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Stuffing a DIY box - does it 'colour' the sound?

post #1 of 30
Thread Starter 
Hey all,

I've been pondering...


In a DIY box (Sealed for the sake of this conversation) I'm aware that you can line it with underlay or similar to reduce high frequency ringing, and also stuff it with polyfill to increase the internal box size that the driver 'sees'.


Going from the links, both are a good idea:
http://sound.westhost.com/articles/boxstuff.htm
http://www.nousaine.com/pdfs/Box%20Stuffing.pdf

but there comes a point where the stuffing reduces the effectiveness.


The second link says that a box of <3 cubic feet you can stuff between 1lb and 1.75lb per cu ft and get increased effective box size by up to 40%, but boxes >3cuft should only have a max of 1lb of stuffing per cu ft and will only gain 25% extra at the most.


So, staying within those limits, does stuffing at the max limits effect the sound negatively? I read about too much stuffing reducing output and muffling the sound, which is not ideal(!) but does the stuffing reduce the subjective 'tightness' of the sound and make it seem a bit slower?

i.e. would, say, a 5cuft box with no stuffing sound 'snappier' than a 4cuft box with 1lb/cuft stuffing to bring it up to 5cuft? (ignoring any particular drivers)


I know popalock stuffed his boxes of 4 SI18s quite heav/iy, going from the pics?, and it is by all accountts awesome smile.gif but have there been any stuffing/no stuffing tests??


Just wondered! smile.gif
post #2 of 30
most of that isn't quite right, but that's ok. :-)

stuffing the box reduces the q of the enclosure, which has the effect of reducing the sensitivity at the natural rolloff corner.

stuffing the box to the point that no air can move is effectively the same as reducing the enclosure size and raising the q.

other than that, it doesn't do a whole lot in the subwoofer range.

josh performed a great test and posted the results on data-bass.com

http://www.data-bass.com/data?page=content&id=79
post #3 of 30
Quote:
Originally Posted by LTD02 View Post

stuffing the box reduces the q of the enclosure, which has the effect of reducing the sensitivity at the natural rolloff corner.
+1. Lower Q reduces a hump in the response, which a larger box also does. But stuffing does not lower Fb, which a larger box also does, so it doesn't give the same result. As for coloration, lining/stuffing doesn't add coloration, it reduces the coloration created by the enclosure.
post #4 of 30
post #5 of 30
Thread Starter 
Thank you all for your replies smile.gif

I need to spend some time getting my head round everything - I think I get it, I just need to get it clear...!!
post #6 of 30
Quote:
Originally Posted by MemX View Post

Thank you all for your replies smile.gif

I need to spend some time getting my head round everything - I think I get it, I just need to get it clear...!!
The main thing is to make the cab as large as possible, so that the modeled Qtc is no higher than 0.85 or so. Then, if you need to, stuff it to get rid of any response hump. I would not make the box 20-25% smaller than that under the assumption that you'll be able to get the final Qtc below 0.8 by stuffing it. If using WinISD go to the Box parameters, click on Advanced. the Qa figure gives an idea of what stuffing will accomplish. The default figure is 100, change it to 5 to see the maximum effect stuffing might have. An interesting comparison is to see what happens with two cabs, one with a Qa of 100, the other with a Qa of 5, their volumes adjusted so that both have a Qtc of 0.7. The response is not the same.
post #7 of 30
Thread Starter 
Ahhhh, so stuffing doesn't change the box size that the driver(s) see per se, but it does alter the response curve? Dammit, I should have known this wouldn't be as easy as it seemed in my head!

:insert facepalm smiley here: lol


I will have another play with WinISD as suggested, thank you, and may well post some graphs up smile.gif

I may have to abandon my originally planned maximum box dimensions! The plan was to leave space to locate another two boxes next to the two I'm planning in the only really viable location, in case I suddenly come into money (hahahaha!!) but I think we all know our individual likelihood of that happening is slim to none lol, so I guess I may as well use the space now!!

I suppose the one benefit of an undersized sealed box is greater protection from bottoming out, which I will need if I run the planned CV5000 and 4xSI18s without employing any limiter and only a planned AntiMode to go with my XT32'd AVR!



Literally every day on here is a school day smile.gif lol
post #8 of 30
Quote:
Originally Posted by MemX View Post

I will have another play with WinISD as suggested, thank you, and may well post some graphs up smile.gif
They'll look like this:


The smaller box bare with a Q of .9 has a response hump. Adding enough stuffing to reduce Q to .7 removes the hump, but stuffing is a passive component, just as a resistor or capacitor is passive, so it can't add something, it can only remove. It removes the hump, but it also lowers overall sensitivity. A box large enough to give a Q of .7 bare also removes the hump, but a larger box is an active component, so it also increases low frequency sensitivity.
post #9 of 30
Quote:
Originally Posted by Bill Fitzmaurice View Post

The main thing is to make the cab as large as possible, so that the modeled Qtc is no higher than 0.85 or so. Then, if you need to, stuff it to get rid of any response hump. I would not make the box 20-25% smaller than that under the assumption that you'll be able to get the final Qtc below 0.8 by stuffing it. If using WinISD go to the Box parameters, click on Advanced. the Qa figure gives an idea of what stuffing will accomplish. The default figure is 100, change it to 5 to see the maximum effect stuffing might have. An interesting comparison is to see what happens with two cabs, one with a Qa of 100, the other with a Qa of 5, their volumes adjusted so that both have a Qtc of 0.7. The response is not the same.


Bill, what does that default number that you mentioned in winISD, at 100 by default, represent in terms of stuffing? You said that the default is 100 but to change it to 5. What does that number represent?

What happens if you model a subs response, and in the model it looks flat with say, 1lb of stuffing for sealed 3cuft SI18, but then when it gets placed in the actual room and measurements are taken, it has a hump due to the room, can you then pull the driver and add more stuffing to help reduce a peak?
post #10 of 30
Quote:
Originally Posted by Martycool007 View Post

Bill, what does that default number that you mentioned in winISD, at 100 by default, represent in terms of stuffing? You said that the default is 100 but to change it to 5. What does that number represent?
It's explained in the WinISD help file,
file:///C:/ProgramData/winisd/help/usingwinisd/boxdesign.html
Quote:
What happens if you model a subs response, and in the model it looks flat with say, 1lb of stuffing for sealed 3cuft SI18, but then when it gets placed in the actual room and measurements are taken, it has a hump due to the room, can you then pull the driver and add more stuffing to help reduce a peak?
I suppose you could, but at the cost of low frequency sensitivity. I'd use EQ instead. For that matter you might consider a combination of moderate stuffing plus EQ as a better overall method than just heavy stuffing. Using EQ to reduce the hump conserves amplifier power and driver excursion, as does maintaining as much sensitivity as possible below the hump.
post #11 of 30
Thread Starter 
Thank you very much for all your help, Bill, it is much appreciated smile.gif

Looks like I will either have to aim for a redesign to maximise internal space within my desired build dimensions, or throw the dimensions out of the window and go for the design I want but in a bigger space!
post #12 of 30
Quote:
Originally Posted by Bill Fitzmaurice View Post

I suppose you could, but at the cost of low frequency sensitivity. I'd use EQ instead. For that matter you might consider a combination of moderate stuffing plus EQ as a better overall method than just heavy stuffing. Using EQ to reduce the hump conserves amplifier power and driver excursion, as does maintaining as much sensitivity as possible below the hump.

Bill,

So does this imply that it would be better to use no stuffing at all as to not reduce sensitivity and use external EQ to reduce the hump?
post #13 of 30
Thread Starter 
Quote:
Originally Posted by duc135 View Post

Bill,

So does this imply that it would be better to use no stuffing at all as to not reduce sensitivity and use external EQ to reduce the hump?
I'm probably wrong biggrin.gif but I think a small amount of stuffing helps reduce any internal ringing that may be present while not effecting the efficiency of the driver?
post #14 of 30
Quote:
Originally Posted by MemX View Post

I'm probably wrong biggrin.gif but I think a small amount of stuffing helps reduce any internal ringing that may be present while not effecting the efficiency of the driver?
Lining the sub with a couple of inches of foam, polyester batting, or Type 700 Fiberglass, or lightly filling it with polyfill, is beneficial, and doesn't affect low frequency response.
post #15 of 30
Quote:
Originally Posted by Bill Fitzmaurice View Post

Lining the sub with a couple of inches of foam, polyester batting, or Type 700 Fiberglass, or lightly filling it with polyfill, is beneficial, and doesn't affect low frequency response.

Not trying to call you out, but I'm assuming we are talking about sub boxes here and not satellite speakers. I could be wrong, but in a different thread I thought you had stated that lining the walls of a sub enclosure provides no benefit so long as they are properly braced. This being that the vibrations of enclosure walls are well outside the frequency range of subwoofers. With this in mind, what benefits would lining or polyfill be at this point if the same can be better achieved by external EQ. If you are referring to lining the enclosure walls of satellite speakers then I can see the benefits there.

This is an honest question as I have several subwoofers already and am building four more. I already have plenty of external DSPs to provide EQ for them all and would like to know if I even need to stuff the boxes with polyfill at this point. If it's better to use just EQ then I'll go back and remove the stuffing out of the existing boxes.
post #16 of 30
^^^^^

post #17 of 30
Quote:
Originally Posted by duc135 View Post

I could be wrong, but in a different thread I thought you had stated that lining the walls of a sub enclosure provides no benefit so long as they are properly braced.
I never would have said that. Perhaps you're thinking about something other than the materials I mentioned. Nearly all speaker cabinets should be lined with damping material to absorb internal reflections.
post #18 of 30
Quote:
Originally Posted by Bill Fitzmaurice View Post

I never would have said that. Perhaps you're thinking about something other than the materials I mentioned. Nearly all speaker cabinets should be lined with damping material to absorb internal reflections.

You have said reg insulation is not the way to go correct? I need to change my ways if so with my next project.
post #19 of 30
So what would be considered lightly stuffing the enclosure if you are using polyfill? I think there is a rule of thumb (oz. per cuft) but I forgot the specific numbers
post #20 of 30
Quote:
Originally Posted by orcarola25 View Post

So what would be considered lightly stuffing the enclosure if you are using polyfill?
Leave it very fluffy, with no compression. A stuffed cab is just that, not only totally filled but compressed.
post #21 of 30
Quote:
Originally Posted by MemX View Post

Ahhhh, so stuffing doesn't change the box size that the driver(s) see per se, but it does alter the response curve?

It does both. From the article I referenced earlier:
Quote:
Originally Posted by Marshall Leach 
This equation [equation (7)] can he interpreted as saying that the effective volume of the box varies inversely with gamma. It thus follows that a filled box can appear bigger than an unfilled box by the factor of 1.4, or by 40%.

There's really two things going on here. The stuffing increases the box losses, which is the same as decreasing the box Q (Qa) as Bill says. But also, the stuffing increases the acoustic compliance of the medium inside the box for a given volume, as shown by equation (7) and others in the paper. When gamma is smaller, the box "looks" bigger, and the opposite is true for larger gamma. The two extreme cases of gamma (in the denominator of the acoustic compliance expression in equation 7) are gamma = 1.4 for the adiabatic case (no stuffing) and gamma = 1.0 for the isothermal case (an idealized situation that can't be reached in practice).

One problem with simulating this is knowing the exact effect, which depends on the thermodynamic properties of the stuffing, and whether it's compressed. WinISD doesn't have a facility to simulate the varying acoustic compliance other than the user making the simulated volume slightly higher than the physical volume. Reducing Qa as Bill has done is a good idea, since stuffing also increases the box losses, but the effect of stuffing is not limited to just changing Qa.
Edited by andyc56 - 8/29/13 at 4:53pm
post #22 of 30
Quote:
Originally Posted by andyc56 View Post

Reducing Qa as Bill has done is a good idea, since stuffing also increases the box losses, but the effect of stuffing is not limited to just changing Qa.
True, but it does give a pretty good approximation. I've measured the effects of stuffing as well, on impedance, Qtc and response. It jibes with what WinISD shows. The only issue with WinISD is that there's no method of calculating how much stuffing is necessary to achieve a given Qa figure. But knowing that a Qa of between 3 and 5 is about as low as you can realize you can get a good ballpark estimate of what you can, and can't, accomplish from a given box size.
As for the Leach paper, the results I measured don't agree with his calculations, and other sources, like data-bass.com, have also gotten the same results as I have. I didn't see any mention of the cab Qtc, which seems odd, nor any actual measured response and impedance charts of stuffed versus unstuffed cabs. Not that I'm in the least bit anti-math, but IMO and IME the math explains the measured results, not the other way around, and when the measured results and the math disagree it's the math that needs to be re-examined, especially since Leach's paper dates to 1989.
post #23 of 30
Quote:
Originally Posted by Bill Fitzmaurice View Post

As for the Leach paper, the results I measured don't agree with his calculations, and other sources, like data-bass.com, have also gotten the same results as I have.

Which specific calculations are you referring to, and what specific results don't agree with them?
post #24 of 30
Quote:
Originally Posted by andyc56 View Post

Which specific calculations are you referring to, and what specific results don't agree with them?
His basic model seems to have omitted a resistor or two. The specific results that don't agree is the contention that a stuffed box 'appears larger to the driver'. It doesn't. If it did then the measured results with a larger unstuffed box and a smaller stuffed box would be the same. They aren't. There are similarities, mainly the lowering of Qtc that both stuffing a box and making a box larger both realize, but only similarities, not an identical result.
post #25 of 30
Quote:
Originally Posted by Bill Fitzmaurice View Post

Quote:
Originally Posted by andyc56 View Post

Which specific calculations are you referring to, and what specific results don't agree with them?
His basic model seems to have omitted a resistor or two. The specific results that don't agree is the contention that a stuffed box 'appears larger to the driver'. It doesn't. If it did then the measured results with a larger unstuffed box and a smaller stuffed box would be the same. They aren't. There are similarities, mainly the lowering of Qtc that both stuffing a box and making a box larger both realize, but only similarities, not an identical result.

Have a look at the data-bass.com measurements of the closed-box system with and without stuffing that was linked earlier in the thread. They unfortunately don't provide info as to which driver was used, nor its T/S parameters nor the volume of the box. However, it must be a pretty large box for the system to have an fc of 25 Hz. With stuffing, fc drops to 23 Hz. If you make QL infinite in simulation, you'll find that changing QA does not change fc at all. That's also clear by inspection of the topology of the equivalent circuit. When QL is some moderate value, non-zero changes in fc occur when changing QA, but they are negligible, a couple tenths of a Hz. Since varying QA does not change fc, it follows that the measured change in fc must be due to some other factor. That is, something more than the enclosure loss must be changing. I can only think of two possibilities: compliance shift or measurement inconsistency.
post #26 of 30
Thread Starter 
Thanks for your replies, all smile.gif


I think I get the basics but I'm going to have to download that paper at home - stupid work blocks any sites that host documents or photos... rolleyes.gif


Every time I think I'm getting a handle on this stuff, it just gets more complicated biggrin.gif lol
post #27 of 30
Quote:
Originally Posted by andyc56 View Post

I can only think of two possibilities: compliance shift or measurement inconsistency.
The third 'possibility' is the law of diminishing returns. Stuffing has the greatest effect when the initial Q is high, and the least effect when it's low to begin with.
post #28 of 30
So how does stuffing affect a ported enclosure? Is it still a good idea to lightly stuff a vented box?
post #29 of 30
Quote:
Originally Posted by Martycool007 View Post

So how does stuffing affect a ported enclosure?
The same way it does a sealed enclosure. But since well designed ported cabs tend to have lower Q values to begin with there's seldom any benefit to be gained with stuffing them, and potentially much to be lost.
Quote:
Is it still a good idea to lightly stuff a vented box?
There's no such thing as 'lightly' stuffed, as stuffed means that the fill is compressed. One can lightly fill a VB, leaving the material fluffed with no compression.
post #30 of 30
Quote:
Originally Posted by Bill Fitzmaurice View Post

Quote:
Originally Posted by andyc56 View Post

I can only think of two possibilities: compliance shift or measurement inconsistency.
The third 'possibility' is the law of diminishing returns. Stuffing has the greatest effect when the initial Q is high, and the least effect when it's low to begin with.

"Possibilities" in the context of what I wrote refers to "possible reasons for the measured reduction of fc with the addition of filling in the data-bass.com measurements referenced above". Since changing QA does not affect fc, something else must be going on besides just increasing enclosure loss. That is, assuming the only effect of the addition of filling is an increase in enclosure loss leads to a contradiction.
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