Q&A - Stuffing a Down Fired Box

[Warmon]

Hi guys.

Once again I need some advice. What's the best way to poly-fil a down fired box? Is fibers falling down into the voice coils really a concern? I have a few ideas, but don't have the ideal solution in mind yet...

Warmon -

[jpmst3]

Just tape a piece of screen over the hole in the motor (to the VC), then fill away...

[Mike P.]

My son inlaw used nylon screen over his. He hasn't had a problem.

[Warmon]

The driver I'm using already has a screen at the pole vent. Is this all I need?...or do I need to screen off the everything inside the frame; ie the back of the cone?

[jpmst3]

Yep, you are good to go. It is fine for the stuffing to touch the driver. Of course, within reason, you can't jam pack the thing to where it is solid stuffing.

[Warmon]

Okay...cool!

Now I have a second question. Stuffing up to 1.5 lbs per CF makes the box look bigger - which is a good thing. After that, diminishing returns and too much is definitely not good. What's happening with cone excursion and group delay with optimal fill? Does it remain unaffected? In other words, is it all good until the point where box resonance starts to rise again due to over fill.

Also, is it worth putting closed cell or open cell foam in a sealed box you plan to stuff at the 1.5 lb rate?

[jpmst3]

Here is an article that might answer some of your questions.

I don't think lining the walls will do much of anything with the stuffing in place.

[Warmon]

I've read it before and it's a great article. I ask the question, because in it, he says:

few rules of thumb: Stuff small enclosures – those with up to about 3 cubic feet of internal volume or less – with 1.5 pounds of fiberfill for each cubic foot of internal volume and you should get about a 30-percent increase in box volume without seriously affecting other performance variables. For larger enclosures, add stuffing at a rate of approximately 1 pound per cubic foot and you should get a virtual-space boost of about 25 percent. One thing's certain: You'll impress the out of your friends at the art gallery and bistro.

He does say "without seriously affecting other performance variables"...

[jpmst3]

Well, nothing is for free so to speak. But, stuffing is a universally utilized practice so I doubt there would be any serious negatives with regard to sound quality by stuffing at reasonable amounts. So, I would not be overly concerned with the side-effects. I am sure they would be mentioned if there were any considerable issues in play.

[Warmon]

I'm golden...thanks!!!

[DevilDriver]

Warmon wrote:

Okay...cool!

Now I have a second question. Stuffing up to 1.5 lbs per CF makes the box look bigger - which is a good thing. After that, diminishing returns and too much is definitely not good. What's happening with cone excursion and group delay with optimal fill? Does it remain unaffected? In other words, is it all good until the point where box resonance starts to rise again due to over fill.

Also, is it worth putting closed cell or open cell foam in a sealed box you plan to stuff at the 1.5 lb rate?

You can expect excursion to increase as the effective Qtc drops. That could be a benefit, or a detractor depending on the driver, power on tap, and bandwidth you're looking at. Group delay should flatten out a bit and that's the important part. When it comes to group delay, high or low isn't as much of a concern as variation is.