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Damping Materials for Subs???

post #1 of 27
12/23/10 at 9:21am
Thread Starter 
I'm trying to learn a bit more about damping materials used for subwoofer enclosures. So far my reading has lead me to believe damping is used for two purposes... dampen enclosure reasonances and to counter standing waves. Different purposes mean using different materials. Also it seems the effective enclosure size and tuning are affected to some extent. So I'm wondering are there different strategies used in ported enclosures vs sealed boxes? Any info would be appreciated as I know very little about this subject at this point and would like some input from the 'experts' and experienced folks here since this is after all the DYI forum. Thanks ya'll.
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post #2 of 27
12/23/10 at 9:26am
Damping is used in vented boxes to absorb midrange reflections that otherwise would reflect back to the cone and/or out the port, causing response zits. Even if it's a sub damping midrange is still a necessity, as the movement of the cone creates midrange harmonics. The same applies to sealed cabs, and additionally those may be stuffed to reduce Q when the box is too small. Standing waves are seldom an issue as most cabs sizes are too small to support them.
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post #3 of 27
12/23/10 at 11:45am
What should be used, fiberglass or that polyfill stuff? Or is there something else?

I am building 4 sealed subs with (2) 15"s each. Box sizes are:

12 1/2" x 21 1/2" x 69 1/2"

20" x 22 1/2" 40"
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post #4 of 27
12/23/10 at 1:15pm
Quote:
Originally Posted by UNICRON-WMD View Post

What should be used, fiberglass or that polyfill stuff? Or is there something else?

Those, open cell foam, cotton, wool, felt, there are many choices.
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post #5 of 27
12/23/10 at 1:58pm
Quote:
Originally Posted by Bill Fitzmaurice View Post

Those, open cell foam, cotton, wool, felt, there are many choices.

How thick? 1", 2"?

I have (2) Dayton 15" HF in each box and will be 8.3CuFt.
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post #6 of 27
12/23/10 at 2:20pm
Thread Starter 
Bill thanks for your responses so far. I read somewhere that polyfil and foam also served an isobaric purpose as well... apparently pressure changes within the enclosure will dissipate somewhat as heat (according to the ideal gas law).

Okay so... What about the laminates like no-rez and BH5 are they even necessary? are they better? worth those high asking prices? Are their cheaper subsitutes or alternates? How do you use damping for tuning purposes? How do you know when you've added too much? What parameters does one measure or what do you listen for? Again, as always, much thanks.
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post #7 of 27
12/23/10 at 2:30pm
You can increase the apparent volume by about 20%. It should fill the volume without being packed, just barely compressed.
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post #8 of 27
12/23/10 at 3:00pm
Quote:
Originally Posted by dwomd View Post

You can increase the apparent volume by about 20%.

No, you can't. That erroneous conclusion was drawn a few years back based on incomplete data.
The amount of fill with a sealed box depends on the system Q. The higher the box Q above roughly 0.8 the more stuffing you need, and the more result it will have on lowering Q. OTOH with Q of roughly 0.8 and below stuffing has no effect, and just lining the cab to damp midrange reflections is sufficient.

Quote:


I read somewhere that polyfil and foam also served an isobaric purpose as well... apparently pressure changes within the enclosure will dissipate somewhat as heat (according to the ideal gas law).

Heat is a by-product of the process, not the cause. The cause is the difference in the acoustic impedance load offered by a stuffed cavity versus unstuffed. The stuffing has a much higher friction coefficient than air, so when compressed and decompressed the friction between the stuffing particles results in much more heat than air.
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post #9 of 27
12/23/10 at 5:30pm
Quote:
Originally Posted by Bill Fitzmaurice View Post

Those, open cell foam, cotton, wool, felt, there are many choices.

If you REALLY want to absorb something do not use polyfill, loose cotton batting or plain old open cell foam. They all do next to nothing compared to dense fiberglass or better still, Bonded Logic cotton insulation. Much higher STCs than foam or polyfill.

Greg
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post #10 of 27
12/23/10 at 6:55pm
Quote:
Originally Posted by gbegland View Post

If you REALLY want to absorb something do not use polyfill, loose cotton batting or plain old open cell foam. They all do next to nothing compared to dense fiberglass or better still, Bonded Logic cotton insulation. Much higher STCs than foam or polyfill.

Greg

Gerry Koonce measured the effects of all the various damping materials a few years back, the results were published in Speaker Builder or AudioXpress (I don't recall which). Rigid type 700 series fiberglass does have the highest resistivity index, but not by a huge margin.
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post #11 of 27
12/24/10 at 7:36am
Quote:
Originally Posted by Bill Fitzmaurice View Post

No, you can't. That erroneous conclusion was drawn a few years back based on incomplete data.
The amount of fill with a sealed box depends on the system Q. The higher the box Q above roughly 0.8 the more stuffing you need, and the more result it will have on lowering Q. OTOH with Q of roughly 0.8 and below stuffing has no effect, and just lining the cab to damp midrange reflections is sufficient.

Ok, with my (2) Dayton Ref HF 15"s in one sealed 8.3 cuft. Do you know what the Q is? I don't have or use a measuring program.

I can get some poly fill or fiberglass somewhere.

I also have alot of extra OC 703 ridged fiberglass. Would that work or be a waist?
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post #12 of 27
12/24/10 at 7:53am
Quote:
Originally Posted by Bill Fitzmaurice View Post

No, you can't. That erroneous conclusion was drawn a few years back based on incomplete data.
The amount of fill with a sealed box depends on the system Q. The higher the box Q above roughly 0.8 the more stuffing you need, and the more result it will have on lowering Q. OTOH with Q of roughly 0.8 and below stuffing has no effect, and just lining the cab to damp midrange reflections is sufficient.

Heat is a by-product of the process, not the cause. The cause is the difference in the acoustic impedance load offered by a stuffed cavity versus unstuffed. The stuffing has a much higher friction coefficient than air, so when compressed and decompressed the friction between the stuffing particles results in much more heat than air.

Informative, thank you Bill.

For lining the walls, simply damping resonance, not stuffing, what is a cheap solution? Any links to buying large quanities would be greatly appreciated
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post #13 of 27
12/24/10 at 8:34am
What's the preferred method of securing the fill?
Staples, glue, netting?
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post #14 of 27
12/24/10 at 9:28am
Quote:
Originally Posted by UNICRON-WMD View Post

Ok, with my (2) Dayton Ref HF 15"s in one sealed 8.3 cuft. Do you know what the Q is? I don't have or use a measuring program.

How did you design the cab?
I'm sure someone can do it for you, my schedule is a bit tight to do simulations.


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For lining the walls, simply damping resonance, not stuffing, what is a cheap solution

Felt carpet padding. Go dumpster diving at a carpet store, get all you want for free.
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What's the preferred method of securing the fill?
Staples, glue, netting?

Any of the above. You do want to keep the driver structure clear, for ventilation. Cross-bracing the cab, which should be done anyway, to create a virtual cage around the driver is a good way to keep stuffing away.
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post #15 of 27
12/24/10 at 10:03am
Quote:
Originally Posted by Bill Fitzmaurice View Post

Felt carpet padding. Go dumpster diving at a carpet store, get all you want for free.
Any of the above. You do want to keep the driver structure clear, for ventilation. Cross-bracing the cab, which should be done anyway, to create a virtual cage around the driver is a good way to keep stuffing away.

Now just for the cab's walls, would a foam bed pad work? Something found at WalMart for $15? Or something more dence?
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post #16 of 27
12/24/10 at 11:16am
Quote:
Originally Posted by UNICRON-WMD View Post

Now just for the cab's walls, would a foam bed pad work? Something found at WalMart for $15? Or something more dence?

Just fine, in fact.
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post #17 of 27
12/24/10 at 12:00pm
Quote:
Originally Posted by Bill Fitzmaurice View Post

Just fine, in fact.

Great, my Q is right at .791.

I'll see what I can find at WallyWorld. I plan just lining the walls but I think I'll do it while building the sub. Should be much easier.
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post #18 of 27
12/24/10 at 12:10pm
I stuff my ported sub with the poly stuff from wally world at about 3 bucks a bag. Its seems to work well, but some of the of it will blow out the port tubes.
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post #19 of 27
12/24/10 at 1:10pm
Quote:
Originally Posted by pocketcash View Post

I stuff my ported sub with the poly stuff from wally world at about 3 bucks a bag. Its seems to work well, but some of the of it will blow out the port tubes.

Ported cabs should only be lined, not stuffed. Doing so will alter the cab tuning.
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post #20 of 27
12/24/10 at 4:15pm
Well, the checkout girl at wally world clearly said this is the stuff to use when I asked about ported subs.
If I have any problems or any other reason to open it up, I will try lining the walls. Thanks
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post #21 of 27
12/26/10 at 8:38am
Well it happened, I blew the driver, PYLE PLQB12 12-Inch 600 Watt Bandpass using a crown xls 1000 bridged. I knew the pyle was cheap crap @ 50 bucks, but with its 600 watt logo, I thought it would of held up to 1000 watt output of the crown. I'm glad it fried, I replaced it with a Polk DXI124, it sounds so much better and deeper, no more speaker rattles, thus no need to over stuff the box. I never learn, I got to stop being so cheap, and to remember I get what I pay for.
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post #22 of 27
12/26/10 at 9:16am
Quote:
Originally Posted by pocketcash View Post
I'm glad it fried, I replaced it with a Polk DXI124, it sounds so much better and deeper, no more speaker rattles, thus no need to over stuff the box. I never learn, I got to stop being so cheap, and to remember I get what I pay for.

With the amp you're using, you're likely to fry that Polk pretty soon as well. It's not just the sub drivers, it's the box you're using them in too.
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post #23 of 27
12/26/10 at 9:44am
Quote:
Originally Posted by pocketcash View Post
with its 600 watt logo, I thought it would of held up to 1000 watt output of the crown. .
A driver's thermal rating says nothing about how much power it can make use of, that's the product of the driver displacement and the cab it's loaded into. By the same token an amp's rated output, measured at a very low THD percentage, says nothing about how much power it will put out when pushed to even moderate distortion levels, let alone full out clipping.
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post #24 of 27
12/26/10 at 10:31am
Quote:
Originally Posted by Erich H View Post

With the amp you're using, you're likely to fry that Polk pretty soon as well. It's not just the sub drivers, it's the box you're using them in too.

The box is ported, thus the resistance should be low, all the power is going to cone / air movement, so heat build-up shouldn't be as high a sealed system. Polk sells the same driver in a ported box, a much better build quality than the plye, but not much bigger, so I see no reason why it shouldn't last.


Quote:
Originally Posted by Bill Fitzmaurice View Post

A driver's thermal rating says nothing about how much power it can make use of, that's the product of the driver displacement and the cab it's loaded into. By the same token an amp's rated output, measured at a very low THD percentage, says nothing about how much power it will put out when pushed to even moderate distortion levels, let alone full out clipping.

I have the clip protection turned on, always did from the first day. The drivers db per watt is not rated, but I'm sure pyle specs are more BS and less fact.
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post #25 of 27
12/26/10 at 1:58pm
Quote:
Originally Posted by pocketcash View Post

The box is ported, thus the resistance should be low, all the power is going to cone / air movement, so heat build-up shouldn't be as high a sealed system. Polk sells the same driver in a ported box, a much better build quality than the plye, but not much bigger, so I see no reason why it shouldn't last.

That is not how ported subs work. If the air in the sealed box compresses and rebounds like a spring as the driver moves in and out, then the air in the port acts like a weight on the spring which causes it to resonate at a certain frequency, so the box is tuned to that resonant frequency, which is usually just below the free air resonance of the driver. The "mass" on the "spring" limits driver excursion while helping maintain a flatter response, and below tuning the port causes rolloff. The resistance of the spring is determined by the size of the box, smaller box, stiffer spring, larger box less stiff spring.
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post #26 of 27
12/26/10 at 3:26pm
Quote:
Originally Posted by dwomd View Post

That is not how ported subs work. If the air in the sealed box compresses and rebounds like a spring as the driver moves in and out, then the air in the port acts like a weight on the spring which causes it to resonate at a certain frequency, so the box is tuned to that resonant frequency, which is usually just below the free air resonance of the driver. The "mass" on the "spring" limits driver excursion while helping maintain a flatter response, and below tuning the port causes rolloff. The resistance of the spring is determined by the size of the box, smaller box, stiffer spring, larger box less stiff spring.

I wasn't doing a sealed vs. port thing. At this point I just want the sub driver to last, and not burn out. I was just wanted bring up the point that a ported sub should run a little cooler and needs little less power than its sealed counter part, if there was one. I think overall a sealed sub does sound better and won't have that port noise which can be such an embarrassment if your showing-off. I'm sure I'll be in the market for a good heavy sealed box real soon. The PYLE was just to get things started for me, but the PYLE driver didn't last, so now I have to pick-up the pace.
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post #27 of 27
12/26/10 at 6:25pm
Itchy, scratchy, fiberglass insulation...... KG
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