Undercoating for dampening?

[seattle_ice]

Has anyone thought of using the spray-on automotive rubberized undercoating for cabinet dampening?

Some brands are basically asphalt/rubber compounds.

Sure would be easy to apply, and you could build the layer thickness up fairly rapidly.

[slayer]

I coated the entire inside of my sub enclosure with a flexible caulk. I went through I think 6 tubes of it and used a spreader to smooth it out over the entire surface. Ended up about a 1/4" thick. It took about 2 days to dry with a fan blowing in it. I waited about a week and a half before mounting the driver just to be safe. The knock test on the box was night and day over the box without it. I have used liquid nails before and it worked well too.
As far as the spray-on automotive rubberized undercoating, I have tried it and it just didn't work for me. I couldn't get it on thick enough to make a big difference. It would have taken too many cans to get to the thickness needed and they are not cheap. The caulk is always on sale and is a much cheaper way to go and is cleaner to work with. But it takes longer to dry. I'm sure there are other options as well. My suggestion is that whatever you decide on, let it totaly dry before proceeding. Do not rush it.

[seattle_ice]

Sometimes I forget that other people don't have the same tools. Doing it by the spray can would probably be cost prohibitive.

I have a heavy sprayer that I can put a 1/4" nozzle in and spray the commercial stuff. Not only does it go on much thicker, it costs a lot less to buy it at the autobody store.

I have this because I work on a lot of classic cars.

[RandyMidd]

I've read of others using undercoating products before...I was thinking of using Henry 307 for a project of mine. Sprayers can be cumbersome to use on an enclosure that's already built, besides requiring a respirator for use, and being messy... I prefer the idea of using a brush over a sprayer.

I'd also prefer water based products over solvent based...I don't want my drivers exposed to off-gasing solvent based products in a sealed container! :cry:

In lieu of caulk the big box home improvement stores carry duct sealant. ( Usually kept near the air duct) The advantage is that it comes in a gallon container (wholesalers to the trade sell for $12, H Depot around $17) you paint it on with a brush. It's water based like caulk but maybe more difficult to cleanup if you let it dry on you. Dries quickly but not too quickly for recoating. It's an "elastomeric" meaning it stays flexible.

[ThomasW]

All these coatings do is add weight to the box. They have no direct damping effect on soundwaves.

Be careful with any coating that off-gasses solvents, those have the potential to damage the drivers.

[RandyMidd]

Thomas,

Can you suggest anything that would help to deaden a box..short of expensive products like Blackhole-5?

I was thinking of APP (torch-down) roofing material (aka cap sheet)...but then I though I'd end up burning myself and making a mess trying to feed hot pieces of material through a 10" opening in my cabs...I think there is a peel-n-stick equivalent...good idea...bad idea?

[Kal Rubinson]

All these coatings do is add weight to the box. They have no direct damping effect on soundwaves.

Be careful with any coating that off-gasses solvents, those have the potential to damage the drivers.

Right. Adding mass without compliance won't absorb much energy.

I used, in the old days, such a compliant undercoating as an adhesive to line the insides of the box with bitumenous felt pads; roofing felt pads about 1/2" thick. This was standard procedure in the old British DIY set.

Of course, I kept them for about a month in the garage before proceding to let them blow off as much as possible.

Kal

[ThomasW]

Thomas,

Can you suggest anything that would help to deaden a box..short of expensive products like Blackhole-5?

You can make your own low cost BH5 using generic acoustic foam and the bituminous sheets PE sells.

http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=268-020

If one wants a 'dead' cabinet that should really be done during the building process not after the fact. Constrained layer damping makes really dead boxes. Take the same sheet stuff from PE and put it between the walls of the box.

[seattle_ice]

All these coatings do is add weight to the box. They have no direct damping effect on soundwaves.

Be careful with any coating that off-gasses solvents, those have the potential to damage the drivers.

I find that a little bit surprising, considering the difference the undercoating can make on audible road noise from inside a car. Of course, that is going from thin sheet metal to a substantially thicker rubberized coating, not 3/4"-1-1/2" MDF.

Undercoating
Specs

Those specs show an 11.8 decibel drop in noise at 1khz over plain steel.

North Creek Advises using a mixture of Drywall mud and 'Soft Glue' (like tacky glue).

[Kal Rubinson]

I find that a little bit surprising, considering the difference the undercoating can make on audible road noise from inside a car. Of course, that is going from thin sheet metal to a substantially thicker rubberized coating, not 3/4"-1-1/2" MDF.

Undercoating
Specs

Those specs show an 11.8 decibel drop in noise at 1khz over plain steel.

North Creek Advises using a mixture of Drywall mud and 'Soft Glue' (like tacky glue).

Simply adding mass will change the resonant characteristics if the added mass is significant compared to that of the original material. With a thin metal, adding the undercoating can be significant. With a 15-25mm MDF, the effect is slight, at best.

Kal

[slayer]

The enclosure I made has minimum 3" wall thickness. I laminated 1.5" MDF with Gorilla glue to make my panels. The front baffle had an extra piece of 1" MDF added to it. The box was put together with the Gorilla glue and and screws. I used 1" MDF for the internal bracing. Without the driver mounted, I gave the knock test. I noticed on the largest walls that they gave off the most noise. After using the flexible caulk on the inside of the enclosure, the same knock test revealed that the noise I heard before was gone. It just sounded dead. Call it what you want, but it helped my enclosure.

[Kal Rubinson]

When you did the knock-test was the enclosure open, empty and devoid of any acoustic stuffing? I suspect that the glop was having a greater effect on the cavity resonance (by changing the inside reflective surfaces) than on the panel resonance of your substantial construction. OTOH, it cannot harm.

Kal

[slayer]

That was my thought exactly. It knew it could not harm anything to apply it to the inside surface so I did. And yes, the enclosure was open when I did the before and after. After about a week and a half of drying to be safe, I filled it with fiberfil stuffing and mounted the driver. The sub is great, it's my room that has issues.

[Kal Rubinson]

That was my thought exactly. It knew it could not harm anything to apply it to the inside surface so I did. And yes, the enclosure was open when I did the before and after. After about a week and a half of drying to be safe, I filled it with fiberfil stuffing and mounted the driver. The sub is great, it's my room that has issues.

Great. I tend to overkill, too. It seems right to do whatever you can before you seal it up and going back becomes a pain. That's why I have 8 unused video cables in the wall between my equipment rack and PDP! :??

Kal

[thylantyr]

Has anyone thought of using the spray-on automotive rubberized undercoating for cabinet dampening?

As already mentioned, just design a proper speaker box
using thick wood with great bracing and forget the esoteric
dampening ideas, waste of money and time, but it's ok
to waste your money and time on DIY projects for the cool
and fun aspect, but don't fool yourself that it's working a miracle.

For every esoteric dampening recipe shown, a counter
recipe can be made using plain wood to get the same
performance.

It's the skill of the carpenter that makes the box work
well.

[seattle_ice]

Has anyone thought of using the spray-on automotive rubberized undercoating for cabinet dampening?

As already mentioned, just design a proper speaker box
using thick wood with great bracing and forget the esoteric
dampening ideas, waste of money and time, but it's ok
to waste your money and time on DIY projects for the cool
and fun aspect, but don't fool yourself that it's working a miracle.

For every esoteric dampening recipe shown, a counter
recipe can be made using plain wood to get the same
performance.

It's the skill of the carpenter that makes the box work
well.

IMO that is a simplistic view of cabinet building. I do not believe every high end speaker manufacturer that has ever used materials other than wood for this purpose to be wasting their time and money.

I have been building anything and everything made from wood since I was 14 years old. But wood is wood. It has specific physical characteristics, no more, no less. It cannot accomplish everything always on its own.

Many times there are size and weight constraints that those of us building cabinets sometimes need to adhere to, and exploring other options is simply expedient, not a waste of time.

[ThomasW]

seattle_ice

A difference of opinions is fine.

Personal attacks and ridicule, end up get people a vacation from the forum...

[seattle_ice]

I apologize.

Wrong Forum for that. Not the kind of person I am either.

Darryn

[RandyMidd]

OK, to reveiw...
It's good to design and build a dead cabinet in the first place, being liberal with matierial thickness and bracing.

additionally you can...
Employ a constrained layer design that sandwiches MDF or masonite with bitimen or similar product in the middle.

But...
What if your rebuilding an existing cabinet like me... I've added length to it, some bracing, (and will be replacing the drivers and XO) and I want to know what else short of a constrained layer can be done?

Would the acoustic foam and the bituminous sheets Thomas suggested help make the cabinet walls less active or serve to stop sound from reverberating inside the cab and possibly back through the drivers thin cone material? What would you do? These are sealed cabs and I'm already stuffing them with fiberglass.

[RandyMidd]

Pour-in Constrained Layer

I’ve been doing some very limited experimenting to determine appropriate materials that may be used for adding a constrained layer to a new project or in my case existing ¾” MDF, 2 Cu Ft speaker cabinets.

It would be entirely unpractical to use conventional materials like ¼” HDF in my case because I can only work through a small 10” woofer opening, so looking into materials that can be applied / poured in place seemed obvious.

What I’ve come up with so far is 1/16 to a 1/8” thick layer of elastomeric that’s applied directly to the interior of the cabinet (with a chip brush), and a mixture of Plaster of Paris and Perlite over it.

Material Preferences
No off-gassing, no esoteric materials costing $50 per 10 oz. tube, constraining layer can’t expand or shrink when setting and should have a reasonable working time.

Constrained Layer
The elastomeric is called Glenkote it’s a water based product used to seal joints in metal air ducting. Cost is $20 for a one gallon pail, available from (some) HVAC industry wholesalers. It’s approved for the thickness I intend to apply, stays flexible after years of weather exposure- so it’ll probably be fine inside of speakers…the main concern I had was how well it would adhere to the MDF walls and what product can I apply over it as the constraining layer that would adhere to it?

Constraining Layer
From experimenting with DAP brand Plaster of Paris, I found that it adheres well to the Glencoat, doesn’t expand or shrink when hardening but has minimal working time and will add considerable weight to the cabinets. DAP says it sets in 20-30 minutes but it’s more like 10 minutes from the time you add water – which doesn’t leave much left for mixing and application. By varying the % mix of ingredients, calcium carbonate and calcium sulfate, DAP offers another product called Plaster Wall Patch that has a longer working time- I believe that the mix and setting times determine the final harness for Plaster of Paris, lower set times equals harder material…anyway I wanted something lightweight if available so I turned to other products that use either Perlite or Vermiculite in their mix.

The two products I found readily available that use a fine crushed mix of Perlite and Plaster of Paris are called Gypsolite by National Gybsum and Structo-lite by USG. Aside from being lightweight, Perlite is touted to have good sound deadening qualities as far as plaster goes. These products weren’t available at my local big box home improvement stores, a local building material supplier had Structo-lite, 50lbs for $15 so that’s what I got. This material really is pretty light when set by comparison and sets much more slowly, maybe a couple hours- I got tired of checking on it so I left. It seems hard enough by itself, but I’ll probably mix the 25 lb bag of DAP Plaster of Paris in with it (I need to use it somehow) which will give me a 2:1 mix of Structo-lite / Plaster of Paris. I’m testing that mix at this time.

Testing
As far as dampening vibrations, it seems a 3/8” thick layer of constraining material over the elastomeric is needed to get a decent reduction in vibes from my very unscientific impact testing, what this will translate to once inside speaker cabinets is unknown and I don’t want to do one speaker at a time to AB them because I’m already way too slow on projects.

Application Details
There are six panels in a simple box but in a speaker cabinet those panels are separated by bracing and other things like midrange and tweeter enclosures. My intention in dealing with these obstacles is to have any change in direction (from a flat panel) define the end to that sandwich, and the obstacle will be have its own sandwich applied to it, separated from contact with the adjacent sandwich by the elastomeric material. The idea is not to hinder “movement” of the butting sandwich layers. I don’t know if this is necessary but it seems logical to avoid cracks as well.
The constrained and constraining layers (the sandwich) will be applied to the top of the cabinet first so that the side walls will assist in its support.

[elambert]

Just as a follow up to the sheet deadener. Long time ago someone had recommended to me a product called peel-n-seal as a substitue for dynamat and the like. It's a rubberized asphalt with an aluminum skin that runs about 40mil thick. Can be found in your local lowes for about $7 for a roll 6in wide and I think 25ft. long. I have used it in several of my cars for dampening, using 2 layers of the stuff.

Whether or not it would be useful in a constrained layer type of situation I don't know. Haven't tried it there myself yet.

[noah katz]

CLD works best with a thin layer.

I doubt plaster of Paris is very stiff, and is bulky.

If I was in your situation, I'd paint the inside of the box with the viscoelastic compound, and then add a layer of fiberglass mat and resin.

[Martyn]

This is a complex subject with no single simple answer. As it happens, I have been doing a few calculations recently on the vibration of flat plates; the results so far are quite surprising, but I still have more work to do.

So far, I'd summarise by saying that the most problematic area is for vibrations up to 50 Hz. Above this frequency, the application of felt or similar materials to the interior surfaces seems to be a simple, effective solution. Below this frequency, felt isn't effective and we need to take other measures. Stiffening the enclosure will raise the resonant frequencies, but there's an infinite number of vibration modes and it's surprisingly difficult to move them above 50 Hz.

We can do a few basic things to maximise stiffness in addition to bracing. For example, we can use a material with a high elastic modulus (Young's Modulus 'E') - baltic birch ply is significantly higher than MDF, for example. Somewhat counter-intuitively, we should use a high 'E' material with low density. This makes more sense when we think in terms of thickness: for two materials with the same 'E' and weighing the same amount per square metre, the thicker one will be stiffer (thicker is always better). All other things being equal, using a denser material actually lowers the resonant frequency of the panel.

I'll bet that there's a vibration engineer somehwere in this forum who could write a better guide to vibration control than I can...

[knifeinthesink]

Here is an outstanding site that has tons of information.

http://personal.cityu.edu.hk/~bsapplec/newpage3.htm

http://personal.cityu.edu.hk/~bsapplec/control.htm

[RandyMidd]

Well, I finished my pour-in CLD speaker mods as planned. I will say it wasn't as easy or as quick as I thought it would be... but nothing is.

I doubt plaster of Paris is very stiff, and is bulky.

Yeah it's stiff alright..it dosen't have a lot of tensile strength, it shouldn't be needed, the material (perlite and plaster = Structolite), when cured, reminds me of a dinner plate. Bulky? I applied it on about 1/4" thick. Yeah it would be nice if it was thinner but I was targeting a general rule of 1/2 the thickness of other constraining layer.

I've turned my free time towards building my sonosubs...I'l report my impressions of the cabs under high SPLs when they're painted and done.