# Fiberglass compared to MDF enclosures ....



## JAG (May 6, 2006)

Just had a customer ask me TWO very interesting questions ..... Something I don't hear very often coming from a consumer ....So I thought I'd post it up here and see what kind of comments stem from it. I gave him the answers , but will withold what I told him until you guys post up some replies ....
*Question # 1* - What kind of inert resonance does fiberglass sub enclosures impose compared to 3/4 MDF enclosures ( He's talking sealed ) ..... Since this is dependant to _some_ degree on the thickness of the fiberglass , I'll head straight into the second part of his question ..... *Part two :* How much thickness with fiberglass does it take to equal the acoustic properties of 3/4" MDF ....
Good stuff coming from a customer huh ?


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## AzGrower (May 30, 2005)

#1since the resin once its cured creates totally air tight structure, it reflects all sound. It absorbs none unlike mdf which has air pockets and is not totally airtight compared to fiberglass. Of course this is based on pure resonance characteristics and not strength

#2 I have to say it all depends on the type of cloth you use. If its woven kyntex or cloth then 1/4" thicke is pretty strong. If you use something like chop matting, then per square inch it may not be as strong and you might need up to 1/2". Also, the fiberglass will flex more than the mdf, but the strength of it should be the same

Am I right? I just thought of a total ******** answer based on the little I know about fiberglassing


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## Derek (Jun 4, 2005)

i've always heard like 3/8" fiberglass is good to go....


it's best used in curvy parts....not flat parts....


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## B&K (Sep 20, 2005)

It will all depend on the "fabricator". Resonance is based on the material properties of either component and the tension that the parts are under--this includes both the properties of the materials and the design of the strucutre. You could easily fabricate two identical structures with the exact amount of fiberglass with different resonances, mdf should however be a lot more consistent of course per unit shape/size. Both are fairly easy to measure though--if you want comparisons, send me some to measure and I will do it for you.


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## JAG (May 6, 2006)

B&K said:


> It will all depend on the "fabricator". Resonance is based on the material properties of either component and the tension that the parts are under--this includes both the properties of the materials and the design of the strucutre. You could easily fabricate two identical structures with the exact amount of fiberglass with different resonances, mdf should however be a lot more consistent of course per unit shape/size. Both are fairly easy to measure though--if you want comparisons, send me some to measure and I will do it for you.


Great answer ..... The fact is , NO amount of fiberglass can achieve the relative lack of resonance frequencies that MDF has. Even if you made the fiberglass 2" thick , it will still have a higher resonance tendency than MDF will. I have heard VERY well crafted fiberglass enclosures that were at least 3/4" thick , and if you rapped your knuckles on the side it would still make a " hollow " sound. Almost like a high pitched Bongo drum ..... 
Strength and rigidity is not the issue , rather resonant frequency is ....


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## B&K (Sep 20, 2005)

Of course whatever combination gives you the "stiffer" combo the resonant frequency should climb.


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## Finleyville (Jun 17, 2005)

Why is the resonant frequency so important anyway?


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## JAG (May 6, 2006)

Finleyville said:


> Why is the resonant frequency so important anyway?


Different resonant frequncies will make the sub sound different. In a perfect world , a sub enclosure should have ZERO resonance. Be acoustically inert. Dead in other words. Manufacturers have experimented with nearly every material ,from concrete ( thiele ) to exotic hard woods ..... The best speakers are generally made from VERY high grade MDF .....


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## chuyler1 (Apr 10, 2006)

AVI said:


> The best speakers are generally made from VERY high grade MDF .....


Really? An MDF speaker?

j/k

I have noticed in my building that fiberglass adds a certain color to the bass. I couldn't classify it as good or bad, just different.


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## JAG (May 6, 2006)

chuyler1 said:


> Really? An MDF speaker?
> 
> j/k
> 
> I have noticed in my building that fiberglass adds a certain color to the bass. I couldn't classify it as good or bad, just different.


Exactly .... and that different sound is often different with different drivers ...


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## Oak244 (Apr 7, 2006)

Would you fix the problem in a Fiberglass inclosure by adding a sound deadening material to it? Not a Aluminum backed one, but more absorbant one. I know my VMPS sub enclosure was treated inside with something to reduce res.


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## B&K (Sep 20, 2005)

As I stated before adding mass will raise the resonant frequency, so when you deaden the res freq goes up. As for absorbing sub frequencies with a layer, good luck you may kill some really high harmonics but not the natural frequency you are playing.


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## jperryss (Mar 15, 2006)

B&K said:


> As I stated before adding mass will raise the resonant frequency, so when you deaden the res freq goes up...


I thought it was the other way around? Doesn't adding mass lower the resonant frequency, attempting to move it out of the audible range? I would think if you added enough mass using sound-deadener or something similar you could all but eliminate the resonances.

-jperryss


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## JAG (May 6, 2006)

jperryss said:


> I thought it was the other way around? Doesn't adding mass lower the resonant frequency, attempting to move it out of the audible range? I would think if you added enough mass using sound-deadener or something similar you could all but eliminate the resonances.
> 
> -jperryss


There is more than just one resonant frequency when dealing with a sub enclosure. There is the freq of the material itself , or it's own resonant frequencies. You also have the constantly changing frequencies the enclosure makes as vibrations from the car , the woofer movement itself , and any wave/reflection issues dealing with air waves and pressure inside the enclosure itself ......
MDF is affected less by these variables than any other material suitable for a sub enclosure. That's why it is considered so acoustically inert. Changing the size of the MDF enclosure has little affect on the resonant freq. But with fiberglass enclosures , changing the size , thickness , shape , ect of the enclosure is much like a drum head on different sized drums. It all makes a different sound , which is why it is hard to predict just how a sub enclsure is going to sound when made from fiberglass ....


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## B&K (Sep 20, 2005)

jperryss said:


> I thought it was the other way around? Doesn't adding mass lower the resonant frequency, attempting to move it out of the audible range? I would think if you added enough mass using sound-deadener or something similar you could all but eliminate the resonances.
> 
> -jperryss


I should have used the word stiffness, but in this case adding mass = adding stiffness which raises the natural frequencies <--plural this time as indeed there will be many.


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## DS-21 (Apr 16, 2005)

AVI said:


> The best speakers are generally made from VERY high grade MDF .....


That would shock everyone using Baltic Birch...

But seriously, resonance should be a non-issue IF you build the glass up to about a cm thick and _brace the hell out of your enclosure_. The bracing bit applies equally well to MDF enclosures. The bare minimum, IMO, is corner reinforcement and braces that connect all walls to each other. Tannoy's consumer and pro lines as well as KEF and other loudspeaker makers also tend to apply bracing immediately behind the driver in their top speakers connected to it by a poster-tak like putty compound. I've started doing that on all my subs recently, and I think (no blind testing done) it results in an improvement. For woofers with pole vents, I tend to use dowels with a few rings between them stiffening the brace. On woofers such as the Peerless XLS line with solid backplates, I'll build a rib in the enclosure as Tannoy does with their pro enclosures. Obviously, that limits future enclosure flexibility to woofers with equal or lesser depth, but otherwise there are few other drawbacks.


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## JAG (May 6, 2006)

DS-21 said:


> That would shock everyone using Baltic Birch...
> 
> But seriously, resonance should be a non-issue IF you build the glass up to about a cm thick and _brace the hell out of your enclosure_. The bracing bit applies equally well to MDF enclosures. The bare minimum, IMO, is corner reinforcement and braces that connect all walls to each other. Tannoy's consumer and pro lines as well as KEF and other loudspeaker makers also tend to apply bracing immediately behind the driver in their top speakers connected to it by a poster-tak like putty compound. I've started doing that on all my subs recently, and I think (no blind testing done) it results in an improvement. For woofers with pole vents, I tend to use dowels with a few rings between them stiffening the brace. On woofers such as the Peerless XLS line with solid backplates, I'll build a rib in the enclosure as Tannoy does with their pro enclosures. Obviously, that limits future enclosure flexibility to woofers with equal or lesser depth, but otherwise there are few other drawbacks.


Are you speaking of solid Birch , or Birch plywood ? I've seen a lot of people using birch plywood ..... This is just terrible. Plywood is one of the most inconsistent wood based materials on earth. It has air pockets everywhere , different densities , and layers that do not always stay together well. Using solid Birch will work , but will have MUCH more tendency to display it's own resonant frequencies ..... Just about every type of hardwood has been experimented with by manufacturers , and MDF is preferred due to t's non resonant properties ...... But I'm sure whatever each person is using , it is always going to be " the best "


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## B&K (Sep 20, 2005)

13 ply birch is pretty much as dead as mdf


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## AzGrower (May 30, 2005)

what about 13 ply balsa wood?


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## chuyler1 (Apr 10, 2006)

I built a box out of 3/4" birch ply once. I wasn't impressed. The supposed weight savings was minimal and it is more expensive so I've been sticking with 3/4" MDF ever since.


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## AzGrower (May 30, 2005)

I usually build my boxes out of 3/4" mdf on the sides and rear wall, but the baffle I use 1.5" mdf. What this does is allow me to flush mount the sub by using a rabbetting bit (also known as flush bit) to create a recess. It does a hell of a job supporting the weight of the subwoofers as well.


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## DS-21 (Apr 16, 2005)

AVI said:


> Are you speaking of solid Birch , or Birch plywood ? I've seen a lot of people using birch plywood ..... This is just terrible. Plywood is one of the most inconsistent wood based materials on earth. It has air pockets everywhere , different densities , and layers that do not always stay together well.


Good void-free 13-layer 19mm Baltic Birch ply is hard to find. HD and Lowes don't carry it. But it does exist, and lots of extremely high end speakers are made out of it for both the consumer market and the professional market.



AVI said:


> But I'm sure whatever each person is using , it is always going to be " the best "


Oh, I use MDF for boxes (generally two 19mm pieces with a layer of Liquid Nails between for baffles, and lots of bracing as discussed above) with regular sides and MDF+glass for boxes with irregular shape exclusively. But especially for subs, the superior stiffness of Baltic Birch ply has some theoretical advantages. The difference, to me, is not worth the added cost. Now, if Corian countertop material ever comes down to the price of Baltic Birch ply I'll switch to that in a heartbeat for my home audio stuff...


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## lbridges (Jan 4, 2006)

DS-21 said:


> ...snip. Now, if Corian countertop material ever comes down to the price of Baltic Birch ply I'll switch to that in a heartbeat for my home audio stuff...


Great stuff IMO.

The last three sets of door baffles I built I used Corian, - used the sink cutouts - scraps from the local kitchen counter-top shop. No worries about moisture effects using MDF, etc. Can't beat the price (all it involved was a bit of dumpster diving).


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## ClinesSelect (Apr 16, 2006)

This is the Corian center console I built for my Dodge Ram. It held two Brahma 10s.


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## chuyler1 (Apr 10, 2006)

MMMM, so clean I could eat off it.


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## B&K (Sep 20, 2005)

Or just prepare your food on it. How do you cut/glue that stuff together?


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## ClinesSelect (Apr 16, 2006)

You cut and router it just like it was MDF. You seam it with Corian Joint Adhesive which fuses the pieces together. 

The hardest part was keeping it shiny. 

Just built:











After way too much time spent sanding and polishing:


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## DS-21 (Apr 16, 2005)

You wouldn't happen to know anyone in Georgia with access to Corian and an inclination to DIY audio, would you?


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## ClinesSelect (Apr 16, 2006)

DS-21 said:


> You wouldn't happen to know anyone in Georgia with access to Corian and an inclination to DIY audio, would you?


I do not know of anyone who fits that description. Sorry.


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## lbridges (Jan 4, 2006)

DS-21 said:


> You wouldn't happen to know anyone in Georgia with access to Corian and an inclination to DIY audio, would you?


If you're just looking for a Corian source - most any kitchen counter-top place can get the stuff....you just have to be willing to pay like $30 a sq ft for raw materials...they'll sell it like that as long as you say it's for a speaker cabinet - at least they will locally. The joint material is just color matched epoxy (or so they told me), and like mentioned above - it cuts & shapes like wood - well like solid maple anyway, not soft pine.


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## chuyler1 (Apr 10, 2006)

Does it weigh the same as MDF?


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## ClinesSelect (Apr 16, 2006)

Close to or a touch heavier. That box loaded was over 100 pounds.


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## JAG (May 6, 2006)

Jim Thiele used a 2" concrete ( special blend ) front baffle on his CS-7 Thiele home speakers. He mated it with 1.5" MDF on the other sides of the speaker. That thing was DEAD .... Mass definitely helps ....
As far as no-gap plywood ..... NO-ONE knows what is really between those layers


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## Brsanko (Nov 27, 2009)

AVI said:


> Plywood is one of the most inconsistent wood based materials on earth...different densities , and layers that do not always stay together well. MDF is preferred due to t's non resonant properties ...... But I'm sure whatever each person is using , it is always going to be " the best "


Okay first of all "inconsistancy" is what gives MDF it's "non resonant properties" and the reason solid wood is bad is because of it's consistancy. Baltic birtch plywood is considered by many to be the primium material for speaker boxes. It has many layers that are very tightly bonded i.e. minimal air pockets. So according to many it is superior to MDF, but not by much and it is far more expensive.


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## Brsanko (Nov 27, 2009)

Oh and I almost forgot my question. A sandwich of fiberglass and some type of nonresonant foam I would think would make for a very versital and nonresonant construction. any comments? or you could even encorperate MDF into the "large panels" of your fiberglass structure.


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## 03blueSI (Feb 5, 2006)

lbridges said:


> Great stuff IMO.
> 
> The last three sets of door baffles I built I used Corian, - used the sink cutouts - scraps from the local kitchen counter-top shop. No worries about moisture effects using MDF, etc. Can't beat the price (all it involved was a bit of dumpster diving).


I believe that Wilson Loudspeakers makes the Watt Puppy and some of their other speakers out of Corian.


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## sam3535 (Jan 21, 2007)

Brsanko said:


> any comments?


Really, only one: this thread is 3 1/2 years old.


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## jon w. (Nov 14, 2008)

gentlemen,

i stumbled upon this thread only moments ago. part 5 in my series of articles entitled, ""the car stereo nobody would build" answers most of your questions directly. please visit my website at whitledgedesigns.com and download part 5. the answer will surprise you!

best regards,
jon


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