Cabinet Damping

[patherb]

Hi All,
I am building a 2 way sealed bookshelf speaker. My drivers are the ScanSpeak 18W/8531G Revelator and the Raal 140-15D Ribbon Tweeter. The cabinet is constructed of 3/4" MDF (Double thickness in the baffle). I have some questions about cabinet damping. I have acquired a roll of 1/24" thick lead sheeting that I was planning on using internally. I have heard mixed reviews on using lead as a damping material. I'd like to know your thoughts. I was also planning on using Acousta-Stuf to fill the inside. I've now read that wool is the preferred fill. I would prefer to use the lead sheeting (since I already paid for it), but I'd be willing to ditch the Acousta-Stuf in favor of wool. Any thoughts?

[sreten]

Hi,

The issue with lead sheeting is whether to use it as constrained or
restrained layer damping or a combination of both. As restrained
requires it to be sandwiched your question is a little late for that.

Lead is certainly effective ( if glued correctly .... how ? ... I do not
know ... ), ideally you would use 2 (or more) layers for 18mm MDF.

Probably 2 layers on the side walls and 1 layer on the back ?

/sreten.

[patherb]

I was planning on using the lead similarly to the last picture in the link:

http://www.humanspeakers.com/howto/bracing.htm

[sreten]

Hi,

The lead looks PVA glued ? You may need to prime it first .....

/sreten.

[MJL21193]

You could use clear silicone caulking (my choice) or a construction adhesive like PL-200 by Bulldog Grip.
Contact cement will probably work too.

[patherb]

I was planning on using glue and staples to secure the panels. I'll also be sealing the inside edges with silicone.

My concern is that I had been in contact with someone who told me that using lead sheeting would cause my cabinet to be/sound overly damped. Has anyone had experience with this?

[sreten]

Originally posted by patherb


... would cause my cabinet to be/sound overly damped ...

Hi,

And what does the pseudoexpertidiot suggest this sounds like ?

/sreten.

[pjpoes]

I'm hoping to post results soon, but I recently acquired a tape accelerometer to begin taking cabinet measurements. I don't have any lead sheeting yet, so I can't really comment on the best way to use it, or its comparative effectiveness with tar based products, etc, but I do have results comparing the PE constrained layer dampening product vs felt and polyvinyl dampening sheet. I also compared number of braces.

I know this won't directly answer your question, and it will take me some time to compile everything I measured into something I can post, but basically, bracing made a bigger difference in reducing cabinet resonance as compared to any other single change. I have done measurements on two different cabinets, one is a 1.85 cubic foot tower, and the other is a .5 cubic foot bookshelf. In both cases, the double front baffle has an extremely minor effect on the panel resonances. No effect on the sides, and minimal effect on the front. A constrained layer front baffle made a huge difference however. It not only reduced the resonance on the front baffle, but also reduced the resonance on the sides as well. My best guess is that some of the vibrations the drivers produce is being isolated so as not to run directly from the front baffle to the sides.

Another thing, while not conclusive as I'm still playing around with distortion measurements, THD is lowered with a less resonant enclosure. I would speculate that some people might prefer the sound of a speaker which has more resonant panels because of greater linear distortion. Like I said, thats a speculation, I can easily read a THD, its a number that flashes in the left corner of the program. However comparing linear and non-linear distortion is a bit more complicated and not something I'm experienced with, so while I suppose I could post plots, I can't yet understand what they are telling me.

[MJL21193]

Originally posted by pjpoes
but basically, bracing made a bigger difference in reducing cabinet resonance as compared to any other single change.

I came to this conclusion recently also. Effective bracing splits the surface area of the panels, thereby driving up the amount of energy required to make them resonate. The point is reached where there isn't enough energy to excite the resonance.
Planet10 has been preaching this and I finally caught on.

[pjpoes]

yeah I to had heard experienced designers preach the importance of bracing over dampening material, and I kept thinking, ok, but double thickness walls and baffles and dampening are good too right? Well, it looks like from my measurements, no, they aren't worth the weight and effort. You will find that tons of bracing, and I can give you some ideas of what I am finding works best, will do the most to deaden the box. Yes adding dampening further helps, but its by a small amount, and mostly in the higher frequencies where its most useful.

The only dampening methods using limp barrier methods, such as lead, that I find makes a big difference, and still nowhere near as big as the bracing, is to do constrained layer methods. This does have an impact on absorbing more low frequency resonances, and when used with proper bracing, will probably give you that nth degree. My problem is the amount of effort to do it right is so great, and the weight increase is so great, I kind of question if I should bother. I can eliminate enough resonances with bracing as to be unmeasurable in the FR or impedance plots. I will guess it wouldn't be audible either.

To give you a better idea what I am saying about bracing, that example you gave about how you want to use the lead, that box doesn't have anywhere near enough bracing to reduce resonances in that box, especially given the energy created by the drivers that were to be used in it. So far I am finding that having braces every 5-6" is best, and really not unreasonable. Also, I think that horizontal and vertical braces are best. I often see bracing that connects the sides, front, and back, but not the top to bottom, or brace to brace.

JmLabs actually did something interesting that I tried, and is a good idea. I don't know if they invented it, they claim its patented, but basically, you make your box like you normally would to start. But all the walls, front, and back are actually braces, and have holes cut in them. This is also where all your internal bracing will be attached to. Then you attach your actual side panels, rear baffle, and front baffle to this. I'm not really sure how else to describe this, its not very good. I need to figure out some 3d software or something one of these days so I can begin giving pictures with my explanations.

Another thing I have read, but am yet to test, is angled braces. I forget who did the work, but some company researching cabinet bracing claimed they found that angled braces eliminated the problems better than straight braces. It's an interesting idea, and if it works, great, but is much harder to implement.


I would call this a good start by the way. This is the bracing used by Elemental Designs in their top subwoofer. I would add to this two more braces where the woofers are themselves.


here is another drawing. Again, notice the angled brace, along with the fact that pretty much every wall is connected to every other wall. This is the minimum level that bracing should be taken to for good energy absorption, or draining (Not sure which braces would fall into).