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Damping Material In Subs. Why Bother?


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36 replies to this topic

#1 of 37 OFFLINE   Travis_G

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Posted October 28 2005 - 02:01 PM

The subwoofer I'm building will be 12 ft^3 and it's longest dimension will be 4 ft. The lowest standing wave mode will be just above 140 Hz. If the 24dB/octave xo is at 80 Hz then the response will be ~ 20 dB down @ 140 Hz. Obviously it's big box, and a smaller one would have it's lowest standing wave mode at an even higher frequency. So knowing all this, why even bother adding damping material, unless it is being used to make the enclosure perform like a larger one? Travis

#2 of 37 OFFLINE   PaulDF

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Posted October 28 2005 - 03:45 PM

You pretty much nailed it, as far as I know.

#3 of 37 OFFLINE   ThomasW

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Posted October 29 2005 - 02:33 AM

The use of damping in a subwoofer has nothing to do with the elimination of standing waves. If you want to find out why it's used build your box and run it without damping. Then install the recommended amount of damping for your alignment. I'm reasonably sure you'll be able to figure why it's only omitted in the cheapest subs money can buy.

#4 of 37 OFFLINE   Travis_G

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Posted October 29 2005 - 10:53 AM

Thank you Thomas, but that still does not answer my question. You say that damping material is not added to eliminate standing waves. Fair enough, but why then IS it added?
Are you sure? I thought that Siegfried Linkwitz' THOR did not use any. I could be wrong though. Besides you know engineers, they all suffer from OCD.:P Travis

#5 of 37 OFFLINE   Leo Kerr

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Posted October 30 2005 - 12:57 AM

If I remember the rec.audio faq properly, the big uses of damping material are to change the resonant frequency of the box walls themselves, and/or/by stiffening the walls, with an added benefit of reducing side-of-box emissions in favor of the driver itself, which, I'm really stretching my memory now, improves the efficiency and 'sharpness' of the woofer, because the stuff coming off the sides may not be completely in phase with the air coming off the cone. Don't trust anything in that paragraph, though. Leo

#6 of 37 OFFLINE   Travis_G

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Posted October 30 2005 - 04:31 AM

Thank you Leo, That is true for extensional damping compounds like roofing felt and tar. I am asking about fibrous material like dacron and fiberglass. I should have been more specific. I have never used any damping material in a subwoofer, except a car sub where space was a concern. They all sounded great, but I never heard the same subs with damping material in them. I still have the car sub, the other subs I sold. I could pull the damping material out, but I would change the Q as well making it under damped.
And this subwoofer was crossed at 100 Hz with a 12 dB/oct XO. Internal volume was 13.75 X 13.75 X 13.75 putting it's lowest resonance at almost 500 Hz. Response would be over 24 dB down at that frequency (correction, math was off earlier). Also if standing waves are a concern why make the enclosure a cube? Now I am utterly confused. If I was designing subwoofers commercially I suppose I would put damping material in them; not because I believe it improves performance, but for marketing purposes. Many people expect to have damping material in their subs. I once read Dan Wiggins say that it isn't necessary to use low impedance drivers in cars anymore because amplifiers that can deliver more voltage at a lower cost then has been possible in the past. He said that "market inertia" is the primary reason for the prevalence of 4 ohm systems. Maybe loudspeaker manufacturers put damping material in their subwoofers because of market inertia. Just some food for thought, Travis

#7 of 37 OFFLINE   ThomasW

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Posted October 30 2005 - 06:50 AM

Dickason is an idiot at times.....There are no standing waves in a 14" cube XO'ed at 100Hz. One puts (frictional loss) damping in a sub to absorb the higher frequencies that 'leak' through the crossover. This would be particularily important with Dickason's use of a 2nd order XO. If you don't want to do this fine. Any sub I've ever built (I've been building them since the mid 1970's) has sounded better with some damping material added. Since you haven't specified the alignment of your box, recommendations about damping are a bit difficult.

#8 of 37 OFFLINE   Travis_G

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Posted October 30 2005 - 08:12 AM


I don't mind doing it if there is a valid reason to. I just want to understand what that reason is. To be perfectly honest, I am still not convinced that there is a reason for fibrous damping material in subwoofers. However, if I am wrong than I want to know. This forum is after all a place to share knowledge. Posted Image

Travis

#9 of 37 OFFLINE   ThomasW

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Posted October 30 2005 - 10:50 AM

Fine by me, I don't have a responsibility to prove anything. And obviously it's easier post dozens of questions instead of taking an hour to run an experiment.

#10 of 37 OFFLINE   Travis_G

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Posted October 30 2005 - 11:57 AM

Absolutely, I wouldn't expect you to. And you are right, it is easier as well as cheaper to ask the questions here. After all, isn't that what this forum is here for? Besides that, some other folks may have the same question that I do. If you have some reference for me to read I would be happy to read it. If you don't have any, or don't feel like digging through your sources, that's fine too. I won't hold it against you. Thank you, Travis

#11 of 37 OFFLINE   ThomasW

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Posted October 30 2005 - 03:08 PM

That statement is simply not true.

Damping material like fiberglass/dacron/whatever, is use to attenuate sound. Regardless of where you put it sound is attenuated. In this instance it's put inside the enclosure, to absorb the higher frequencies that aren't fully attenuated by the XO and are being reproduced by the driver.

Good luck with your project.....

Regards
Thomas

#12 of 37 OFFLINE   Travis_G

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Posted October 30 2005 - 04:25 PM

Sorry Thomas, I edited right after your post to make mine clearer.
Let me clarify. In full range speakers, standing waves inside the enclosure as well as waves reflected from the enclosure walls may be reflected out through the cone of the speaker. This WILL cause response deviations as many of the wavelengths will be comparable to the enclosure's size. At some frequencies the reflections will combine constructively and at other frequencies the reflections will combine destructively thus causing response deviations. However, this can ONLY happen when a wavelength is double any inside dimension of the enclosure and every multiple of that frequency. In addition sound waves reflecting of the enclosure walls will be in-phase with the drivers direct radiation at some frequencies and out-of-phase at others. It is these deviations from flat response that damping material can correct (because these deviations result from reflections from the walls of the enclosure.) However, the damping can in no way correct XO deficiencies. If that were the case damping material would alter both the shape and frequency of XO networks. Clearly it does not if you examine the many measurements in some of Dickason's books. Typically, in the case of the subwoofer, everything reflected from the enclosure walls will be out-of-phase with the drivers radiation, and damping material will be ineffective at any frequency within the pass-band. This assumes that the subwoofer is not EXTREMELY large. Sound energy in the stop-band will still produce standing waves, the question is are they audible? Travis

#13 of 37 OFFLINE   ThomasW

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Posted October 30 2005 - 04:54 PM

If you put damping material inside a subwoofer cabinet it will improve the sound quality. This is my final post to this thread.

#14 of 37 OFFLINE   Travis_G

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Posted October 31 2005 - 05:54 AM

Thank you for your replies Thomas. Sorry if I failed to communicate effectively. Travis

#15 of 37 OFFLINE   Danny Richie

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Posted October 31 2005 - 02:20 PM

Oh boy what a thread. Posted Image

Think of the damping material as something used to tune a box. Adding more damping to the box means that the woofer has to force the air in the box through the damping material. This slows down the air flow in the box and makes the box appear to be larger than it is.

For instance. Our SW-12A woofer needs an optimal sealed box of 1.363 cubic feet with no damping to have a .7 Qtc.

With a minimal amount of damping it optimal box size drops to 1.268 cubic feet with a .7 Qtc.

With normal stuffing it drops the box size down to 1.175 cubic feet with a .7 Qts.

Stuffed Heavy it drops the enclosure down to .918 cubic feet and still keeping a .7 Qts.

If your box has no damping material in it then it may be larger than it needs to be. Smaller box sizes are typical more desired. Or adding more stuffing to it might give you a little bit more extension by making the box appear larger.

It does little to attenuate higher frequencies as they are already well down in output. With a 12db per octave crossover (pretty low for a sub amp) set at 50Hz it will be 24db down by 100Hz and 36db down by 200Hz.

Sure the damping will catch the internal waves but just as you mentioned, it will do nothing for what passes from the front of the woofer.

Then again it is well down in output anyway.

More importantly then anything else is damping the enclosure walls with something to eliminate wall resonances. A solid, well dampened box goes a long way toward being musical.

Damping material like No Rez or Blackhole 5 can really do wonders for some sub boxes.

#16 of 37 OFFLINE   ThomasW

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Posted October 31 2005 - 02:58 PM

Gee, imagine that. Do you happen to know anyone that sells those products? Posted Image

#17 of 37 OFFLINE   Chris Popovich

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Posted October 31 2005 - 03:42 PM

ThomasW said it best when he stated that you should try it yourself and see the difference. You can argue whatever you want on here, but at the end of the day, adding dampening to the sub box will filter a lot of higher frequency 'noise' out. If your physics doesn't support this, then you aren't doing it right. It also effectively raises the box size, but I don't really build subs that depend on stuffing to gain box volume, they're for myself so I don't need to sell it to someone who wants a small box. ThomasW has been around here a long time and knows his stuff pretty damn well. Do some research and you'll find most of this has been covered many many times. -Chris

#18 of 37 OFFLINE   Travis_G

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Posted October 31 2005 - 04:43 PM

Thank you Danny for the info. Thank you Chris.
I am aware of Thomas reputation, as I have followed this forum for some time. I was a member of this forum about 6 years ago back when Tom Vodhanel and Ron Stimpson were the gurus here and occasionally you would even see posts from Dan Wiggins. This forum used to be a very busy place. You are correct in that damping material can attenuate higher frequency noise. However noise/FR deviations that originate in the XO cannot be attenuated by damping material inside the box (unless it is an indirect radiating subwoofer). Damping material also cannot increase the slope of the XO's attenuation. This is for the same reason that you cannot tame cone break-up modes with the use of fibrous damping material inside the enclosure. If the error radiates from the front of the cone, it won't be absorbed by damping material behind the cone. Travis

#19 of 37 OFFLINE   Mark--M

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Posted October 31 2005 - 05:52 PM

I was also thinking that stuffing a box will make it similer to a larger one (lower q). But do you really "gain" low end extension, or is it just the upper limit auttenated? For example if you had two identical subs (one stuffed and one not) running off the same amount of power, would the stuffed one actually be stronger at 20hz? I have an avalance 15 in a 2.5 cuft sealed box. I have the walls lined with fiberglass, should I go all the way and stuff it with polyfill? How much should I use?

#20 of 37 OFFLINE   ThomasW

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Posted October 31 2005 - 06:22 PM

Ported boxes are lined with damping material, sealed boxes are stuffed at a rate of 0.5-1.5 lb/cu ft. Fiberglass batting is a better choice for subs than polyfill Start with 1 lb/cu ft. Then add more damping in 1/4 lb increments, as long as the bass continues to 'tighten' add more damping. At the point where the bass no longer tightens, and the only effect is a drop in the overall output level, remove the last amount of damping added.




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