need 'merely' ( see how 'easy' one word can make it!) be concerned primarily with accurately determining the physical characteristics of the perforated membrane, many of which are available from high tolerance fabricators- as the perforated membranes require such stated tolerances for other purposes - so you are able to piggy-back and benefit from the economies of scale and of the demands of other industrial processes. Additional factors such as rigid enclosure volumes and the addition of predictable porous absorption are easily determined and managed.

Perforated membranes in metal or wood are readily available - especially perforated metals - with great tolerances (from industrial metal fabricators - e.g.: perforated sifting membranes offering extremely close tolerances are easy to source ). And even pegboard is readily available and useful, if only a bit large and heavy for one to obtain reliable density measurements (meaning you do the measurements correctly and seek out a precise industrial scale and don't try to derive anything from some half baked bathroom variety scale!) and spend the time to accurately calculate the percentage of solid surface area and porosity, etc...

The point is, with a little busywork, the data is there and obtainable with a high degree of reliability - whereas with a rigid membrane resonator there are simply too many complex variables for the average person to address, not to mention achieving a consistent mounting/suspension compliance of the membrane itself. And a non-rigid membrane variant offers still more additional variables needed to be addressed!

Among the better sources of information (beware, you WILL need to understand math as this does involve acoustical engineering & physics!) are the D'Antonio & Cox Acoustical Absorbers and Diffusors and the Ted Schultz Acoustical Uses For Perforated Metals.

But before anyone run off thinking - oh, so the perforated and slotted absorbers are easy, be aware of the additional issue besides simply building a standalone absorber! You will need to precisely measure and calculate the percentage material and percentage porosity/perforation. You will need to be able to accurately measure the mass and thus calculate the density of the material. Precise mass and perforation measurement and calculation is necessary. Thus it is not as simple as running down to Home Depot and simply throwing together a 'creatively designed' assemblage!

Just be aware, that after you get the tuning of the absorbers down, that you allow for adjustability of the internal sealed volumes facilitating a degree of tuning variance. This is required in order to allow for the interactive nature of installing the traps into a space - as their presence and interaction with other tuned traps will result in a slight shifting of resonant modes and Qs - resulting in the need for iterative adjustments and final tuning of the traps in the space.

You see, just when you think you have the solution, you get to begin phase two. Deja vu!


If one is interested in pursuing resonant absorbers, ...unlike the resonant membrane traps.... I would have no problem pointing someone in the direction of the perforated or slotted variants. But I would not enter the challenge without a clear understanding of the full process required. As if you are not up to anticipating the variables, and addressing each factor along the way, overlooked variables can easily come back to bite you.

So do a bit of research into the various industrial and commercial noise control products - and you will find them surprisingly quite common. The irony is that they are more common in professional installations than are porous LF absorbers! (Where reality is quite contrary to the opinion many seem to hold here who are not more aware of them.)

I hope I have not scared everyone away! They are achievable. But they are more complex than simply 'stuffing a mattress' and do require a bit more forethought and precision as well as iterative followup.

[tonio]

Thanks all

I will dampen the poly's, as they will be for the live room.

SAC-I did look into perforated LF absorbers, and agree that many sources are available and used commonly in buildings. With the miriad of calculaters on the web, it should'nt be too difficult to start, yet the tweeking would be prohibitive for a casual user. Beats the price of a RPG Modex for sure.

T

[johndykstra]

Originally Posted by tonio

Sorry if I was not specific enough....

I was reffering : poly as a polycylindrical diffuser(scatterer).

Playing around with dystraster/Avare(Andre)'s design posted here:
D. I. Y. Polys

Which I have cloned-well almost done. Just need some spraytac or the 3M spray adhesive dealio. If I remember correctly dykstraster had a gap(absorbent not touching the panel), but is a sealed unit.

Also I am making a somewhat traditional poly (polycylindrical diffuser) in the same size (2x4') without the ribs and not sealed.

My understanding is that traditional poly's have a peak due to the resonating character. However wouldn't it have more of an broadband nature since it is not flat?

Hello.

my polys were stuffed. That's to say, rock wool was carved so it filled the cavity... I didn't "cram" it in there. The first one I made, the rockwool didn't quite make contact with the very apex of the curve... about a 1/4" or so at the most. When I knock on this unit, there is a bit of a hollow sound where the insulation gaps. In subsequent builds, I filled that remaining 1/4" with scraps.

hoping you post some photos when you are done!

What's the spray tac for?

[tonio]

Hi

I didn't "cram" it in there. The first one I made, the rockwool didn't quite make contact with the very apex of the curve... about a 1/4" or so at the most.

I must have missed that on your post.

On the traditional ply, I can just remove the panel as it is on temporarily.

I was planning to use spraytac or spray glue to attach the rockwool for cutting-so it does'nt move around.

Will post some pictures when its set upthumbsup

[Glenn Kuras]

Originally Posted by tonio

Hi



.

I was planning to use spraytac or spray glue to attach the rockwool for cutting-so it does'nt move around.

Will post some pictures when its set upthumbsup

Not sure if you really need to attach it but GREAT idea.

[johndykstra]

Originally Posted by Glenn Kuras

Not sure if you really need to attach it but GREAT idea.

+1.

[johndykstra]

pictures?

[DanDan]

A little OT but historically interesting perhaps.
The EMT 140 Reverb plate had a variable Decay time. This was achieved by an absorbent panel. This panel could be moved on mechanical arms from extremely close (but not touching) to the plate to several inches out. The range of decay was very wide, from one to quite a few seconds.

DD

[tonio]

Originally Posted by dykstraster@gmai

pictures?

will do, got busy making skyline qrd's.

T

[tonio]

poly in a suspended in wall/ceiling config, and 1st skyline QRD build-got 2 made so far.





T

[johndykstra]

looks great man! did you leave the wood without a finish?

[PaulP]

Originally Posted by tonio

... and 1st skyline QRD build-got 2 made so far.

How much does each skyline weigh and where will you be mounting them ?

Did you put glue between the blocks or only on the ends of each ?

Paul P

[tonio]

Originally Posted by dykstraster@gmai

looks great man! did you leave the wood without a finish?

Thanks!
The poly I used Watco danish oil (dark walnut) - 2 coats.

The QRD is still in a raw state. Most likely paint them for better scatter/diffusion, as a raw state may have more absorptive results.

How much does each skyline weigh and where will you be mounting them ?

Did you put glue between the blocks or only on the ends of each ?

Not sure of the weight-dont have a scale-guesstimating under 20pounds.
Researching on the placement.

I used liquid nails on the ends and in between each block. I tested regular titebond II wood glue, and the curing process is too slow and need constant pressure/clamping. Liquid glue becomes tacky quickly, but the total cure is overnight. So putting down a bead of liquid nails via a caulking gun, I laid out 2 rows of blocks. Squash for the bottom(end) adhesion, add a slathered dab for the in betweens. Onto the next 2 rows, etc, etc.
Liquid glue is tacky enough initially to place the blocks and provide manuevering, yet firmly stays in place. After 30 minutes all bets are off , as its pretty much set!!

T