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10-28-2005, 10:01 PM
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#1 of 37 
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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
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10-28-2005, 11:45 PM
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#2 of 37
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You pretty much nailed it, as far as I know.
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10-29-2005, 10:33 AM
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#3 of 37
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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.
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10-29-2005, 06:53 PM
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#4 of 37
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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?
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I'm reasonably sure you'll be able to figure why it's only omitted in the cheapest subs money can buy.
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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 |
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10-30-2005, 08:57 AM
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#5 of 37
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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
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10-30-2005, 12:31 PM
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#6 of 37
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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.
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The use of damping in a subwoofer has nothing to do with the elimination of standing waves.
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According to Dickason, that is why he used damping material in the subwoofer when he designed his Two-Way Satellite with Subwoofer system found in, "Loudspeaker Recipes Book 1.":
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I damped standing waves by lining the enclosure's three sides and back with R19 fiberglass, excepting the spot directly behind the port.
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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 |
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10-30-2005, 02:50 PM
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#7 of 37
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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.
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10-30-2005, 04:12 PM
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#8 of 37
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| One puts (frictional loss) damping in a sub to absorb the higher frequencies that 'leak' through the crossover. |
Sorry Thomas, but I don't understand. I didn't think that a properly functioning crossover leaked. A crossovers should attenuate at a predetermined rate shouldn't it? Besides, if higher frequencies are leaking through the XO, damping material inside the box is going to do virtually nothing to attenuate it in a direct radiating subwoofer. Perhaps if you could provide me with some reference(s), I might understand.
| If you don't want to do this fine. |
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. 
Travis |
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10-30-2005, 06:50 PM
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#9 of 37
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I didn't think that a properly functioning crossover leaked.
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Filters aren't a brick wall, even though they're attenuated, there are higher frequencies coming through the XO.
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Besides, if higher frequencies are leaking through the XO, damping material inside the box is going to do virtually nothing to attenuate it in a direct radiating subwoofer.
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Don't be silly of course it will. That's what damping material (fiberglass/polyfill/whatever) does, it absorbs sound (actually it converts it to heat). And the frequencies absorbed depend on the thickness of the material
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To be perfectly honest, I am still not convinced that there is a reason for fibrous damping material in subwoofers
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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.
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10-30-2005, 07:57 PM
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#10 of 37
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| Filters aren't a brick wall, even though they're attenuated, there are higher frequencies coming through the XO. |
Of course, that has already been established in this thread. I guess I'm confused as to what you meant by "leak":
| One puts (frictional loss) damping in a sub to absorb the higher frequencies that 'leak' through the crossover. |
Are you suggesting that damping material is used to increase the attenuation/order of the XO? I don't know why this would be necessary.
| Don't be silly of course it will. That's what damping material (fiberglass/polyfill/whatever) does, it absorbs sound (actually it converts it to heat). And the frequencies absorbed depend on the thickness of the material |
That is true for sounds that are reflected from the enclosure walls. You said it was to absorb higher frequencies that leak through the XO. Response deviations in the stop-band that pass through the XO radiate directly from the driver and will not be attenuated by any damping material inside the enclosure (unless of course you have a band pass enclosure or other indirect radiating enclosure). Response deviations caused by standing waves in the enclosure may be attenuated, but I thought that were suggesting that damping material could be used to compensate for XO deficiencies.
| 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. |
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 |
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10-30-2005, 11:08 PM
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#11 of 37
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You said it was to absorb higher frequencies that leak through the XO.
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Yes I did.
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Sounds in the stop-band that pass through the XO radiate directly from the driver and will NOT be attenuated by any damping material INSIDE the enclosure
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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
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