Importance of box dampening...

The main purpose of sound dampening/deadening is the absorbption of the backwave and other mechanical noises that could come back through the cone. The secondary benefit of "stuffing" is that using principals of thermodynamics, it makes the enclosure act like it's larger.
And the benefits are in that order.

At the very least they should be lined to control modulation distortion harmonics bouncing around in the box. If the Qtc of the cab is higher than 0.8 or so they should be stuffed with sufficient material to bring the Qtc down.

Is there a formula for how much you can reduce Qtc based on enclosure volume and stuffing density? Or is it an approximation?

Do you know what thermodynamics is? You got the idea of "stuffing" down but, thermodynamics doesn't apply here...

Perhaps in AkAbak, but I've never looked as I don't do sealed subs. Most plans just say 'stuff with 1lb./cu ft', which is a shot in the dark. IMO any serious designer would use WT3 or the like to measure the finished cab Qtc and know what he's getting.

There has to be a method. Some way to measure where there is too little, too much, or just enough. I do the 1lb per cu ft.......

I just told you. Measure the Qtc of the finished cab, stuff as required to get the desired Qtc, or as close to it as possible.

There are many: foam, polyester, carpet padding, fiberglass. Felt carpet padding you can get for free, just go dumpster diving at a carpet store. They throw away scraps too small for a floor but plenty large enough to line a speaker.

Thermodynamics is why stuffing works. Maybe you don't understand stuffing.

http://en.wikipedia.org/wiki/Thermodynamics
Maybe you don't understand Thermodynamics....
http://en.wikipedia.org/wiki/Thermo
The prefix here is Thermo, meaning related to heat or temperature.

When you put stuffing in a box it does two things:

First of all, it introduces an often desirable resistive loss and secondly it increases
the apparent volume of the box. In this second part thermodynamics comes in:

In a empty box, you get a adiabatic process, meaning that when the cone move
inwards it raises the box pressure and hence the temperature. A gas with increasing
temperature tries to expand which will cause an increased suspension stiffness and
raised resonant frequency as a result.

When adding stuffing you move toward a isothermal process, the stuffing cools the
raising air temperature in the box and give a less increased suspension stiffness then
in the adiabatic case, and a lower resonant frequency as a result.

In other words, yes the prefix is thermo, and it got everything to do with it!

Hope I made myself understood, my english is a little rusty...

Not really. Stuffing lowers Qtc and alters system impedance by offering a higher impedance load to the driver than air alone. More heat is produced because the friction between fibers excited by the cone movement is greater than that of air alone. But the heat does not cause the alteration of Qtc and impedance, it's just a byproduct of the process.

I do. This shows measured response, Qtc and impedance of the same driver in the same box, bare and stuffed.

Tom Nousaine did some experiments with stuffing yours ago.
www.nousaine.com/pdfs/Box%20Stuffing.pdf

What method

Bill, you're just saying that box stuffing works because it does, and heat is just a coincidence. Thermodynamics provides a "why" it works which results in an altered Qtc.

Bill, you dont agree with my explanation then, nor Tom Nousaine's?
It seems we have different views on where the increasing temperature is derived from.

Briefly from Nousaine's paper: "The air inside your enclosure actually heats
up as your woofer moves, making the air stiffer. When the enclosure is stuffed
with fiber, the fibers wiggle, dissipating some of the heat and making the system
work as though the box were larger."

Nousaine's work did not include the all important Qtc and impedance measurements required to correctly identify what was occurring. He only measured F3, and didn't get that right either, as he didn't reference it to transfer function magnititude. His errors are easily forgiven, as the tools required to quickly, easily and accurately measure Qtc and impedance weren't available then.

I have read that stuffing larger enclosures has little effect on qtc. Is there a point in enlcosure volume where stuffing makes no difference at all.

I have always wondered because my Q18's need to be in a 30 cu ft plus box for optimum performance but I have them in 15 cu ft boxes. Will stuffing them make any difference to the alignement of a specific tune.

Yes. Just as there comes a point of diminishing returns with making a box larger there also comes a point of diminishing returns adding stuffing to a box that's already large enough to reach the desired Qtc, or very close to it, without it. But with a huge box internal resonances can become an issue, and you may need to fully fill the box to suppress them even if lowered Qtc isn't a goal.


What's their Qtc? What would it be in 30 cu ft? A difference of 0.1 or less would be hardly noticeable.

Air stiffer? that makes no logical sense.

As air heats it expands, which would increase pressure, not stiffness. Actually it would do otherwise, by decreasing density. But since its in a sealed chamber, in the case of a sealed enclosure, the increase of temp, results in increased pressure, in turn would increase the damping factor offerd by air..

(logically)

True. What to consider is not the 'stiffness' (compliance) of the air in a sealed box but its mass. The higher the air mass the lower the Qtc. Stuffing emulates the higher air mass of a larger box by adding fibrous material to the air contained in a smaller box. You could accomplish much the same by using a heavier than air gas, but stuffing is far more practical.

Right, hydrodynamics.. since at this point we are talking about densities, and how air reacts to space, pressure, and obsticales in its path..

Right?

What you're talking about is impedance. Model a sealed cab, look at the impedance. Change the box size, look at the impedance. Stuff the cab, look at the impedance. If you want to alter FR and Qtc you do so by altering impedance. Changing the box size is one method, using stuffing is another. But don't fall into the 'stuffing fools the driver into thinking the box is larger' trap. Drivers aren't so easily fooled. The impedance, FR and Qtc alterations that stuffing results in have similarities to what a larger box does, but it's not identical. Looking at the impedance reveals that as well.

Thanks for the tip, and lessons, I always love learning something new.

Hydrodynamics do enter the discussion, though, in a roundabout way. The reason why sound travels far more effectively underwater is related both to the speed of sound in water and to the impedance of water, which is much higher than that of air. Contrast that to a vacuum, where there is neither speed of sound nor impedance, and therefore no sound at all.

Hi Bill,

I was refering to my ported sub with being in a 15 cu ft instead of a 30 cu ft box which is optimum size. It was more thinking that stuffing could make the alignment better by way off boosting the bottom end without EQ thus making the driver think it is in a bigger box.

With sealed with my Q18 in a 16.5 cu ft box because I am including the port as internal volume is around 0.744 but in a 28 cu ft box it is 0.7 which isn't a massive difference I guess.

I have 5kg off fiber fill in the garage and wasn't sure if stuffing my 15 cu ft boxes would actually make any difference what so ever.

In my mind, air stiffness is a layman's explanation for air pressure. An increase in air pressure would be similar in concept to "stiffer" air, as the air presses with more force on the cone.