# Mid/bass driver damping and bracing



## Skwisgaar (May 31, 2010)

I am designing a slot port enclosure for a mid/bass driver with the assistance of Bassbox Pro. The response needs to be from ~60-5000 Hz. There will be no internal crossover.

I have a couple of questions.

Which is better at killing low frequency standing waves? Blackhole 5 or No Rez? 

Is either material sufficient for killing mid frequency standing waves on its own, or does it need to be used in conjunction with something like fiberglass panels?

I intend to use corner bracing. How wide should the boards I use for bracing be? I will use 13 ply birch throughout, except maybe MDF for the rear baffle.


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## lsiberian (Mar 24, 2009)

Skwisgaar said:


> I am designing a slot port enclosure for a mid/bass driver with the assistance of Bassbox Pro. The response needs to be from ~60-5000 Hz. There will be no internal crossover.
> 
> I have a couple of questions.
> 
> ...


Use rock wool it's cheaper and just as effective. http://www.atsacoustics.com/item--Roxul-AFB--1003.html is what I use. I use 13 ply for all speaker builds it works just fine. For bracing cross braces are best. For this I use Oak 1 x 2


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## Skwisgaar (May 31, 2010)

Do you have any spec sheet showing rock wool's sound absorption? I am under the impression that all these fiberglass type materials bottom out at about 150 Hz. The density on the material you pointed out is low compared to rigid fiberglass.

Do you have a link to a cross bracing speaker plan? I have spent a lot of time looking at sound barrier products for automobiles, speakers, and rooms, but I know almost nothing about bracing. 

From what I have seen of cross bracing from a little surfing it does not look like my cup of tea. Too difficult and it wouldn't be easy to remove the speaker anymore (which I intend to rear mount). 

Is oak better than fir for cleats?

I found an answer to my NoRez v Blackhole 5 question

http://www.skiingninja.com/Articles.asp?ID=157

It's curious that the materials have a neutral effect on cabinet volume. I had wondered about that but not thought to ask.


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## lsiberian (Mar 24, 2009)

The problem with NoRez and Blackhole is the cost. You can get a pack of rockwool for a lot less and have the best sound absorption. To absorb bass you need thickness. 4" thick rockwool has very little reflected sound. 

Materials do effect the tuning of the box, but not enough to create an audible difference in most cases. In any case the positives of the lining outweigh the negatives of the box tuning change. If you are building a ported speakers I'd suggest 4" on the back and 2" on the sides, but no more than that or you will kill the port. For ported designs I actually leave the sides uncovered and just treat the back 4" thick. I prefer AFB because it doesn't effect tuning as much as 6 or 8lb rockwool. So I get the best of both worlds.

http://www.bobgolds.com/AbsorptionCoefficients.htm under Roxul rigid. 0.86 is not significantly less than 1 so 4" of AFB is just right for the rear absorption in a speaker. The stuff is gold in my subs and doesn't itch like fiber when handling it.


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## fbov (Aug 28, 2008)

Here's a quick tutorial. 

Outline
Box resonance
- bracing
- damping

Internal resonance
- surface treatment
- volume treatment
- ported boxes

Related concepts
Wavelengths
- bass >> enclosure dimensions
- mid ~= enclosure dimensions
- high << enclosure dimensions

Nodes
- sound is a combination pressure wave and displacement wave
- a displacement "node" is a point where the displacement wave crosses zero
- surfaces are always displacement nodes and pressure peaks

A box containing a speaker will vibrate when the speaker does. Resonances will occur at certain wavelengths, but they are only standing waves when they "fit": box dimension = n/2 wavelength for n=integer. Thus there are no standing wave issues in subwoofers as subs are smaller than the wavelengths they produce (100Hz 1/2 wavelength is 5.65 ft.). In any speaker (reasonable) that produces bass, the issue has nothing to do with standing waves. You're dealing with mechanical resonances, and you have 2 choices.

Bracing: Make the panels stiffer
Double thickness panels do the same thing as adding runners along a panel or put cross braces connecting panels. It all increases stiffness, driving resonances up in frequency. Higher frequency resonances carry less energy and are easier to damp, all good things. 

Damping: Make the panels absorb energy
This is the goal of the surface treatments you list (although Black Hole is a hybrid). While they do add mass, which has the same effect as bracing (drives resonance frequencies higher), they also add layers that are deformed by the vibration and convert that energy into heat. The most effective are multilayer systems called constraint layer damping (CLD), which is part of the Black Hole hybrid. 

Now let's go up in frequency. Wavelengths are now comparable to the box dimensions, so internal acoustic reflections must be controlled., as well as mechanical box resonances. 

Surface treatment: absorb acoustic energy at reflections
This is sonic absorber like OC703, Roxul, carpet underlayment, and Black Hole top layer. These are somewhat dense materials that inhibit but allow the flow of air through their fibers/cells (i.e. closed cell foams do not work). They are mostly air, which is why they don't subtract from box volume, but they absorb energy from any air that tries to pass through. Effectiveness depends on density and thickness, the latter discussed below. 

Note, however, that surfaces are displacement nodes, so there's no air motion to absorb at the surface. Thus surface treatment's energy absorption depends on thickness relative to wavelength. We've already determined that the longest standing wave resonance supported by the box is twice longest box dimension, so there's a lower limit on frequencies we need to absorb. Practically speaking, there's not much added benefit once the total surface treatment thickness is 10-15% of each dimension, and less may sound better. 

Volume treatment: absorb acoustic energy between reflections
This is "fill," rarified bunches of loose fibers ranging from pillow stuffing to fiberglass insulation. It is used loose, typically in bunches stuffed into sealed enclosures. This is the easiest material to add or remove so it's popular for fine tuning bass response. It has the ability to absorb nearly all the acoustic energy at all wavelengths because it fills the volume. It can also make a speaker sound dead, and will kill the output of a ported box if used improperly. 

Ported boxes: no fill in the port path
A ported box needs to address both resonances and standing waves, while maintaining or enhancing one specific resonance - the port. This means that fill must be used very sparingly in any box with a desirable resonance. In ported boxes, you might put some around the base of the port, away from the opening. In a transmission line, there are specific location and density requirements so only the right resonances are absorbed. 

Specific suggestions (assuming small, full-range driver)
- Line walls with ~1" foam (Use Black Hole if you have money to burn)
- use 1 brace connecting centers of sides (assumed largest panel) 
(Corner braces are ineffective acousticly, but do make the box stronger)
- I'd recommend a double thickness baffle, but that's hard to do rear-mounted.
- Use the speakers placed against a wall or consider adding a contour circuit. 
Baffle Step Compensation (BSC) is required in most all free-standing speakers or you may find the bass severely lacking. 

Have fun,
Frank


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## Skwisgaar (May 31, 2010)

The rear baffle will be the largest panel. 

Will anything be gained resonance wise by making the front baffle non-removable?

Has anyone tried Formica as a baffle material? I saw mention of its use somewhere.


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## fbov (Aug 28, 2008)

If the rear is the largest panel, then so is the front. With one driver, you have plenty of room for a brace. However, it's a lot easier to put a brace between panels that are not removable. Also consider that in most cases, you can add bracing if box resonances are excessive. You finger tips can tell you if and where you'd want to add braces. 

Conversely, as isiberian notes, the back panel benefits from significant thickness of acoustic absorber, but you may be limited if this is a shallow box. Plan on 1-2" at minimum, and consider changing dimensions if you don't have 2" of clearance behind the driver magnet. 

If the baffle fits well, there are no issues making it removable. The concern here is small gaps that might resonate; any gasket tape or sealant you'd use to make it air tight would fill these gaps and eliminate resonances due to mounting. 

Formica(r) is the trade name for a laminate that you can apply to wood, or buy pre-bonded to wood composite. I know of several speakers built with it, but not just a baffle. The only great advantage I see is that it's pre-finished, but you can also buy pre-finished MDF....

In my experience, the key to box construction is straight and true saw cuts - nothing but 90 degree angles in all directions. Get this right and your seams will have no gaps and your boxes will go together square. 

Have fun,
Frank


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## 1Michael (Nov 2, 2006)

fbov said:


> In my experience, the key to box construction is straight and true saw cuts - nothing but 90 degree angles in all directions.


:rofl::coocoo: Straight and true cuts:rofl::rofl::rofl::rofl::rofl: That's a good one :rofl::rofl:
I would not know what to do with myself if I ever had one of those...


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## Skwisgaar (May 31, 2010)

fbov said:


> If the rear is the largest panel, then so is the front.


No, it was going to be shelf ported and now I've even decided to use a big block of wood to raise the ports off the bottom, but still have them be rectangular, so I can control their size. This will help me enlarge the outer dimensions to something that will be easier to work with, and prevent a bare spot in the lining behind the cleats. The lower front brace will be on top of the port cover.

It's going to be too shallow for 4" of material certainly, but it's not going to have problems fitting the driver.


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## fbov (Aug 28, 2008)

That's right, front slot port... mine's on the bottom.


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