Stuffing in vented boxes

[jethari]

Do vented enclosures and bandpass enclosures also need to filled with acoustic stuffing for best results?

[pinkmouse]

Depends...

In general no, but sometimes it helps to put a layer of rockwool or glass fibre insulation on a couple of the internal walls to absorb box reflections.

But one of the joys of building your own boxes is in the experimentation, so feel free to ignore the above rule

Stuffing a box adds accoustic impedance to the system, and so can make a box seem larger to the driver, thus making the bottom end more extended, but as a side effect slowing the apparent speed of the bass, because of a slower roll off and an increase in group delay.

However stuffing a bandpass box can have advantages, as it will increase the amount of out of band cut off, but you need to be careful not to upset the volume ratios of the chambers.

Hope this makes sense!

[MJK]

I think that adding a little stuffing to a vented enclosre is a good thing to do. I use less than 0.25 lb/ft^3 which really has a minimal effect on the apparent box volume. At the same time the fibers damp internal standing waves that can produce output from the port at frequencies well above the tuning frequency.

Martin
www.quarter-wave.com

[Raka]

Hi,

here you are my modest opinion:

I'm now building a small units (you can check the thread MINIMUM VOLUME MATTERS), and as I don't have so much spare time, I have end last week the "housings". I've put the drivers and a provisional 1st crossover and connected to my amp. As expected, the bass is not a thunder (Calculated F3=70Hz). The walls are covered with 1' foam stuff, all but the front panel. Besides I filled the inside with felt rolled with fiber glass, taking care to not cover the bass reflex port. Results: MUCH better bass and mid bass without all this stuff. BTW, what makes more difference is the fiberglass.

I'm even thinking to get out some of the foam and keep only the back stuffed.

Some time ago, I read an article about this point. The conclusion was that stufing a vented enclosure is getting the bad things from the closed and the vented worlds. Maybe John K?

But we agree in the same point: Try and try and try. The theory comes after the experience.

[jethari]

A lot of very good suggestions - thanks a million all you guys. I think the best advise was to try and keep trying. It sure makes a lot of sense. I'll try stuffing the boxes with polyfill or some other equivalent and see what it sounds like. Thanks again.
Hari

[kelticwizard]

The accepted rule for bass reflex is: one inch or so of damping materials attached to the walls. No filling at all. Make sure the port is clear.

You can vary this to taste, of course, but the "no fill" rule is pretty much accepted.

[planet10]

Originally posted by pinkmouse
but as a side effect slowing the apparent speed of the bass, because of a slower roll off and an increase in group delay.

Isn't the other way around... the faster the roll-off, the greater the group delay?

dave

[navin]

from my experience kelticwizard has got it right. i use open cell foam though as it is easier to hendle and control. i also brace the heck out fo the box.

bracing cannot be underrated.

i got 7 braces made of 35mm MDF in my last project.

next week i will post pics of my curved sides project. only i dont want to usurp too much space. if anyone has a place i can post them and then all you guys need is a link let me know. i can mail them to anyone interested in posting them too.

my wife and son have hijack my camera for this week.

[pinkmouse]

[quote]Isn't the other way around... the faster the roll-off, the greater the group delay?[\quote]
Oops...

Yes Dave, you are right, I am wrong!!!( again!!!) I must have had a temporary mindstorm.

To restate the definition, for myself and any other people confused out there:

Group Delay is proportional to the phase response of the box, driver and crossover combination.

Or more simply, as Q gets higher, (sharper roll off), phase change between frequencies increases, leading to a greater Group Delay.

Sorry folks

[pinkmouse]

Just as an extra note- for those that are unfamiliar with phase in reference to loudspeakers,

Phase, for a given frequency, is proportional to the delay between an input to the speaker system and the resulting output, the greater the delay, the steeper the phase angle.