FWIW from Josh Ricci's data-bass site https://web.archive.org/web/20180130173833/http://data-bass.com/data?page=content&id=79
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Does a lot of damping material have any negative effects in a closed box?
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It appears that someone has questions about this quite some time ago:
The Appropriate Use and Effects of Speaker Stuffing - Techtalk Speaker Building, Audio, Video Discussion Forum
I have a very simple question that has me wondering about speaker stuffing - and no, not the kind of stuffing that makes you belch after a meal. I mean
techtalk.parts-express.com
Overstuffing? - Techtalk Speaker Building, Audio, Video Discussion Forum
Want a second or third opinion about your speaker cabinet design or other audio related problem? Post your question or comment on the Technical Discussion Board. Hundreds of technicians, engineers, and hobbyists, nationwide read and discuss electronics related questions each week. We welcome...
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This has been my experience, which includes blind listening tests in the course of product development specifically focused on investigating the type and quantity of damping material for a sealed box.
Ime the most reliable way to fine-tune the amount of damping material can be rather time-consuming, at it involves A/B listening comparisons of two otherwise identical loudspeakers.
The 1lb/ft^3 they mention is what I usually shoot for. You'll also notice that in Tom Nousaine's experiments I linked in post #35, that was about the optimum stuffing density to minimize Fs.
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- #45
Very interesting! Since you didn't write anything about objective data it seems that you can't measure the positive change right? Can you detect in which frequency range the negative impact happen is it around the resonance frequency of the driver or above?
Thank you for the links interesting to read. I had read a bit of the threads but I didn't find an answer to my question here regarding potential harm if you don't do crazy stuffing.
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I think there are two effects which we have to look at separately:
- First the effect on the resonance frequency and damping of the driver. The following link will help with the theory of the equivalent circuit but it hasn't included damping material in it. Does anyone know how accurately modify the equivalent circuit to include damping material in the closed box?
Speaker equivalent circuit / electrical model - Audio Judgement
Cleverly placed electrical components can form a speaker equivalent circuit. Electrically, It will act the same way as a loudspeaker will.
audiojudgement.com
- Second the effect on the frequencies above the resonance frequency. It is well known that all the box resonances and sound from the backside of the driver can be reduced with damping material. Negative effects is maybe the reduction in overall spl since the back wave can't contribute to the front spl as much, but there have to be more negative effects otherwise it would be very wired why most scientific based companies don't use more material.
olieb
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In the simplified theory of a speaker as an electric circuit this can be accounted for by changing (lowering) Rm for the mechanical damping.
I would be surprised if there is an accurate theory about how to account for the amount of different materials in different speaker cabinets in different sizes and for different drivers. This still has to be done and probably will work in simulation (BEM/FEM) only.
Or you measure and adjust Rm.
Here is a nice piece where Linkwitz studies and documents exactly this problem in a practical case.
Woofer equalization in LXmini
He deals (has to) with the back wave problem, too.
Schollaudio
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Does anyone know the 18" woofer he used in these tests?
To dampen means to add moisture to something. To damp means to reduce or discourage something, usually by way of using something dissipative.
The focus was on the midrange, and I haven't found frequency response measurements to be of much use for evaluating the effects of internal damping material on sound quality in the midrange region.
"Damp" can refer to either moisture or vibration, and likewise "dampen" can refer to either moisture or vibration. In both cases the primary meaning has to do with moisture.
Colloquially, yes. Strictly speaking, no.
For many years I would have agreed with you because usage of the term "dampen" in an acoustics context just sounded wrong to me. Then I looked it up.
Imo dictionaries can be wrong, but it's unusual.
Using something that puts the brakes on boomy sound from my speakers I am all for. Pouring water on them I am not.
This is at the end of the audiojudgement.com article you cite above. Damping is part of the enclosure compliance Cab. Cab is shown here as part of a closed box equivalent circuit…
Note the math under the inductor circled with the dotted lines.
Note the math under the inductor circled with the dotted lines.
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Thanks for sharing!
Had not encountered the tech talk thread on overstuffing previously and has some great content. Was considering doing an experiment, but others did in the thread. One clearly shows the reduction in output from adding stuffing with a real world application. This is comparable to the chart I posted in #3 above. If the Qtc is changed from 1 to .5, (potentially by added stuffing) there is a 6 dB decrease in SPL at 50 Hz.
Did not know that thermodynamics was one of Bagby’s skills. Agree with his position that the apparent increase in volume from stuffing has very limited range. It seems to have originated in Leach’s research and has been overextended.
ernestcarl
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I have been looking at the smallest “mini subwoofer” kits on offer at Part Express (say, the smallest 8 inch mini subs) and every time someone asks about stuffing/damping material the staff always answer with:
“Best Answer: We do not recommend it, any kit that requires damping matieral will have it supplied in the kit.”
While some other folks say they use the usual 1lb stuff parts express already sells and that it works fine.
I dunno… I’ve never built a sub/speaker before.
I’m curious, do you think their staff is technically correct in discouraging people from adding any kind of damping material in their subwoofer kits?
I have been looking at the smallest “mini subwoofer” kits on offer at Part Express (say, the smallest 8 inch mini subs) and every time someone asks about stuffing/damping material the staff always answer with:
“Best Answer: We do not recommend it, any kit that requires damping matieral will have it supplied in the kit.”
While some other folks say they use the usual 1lb stuff parts express already sells and that it works fine.
I dunno… I’ve never built a sub/speaker before.
I’m curious, do you think their staff is technically correct in discouraging people from adding any kind of damping material in their subwoofer kits?
On a small solid cabinet, probably matters less. That said, these are woofers with strong motors, would consider some 3/4" sonic barrier to line the inner walls...
Which specific kit? What are they using to fasten the woofer?
ernestcarl
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Mainly either the Hi-Output Passive Dayton Audio Reference 8" Mini Subwoofer Kit or Hi-Performance Passive Dayton Audio Ultimax 8" Mini Subwoofer Kit which are sealed subs.
But The Workhorse Passive GRS 8" Mini Subwoofer Kit would also fit in this compact size/form.
My "folk intuition" tells me that if I'm planning to use the sub only, say, 120Hz and below then adding additional damping material might be of little to no benefit since the driver is only reproducing bass and with little to no midrange energy. However, if I were planning on extending the playback FR to a greater extent of the midrange area, then would most definitely need more damping.
These both have smaller cabinets, so damping is not going to do much (notably for panel resonances). Would at least do some Sonic barrier on the back panel in either case. There is this somewhat larger version...
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Dayton Audio 8" Reference Series HO Subwoofer and Cabinet Bundle 300 WattsAbout the CabinetThis particular cabinet is designed specifically for the Dayton Audio 8" Reference Series High Output subwoofer, but works for any subwoofer with an overall diameter of 8-1/2" or less, and a cutout...
www.parts-express.com
Gives you a little more room to work with and has better cabinet bracing....
Would find out what they recommend for good fasteners (likely t-nuts and bolts).
This is clearly lower cost kit of the bunch and would not do unless budget is an issue.
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- #59
With small subwoofer it seems that the damping material doesn't change much in a small enclosure.
So in this model the enclosure is assumed to be bigger with damping material and there are no other changes?
Bill over on Tech Talk pointed out that damping material in a sub box will absorb harmonics, potentially reducing re-radiation from the cone. Might be true, although I've never tested it. I just stuffed my woofer boxes full, even though they cut off at 110Hz.
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