# beneficial to dynamat the inside of sub box??



## KILROZ (May 20, 2008)

is it beneficial to put dynamat on the inside of a sub box?? i was reading that the sub can create resonance in the box and cause distortion, or somethin like that. 

i have a pretty decent custom box and i'm waiting on a new 12" idMAX to get in. wanted to get the box and everything ready for when it arrives!


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## BEAVER (May 26, 2007)

http://www.diymobileaudio.com/forum/showthread.php?t=34692


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## KILROZ (May 20, 2008)

well after reading that thread, it seems that there's mixed opinions on whether or not to put a lining in the box. 

do you think it'd hurt to line the inside with dynamat xtreme?? i'd give it a try and report back, unless ya think it'd be a total waste of time!


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## BEAVER (May 26, 2007)

I think it's a waste of time... but as you can see, opinions seem to differ. 

Just the other day a guy from Sequin lumber told me that one of the local car audio shops lines the inside of all of their enclosures with epoxy claiming it makes the subs "hit harder, man".


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## AUr6 (Apr 10, 2007)

Zaph Audio said:


> For damping vented enclosures, I use Whispermat, Sonic Barrier or 1/2" carpet padding doubled up. Whispermat is the best value. Sonic Barrier is expensive but occasionally handy since it comes with adhesive and is sold in small (but expensive) quantities. Carpet padding is surprisingly effective and cheap but it varies in performance. I never use egg crate foam because it just plain does not perform well. For sealed, I feel that "Acousti-Stuff" is the best performing, but standard craft store dacron pillow fiber will do oK for about 1/3 the cost.



Got quote from here: http://www.zaphaudio.com/mantras.html
granted, it's for home audio speakers, but it's something... 

I guess it's appropriate to remember what problem yo'ure trying to solve. Are you worried about your box flexing? I'd consider bracing or other ways besides adding a little piece of dynamat. I can't imagine dynamat would have a huge impact being applied to MDF compared to a big improvement when it's applied to sheetmetal. 

Worried about resonance or standing waves inside? dynamat probably isn't a solution for that either... just my thoughts though...


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## KILROZ (May 20, 2008)

BEAVER said:


> I think it's a waste of time... but as you can see, opinions seem to differ.
> 
> Just the other day a guy from Sequin lumber told me that one of the local car audio shops lines the inside of all of their enclosures with epoxy claiming it makes the subs "hit harder, man".


see i was kinda under that impression too... i thought that lining the box with epoxy or fiberglass or even dynamat would make the sub hit harder. 



AUr6 said:


> I guess it's appropriate to remember what problem yo'ure trying to solve. Are you worried about your box flexing? I'd consider bracing or other ways besides adding a little piece of dynamat. I can't imagine dynamat would have a huge impact being applied to MDF compared to a big improvement when it's applied to sheetmetal.
> 
> Worried about resonance or standing waves inside? dynamat probably isn't a solution for that either... just my thoughts though...


no, not worried about box flexing. it's 3/4" thick, and sturdy as a mofo. it's a sealed enclosure, and i was worried about the resonance inside the box.


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## Abaddon (Aug 28, 2007)

KILROZ said:


> see i was kinda under that impression too... i thought that lining the box with epoxy or fiberglass or even dynamat would make the sub hit harder.
> 
> 
> 
> no, not worried about box flexing. it's 3/4" thick, and sturdy as a mofo. it's a sealed enclosure, and i was worried about the resonance inside the box.


Well, the resonance would be due to the box flexing.. so if you don't think it will flex.. then you have no worries.


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## AUr6 (Apr 10, 2007)

KILROZ said:


> see i was kinda under that impression too... i thought that lining the box with epoxy or fiberglass or even dynamat would make the sub hit harder.


How? what does either material do to somehow improve the enclosure? 



If you're worried about resonance, you could make sure you're encloure isn't creating standing waves (don't make a cube) or maybe put extra strips of MDF in the seams to get rid of the 90º angles like here... not sure if the extra strips help though.

Honestly, I'd just shove polyfil in there at most and see what you get... maybe experiment with a few different options and see if you notice a difference.


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## BEAVER (May 26, 2007)

The shop that uses the epoxy method swears that some of the sound is absorbed by the mdf...


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## Abaddon (Aug 28, 2007)

BEAVER said:


> The shop that uses the epoxy method swears that some of the sound is absorbed by the mdf...


I would argue that MDF is indeed porous and airchange can occur through the MDF creating pressure losses. But music is not a "slow" process. It is, infact, isothermal in that it's a quick process, and as such the porousness of the MDF should not come into play.... ie.. MDF is just fine...


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## XC-C30 (Jul 24, 2007)

You need some kind of absorbtion to get rid of standing waves. The fact that epoxying the inside of the enclosure makes it "hit harder" is because of reflection being created (IE fake detail). And yes, MDF DOES absorb the sound. But is it worth mentioning? In some cases, yes, In others, nope. depends on the sub being used and the sound you are after. But in general it doesn't make taht big of a difference.


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## KILROZ (May 20, 2008)

XC-C30 said:


> You need some kind of absorbtion to get rid of standing waves. The fact that epoxying the inside of the enclosure makes it "hit harder" is because of reflection being created (IE fake detail). And yes, MDF DOES absorb the sound. But is it worth mentioning? In some cases, yes, In others, nope. depends on the sub being used and the sound you are after. But in general it doesn't make taht big of a difference.


using a 12" IDmax in a sealed enclosure. looking for SQ, tight and accurate bass but i still want that thing to pound


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## BEAVER (May 26, 2007)

I'd put it in 1.5cft, heavily braced, with a double layered baffle and call it a day.

Later you could experiment with blocks of wood to reduce volume or polyfill to create the illusion of more volume, if you felt the need.


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## BlueSQ (Mar 22, 2007)

I would be seriously surprised if you noticed any difference with the car off, let alone w/ the engine running. Save your $, buy a better sub or more power.


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## 60ndown (Feb 8, 2007)

real easy to know for sure,

run it for a week with no stuffing / deadening.

then add some,

see if you hear a difference.


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## ogahyellow (Apr 16, 2007)

the stuffing will change the system Q right, so you _should_ be able to hear a difference? There are 2 ways to kill the resonance -- adding stiffness (ply) or adding mass (mdf). Epoxy + mdf is doing the weight + stiffness.


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

Abaddon said:


> I would argue that MDF is indeed porous and airchange can occur through the MDF creating pressure losses. But music is not a "slow" process. It is, infact, isothermal in that it's a quick process, and as such the porousness of the MDF should not come into play.... ie.. MDF is just fine...


I know for a fact that MDF IS porous, I mean not to waht we are doing with it, but back in the day when I was making Pre-fab boxes for Orion, we had a CNC we used to cut the baffels, and it used a rather large vacuum pump to hold parts to the table... You could lay a piece of paper on the MDF and watch it get sucked down, THROUGH the MDF. 

But like you said music is not a slow process, otherwise EVERYONE would have a AP enclosure...


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## Mr Marv (Aug 19, 2005)

Sorry I can't be more helpful but some days I read sh!t that just makes my brain hurt and then I can't form a coherent sentence.


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## 60ndown (Feb 8, 2007)

Mr Marv said:


> Sorry I can't be more helpful but some days I read sh!t that just makes my brain hurt and then I can't form a coherent sentence.


lol..


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## Oliver (Jun 25, 2007)

Some want to leave nothing to chance...

For example, me 










I sealed the MDF with polyurethane, then coated it with undercoating.

Will i notice standing wave with 10 ft, 20 ft, ...40 ft long notes, NO, is that what i was addressing, No.

Will the wood last longer enclosed in plastic, YES.

Will it resonate less with all this material covering it, MAYBE

Try bouncing a ball on a hard surface, now a carpet...


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## XC-C30 (Jul 24, 2007)

a$$hole said:


> Try bouncing a ball on a hard surface, now a carpet...


Sorry.... it bounces back

Now if you'd have said a pillow or something, or a blanket that's folded a couple of times.....


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## WRX/Z28 (Feb 21, 2008)

I fiberglassed the inside of my enclosure. I figured A: we had some extra from a fiberglass box we were making. B: what could it hurt and C: my enclosure would be perfectly sealed, including the terminal cup, which is notorious for leaking.


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## BlackSapphire (Apr 16, 2008)

WRX/Z28 said:


> I fiberglassed the inside of my enclosure. I figured A: we had some extra from a fiberglass box we were making. B: what could it hurt and C: my enclosure would be perfectly sealed, including the terminal cup, which is notorious for leaking.


That is the main reason I coat the inside of my boxes as well. It's more to seal it than anything.

If you don't have large side panels, inadequate bracing, etc., I wouldn't worry about trying to use a mass-loader. Properly braced 3/4" MDF doesn't really flex. Like Beaver said, double the baffle and be done. There's no 
quick fix for improper enclosure construction techniques.


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## Attack eagle (Nov 18, 2006)

panel resonance and internal reflectivity can be two different things.
Dynamat isn't going to help much when it is aluminum backed...

I have used UNEVENLY sprayed undercoating inside FG boxes or boxes with parallel face and rear walls to cutdown on reflectivity...
I lot of the turd box subs (kicker especially) people would want installed have poly cones and you can hear the reflections of the backwall thru them.


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## Abmolech (Nov 2, 2006)

This is becoming truly sad.
Standing waves require velocity, (basically every quarter length is possible), a suitable angle of incidence and to be in the same space.

Like a fire triangle, without one of three sides, you don't get standing waves.

http://www.kettering.edu/~drussell/Demos/superposition/superposition.html

http://www.sengpielaudio.com/calculator-roommodes.htm

Point
Standing waves below 60 Hz are not a problem inside a car, let alone a sub box.

Poly fill etc?

As mentioned this will change your compliance (Q), and depending on the amount, how audible this may become.

Point
Polyfill etc can make an audible difference, but remember there is no free lunch, and the some of back wave fill be turned into heat. (Loss in SPL)

Reflective surface versus absorbing?
Against this is decreed by the coefficient of absorption, and the velocity of the wave at the surface for the amount of attenuation. Clue, the velocity is too low to make an audible difference for sub waves.

Point
Your wasting your money on absorbers and diffusers inside a sub box.

Dampening?
This is a decoupler.(It has some blocking ability) Unless the box will resonate inside the useful bass band of the decoupler, the only use you may gain could come from the increased blocking ability or the slight volume decrease of the enclosure.

I suggest 12 dB attenuation of a blocker is sufficient to be useful. (Consider a 12 dB crossover filter) The volume change is inconsequential to be of a suitable change to cause an audible outcome.

Point
Dampening is another waste of money in this area. If you were going for minuscule gains in SPL, there may be some use in considering a different enclosure etc. 0.1 dB is barely audible, and this is perhaps what you may be chasing with "an over built enclosure".


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