Subwoofer bracing not important? What?

[wigginjs]

The vibration of the driver will transfer to the inner box, and this will be damped by the flexible adhesive layer before being transferred to the outer box, with a magnitude that is significantly reduced.

Cool animation, but I'm still a little fuzzy on this point. The driver is mounted to the inner box, but it still has to be exposed to the room, you you'll need a hole for it in the outer box as well right? What does the transition from inner box to outer box look like, or, how is it accomplished? Doesn't this also cause the woofer to be severly recessed into the box? Is that not a problem?

 

[paulspencer]

Here's a plan to describe (actually it's a section).

The driver is attached to the inner box (magenta), but not flush mounted as you would normally do. The red shows the bracing.

The yellow is the outer box. The hole is cut larger to accomodate the outer diameter of the face plate.

The dotted blue line (ignore the little squares in the corners) is the damping layer, which would be the liquid nails adhesive.

No, as you can see, the woofer is flush mounted as you would normally have if you had double thickness walls, as some do. The difference is that the adhesive is flexible and the layers are not rigidly connected.

Basically you build the bracing, then build the inner box around it. You then partly assemble the outer box, apply the flexible adhesive damping layer, and progressively add the outer box, then fix the driver. It requires a little more care, but isn't by any means the most difficult sub box to build.

 

[wigginjs]

Thanks for the drawing, I think I fully understand what you guys were talking about now, but I've got a question (or maybe a suggestion). I think the construction could be made much more simple if the front baffle did not have a layer of damping material between the inner and outer walls (see pic, sorry paul, I kind of mangled yours). Would this still provide much the same effect? I'm guess it would for the most part, but the more isolated the inner box is the better.

 

[paulspencer]

I assume what you are talking about is removing the damping adhesive layer on the front baffle. This is probably not a good idea as the outer and inner box would then be connected on the front baffle and vibrations would transfer through. This would mean a loss in performance without any real simplification in construction. If you want to build a box within a box following this CLD technique, then you pretty much have to do it as I've shown, otherwise you might as well just make the walls thicker or use bracing as you see required. Otherwise you end up with the extra effort of CLD construction (although it isn't that bad) with performance that is no better than a more conventional solidly built box.

Now if I misunderstand your description and you in fact mean make the front baffle just one layer thick, then this becomes a significant weak point in the box, you certainly don't want this.

 

[wigginjs]

Yes, I was talking about removing the damping adhesive layer on the front baffle. Another question for you. How did you decide on using liquid nails as a damping layer? Do they sell liquid nails in large amount (liter/gallons)? The first material that came to my mind would be to use neoprene foam rubber sheets (which can be purchased from www.mcmastercarr.com, search for "neoprene foam rubber"), what are your thoughts on this? Also, how much better does this technique work than simply lining the inside wall of a normal enclosure with liquid nails (or neoprene)?

 

[paulspencer]

Liquid nails adhesive is recommended by Dr Earl Geddes

In suggesting liquid nails I'm effectively passing on his recommendation as it makes a lot of sense. Yes you can use foam but it won't have the same effect. There are 2 reasons to use liquid nails:

1. being an adhesive, the bending of the panels that takes place in vibration transfers shear forces to the adhesive, thereby maximisin the damping effect
2. vibration of the inner panel is not coupled to the outer, thus the magnitude of vibration is damped and reduced before being passed on to the second layer

The first action is often not understood, therefore foam is considered suitable. I believe a flexible adhesive is best, I'm sure there are others like Liquid nails that are just fine.

So you can see if you line the interior with liquid nails it will do virtually nothing. In Australia you can get liquid nails in cylinders similar to those for silicone sealant.

I think CLD is a great technique to use as it maximises effectiveness very efficiently - it will perform significantly better than a conventional box of the same thickness. Compared to a double thickness box, you only have to add adhesive and some extra attention and care in construction.

 

[wigginjs]

Of course you're right. I see how the CLD technique could be very effective at mechanically isolating the inner box. I understand that the liquid nails would do a good job coupling the inner box and the outer box, but I think the same effect could be achieved by using a 1/2" thick sheet of neoprene contact cemented to both the inner and outer boxes without the need for large amount of adhesive (I use liquid nails on my crossovers and think it has a strong smell, I can only image what it would be like pouring it into a speaker enclosure). I'm now strongly considering this technique for my next subwoofer. Any other comments you have would be appreciated, thanks for taking the time to explain yourself.

 

[paulspencer]

I understand that the liquid nails would do a good job coupling the inner box and the outer box, but I think the same effect could be achieved by using a 1/2" thick sheet of neoprene contact cemented to both the inner and outer boxes without the need for large amount of adhesive (I use liquid nails on my crossovers and think it has a strong smell, I can only image what it would be like pouring it into a speaker enclosure).

Yes that would work fine. You could first cut and assemble the inner box. Then place the neoprene over it to get the right gap between the two layers. Then you might trim the outer box when placing the panels over the neoprene, having first made it slightly larger than required.

I'm now strongly considering this technique for my next subwoofer. Any other comments you have would be appreciated, thanks for taking the time to explain yourself.

If you do, it would be great to see some pics on your site of construction ...

My subs are in prototype stage, but when I get around to building the final box, they will be constructed using 3 techniqes to get an inert box:

1. a fairly elaborate bracing mechanism

2. curved walls, mainly to get the look I want, but also allows me to make the walls a little thinner (as seen in the rendering on my site)

3. CLD - trickier to achieve with the box I have in mind as it incorporates 3mm sheets of MDF laminated together to form a curve

 

[Christof]

Ok, I got my dead horse beater in hand and I'd like to get more info from this old thread. As an architectural woodworker I use this liquid nails adhesive on a a semi-regular basis. I know for a fact that this adhesive gets very hard (semi-rigid) after a period of time. I do not understand how it can effectively absorb energy and turn it to heat via friction as we are trying to acheive with CLD.

I'm in the process of building a new HTsub and would really like to hear what other materials are used for CLD construction?

I plan to press my 3 layers (1/2" MDF + 1/4"?? + 3/4" MDF)together in a vacuum press with some sort of layer between. All joinery will be done with mitres such that the three layers will be aligned to form a CLD enclosure.

Thanks
Christof

 

[wigginjs]

Funny you should ask. I'm at this time preparing a webpage that will describe the steps I used to build a subwoofer I just recently finished. It used this so called "CLD" technique, except I used a 1/2" layer of neoprene foam rubber as the mechanical isolator. I'll be sure to make a thread once I finish the guide. I do already have a few working pictures posted in my "Current Theater Pics" section.

 

[paulspencer]

I know for a fact that this adhesive gets very hard (semi-rigid) after a period of time. I do not understand how it can effectively absorb energy and turn it to heat via friction as we are trying to acheive with CLD.

It needs to remain flexible for this to work. I first heard liquid nails recommended for this by Dr Earl Geddes at http://www.gedlee.com/

Are you saying that you don't think it has enough elasticity? Or that it becomes rigid and brittle over time?

 

[Ron E]

that's right about small box, but anyway wood will be resound.
that's because I've use material with large number of damping factor. it's to answer the purpose of first reason.
second one - standing waves - very serios problem and we can't leave it out, using bracing.
form of box, damping materials inside...should be effective
ok. good luck

Marble has a high damping factor?

Take a piece of plywood, and a piece of marble. Hang them from one corner and strike them. Measure the time it takes for the sound to die out. If the marble is not cracked, it will ring like a bell. The wood won't. Feel them after 5 seconds, which is vibrating more?

Marble has high mass and stiffness, but damping is intrinsically bad in most stiff materials. Actually, one guy tested many materials for panel resonance (including stone IIRC) and found particle board (not MDF) had the best properties among easily available materials.

The walls in old buildings are really thick stone, not because that is what is required to resist attack or collapse, but because they had no idea how to decide how thick to make the walls.. Many DIYers are the same way - overbuilding because if 3/4" is good, 2" must be better......

Pressure changes in boxes are about 1Psi or 7kPa for large excursions in small boxes (when Vd = 0.05*Vb)

 

[planet10]

vibration control.

Push-push loading is a huge benefit here. I'm at the point where i usually don't consider a sub unless i have an even number of drivers for it.

dave

 

[paulspencer]

Push-push loading is a huge benefit here. I'm at the point where i usually don't consider a sub unless i have an even number of drivers for it.

Dave, I like this idea but I find it tends to mean smaller cheaper drivers, or big boxes! (If we are talking serious subs). It also makes it very difficult to consider a sub like the Tumult, although I suppose you could look at a pair of 15" Koda drivers using the same XBL2 technology.

 

[planet10]

I like this idea but I find it tends to mean smaller cheaper drivers, or big boxes!

I feel the advantages of push-push (particularily for a sub) give you a better end product.

One can see the effort you are going to to keep the outside of the box from transmiting speaker induced vibration, push-push kills it at the source (and at the same time improves downward dynamic range). My single 3/4" layer push-push box will have less external vibration than your double-layer box, and will have much less problems of energy storage (and later release) that you have to worry about in such a box

dave

 

[Tim Moorman]

Double Up

For a closed box system, I think room effects are so dominant at subwoofer frequencies that two separate subs in solid, well braced boxes will be of far greater benefit than the dual P/P or other single box configurations, no matter how well the box is built.

Dipole open baffle bass is a good solution for the room, as far as they go, perhaps with twin subs below for HT.

Tim

 

[planet10]

Re: Double Up

For a closed box system, I think room effects are so dominant at subwoofer frequencies that two separate subs in solid, well braced boxes will be of far greater benefit than the dual P/P.

You underestimate the effectiveness of push-push loading... but i do agree 2 woofer boxes are better than 1... so my even number of choice is also evenly divisible by 4.

dave

 

[jdybnis]

When you do a sub push-push, where do you put the drivers? Can you position it in a corner or are you restricted to putting it out in the open?

-Josh

 

[planet10]

When you do a sub push-push, where do you put the drivers? Can you position it in a corner or are you restricted to putting it out in the open?

Drivers are placed on opposite sides of the cabinets (ie back & front or left side & right side -- or if lucky one driver on each side)... i have at least one client that uses his in a corner -- set on an angle

If you were designing for a corner, you could make the box 5-sided to have it stick into the room less (in this case the plate amp faces the corner)

dave

 

[Svante]

I missed this thread. Anyway, I thought I should share my view on the original question, wether or not bracing is more or less important in subs.

If we assume that the lowest resonance in the walls of the box is higher than the upper frequency limit of the crossover, the motion of the walls will be spring-controlled and there will be no excitation of the resonance. This means that the walls will vibrate, and the displacement of the walls will be proportional to the pressure inside the box. The box will expand and contract (is that the proper word?) like a balloon. Stiffer walls will reduce the displacement of the walls.

Now, vibrating walls might seem like a bad thing and if they vibrate a lot, it is. However, since the frequency response of the walls is pretty flat (ie below all resonances), the vibration of the walls will not contribute a lot to the overall response of the box. Since there are no sharp peaks (resonances) in the box contribution, it will hardly be visible in the overall response of the system.

There is of course a limit to this reasoning. If the walls vibrate to such an extent that they become non-linear and produce distortion, this will be bad. So, a card-board box will not do, but that is probably obvious. Also, if the vibration is transferred to the floor or some other big surface that has resonances in the sub's frequency range, it is bad too. But I see little need for concerns about vibrating walls in a normal sub's frequency range, since there typically are no resonances in the cabinet there.

For full range systems, on the other hand, there will definitely be structural modes in the frequency range of the speaker, and the need to control the box vibration will therefore be larger. Since a resonance will yield a contribution in a narrow frequency range, the effect on the response will be more obvious. Sandwich damping can be used to reduce the Qs of the structural modes and bracing can be used to move the modes towards higher frequencies, where the excitation is less efficient.

Just my 5 öre...