Damping The vented box

[Aucosticraft]

While reading "Loudspeaker design cookbook" by Vance Dickason, I came across following " traditionaly enclosure damping, to suppress standing waves in a vented-box system, consists of lining one of each opposite side with 1"-2" of fiberglass. It is recommended, however, that you cover all surfaces directly behind, and adjacent to, woofer. Colloms recommends that such damping material be placed with the volume or open area, not on the box walls. "


Does the author mean to say, the damping material should be stuffed like that of closed enclosure design. taking care that vent is not obstructed? That is not just lining the walls of vented enclosure.

[Don Hills]

He means, if you're going to use 1 or 2 inch thick material, hang it in the middle of the box instead of against the walls. I't'll be more effective. Note that you can fill the whole box if you want to, just leave a bit of space around the port mouth. Several box modeller programs such as Unibox allow using damping material in the enclosure design.

[chris661]

Another source of information if you're interested...

Volume filling a reflex box

[Aucosticraft]

Thank you Don and chris.

Chris EPS site is my important bookmark. It had always been a big source of knowledge.

Do you know any site which discuss deep about this with some calculation.

With random surfing, I could understand that, because the stuffing absorbs sound and minimizes reflecting it back avoiding standing waves, this makes it look like a bigger enclosure. but the physical volume of enclosure gets smaller, so the compression ratio changes.

But mathematically how can this be related and calculated just don't know.

[Cokewithlime]

In actuality the biggest problems with sub enclosures is inadequate panel bracing. Lack of sufficient panel bracing allows LF resonances to build up within the enclosure and this is what many refer to as a sub having a boomy character.

Most sub enclosures are just not big enough to allow standing waves of LF to build up, given the wavelength of low frequencies

Fiberglass, Dacron, carpet, etc have virtually no effect on controlling or reducing low frequencies, although they do help somewhat in controlling upper harmonics of the LF content.

You might be surprised to learn that many of the best sounding ported subs have very little to no stuffing/lining but are extremely well engineered to be very "dead" resonance wise by using panel bracing and double panel thickness with a constraining layer between the two panels.

[Aucosticraft]

Thanks Coke...

Hey is it really that big gray area, only one reply..?

[Cokewithlime]

The only thing that stuffing a ported enclosure does is it helps a bit with absorption, (Qa in WinISD) to lower the Q of a undersized(high Q) enclosure - stuffing/lining really has very little to no effect on controlling any standing waves within a enclosure.

If you are concerned(and you should not be with sub enclosures) about standing waves simply make sure no two inner surfaces of the enclosure are completely parallel to each other, that way wave frequencies inside the enclosure can never build and form a standing wave.

[PaleRider]

Damping inside a bassreflex has the same effect as a muffler to a exhaust-system, it reduces the soundlevel and ruins soundquality. The right way to go would be to look at 1/4 wave horns or do like I`ve done; asymetric cabinet w. diffractors + aerodynamic ports pointing forward paralell w. floor.

[Scott L]

Originally Posted by Cokewithlime

The only thing that stuffing a ported enclosure does is it helps a bit with absorption, (Qa in WinISD) to lower the Q of a undersized(high Q) enclosure - stuffing/lining really has very little to no effect on controlling any standing waves within a enclosure.

If you are concerned(and you should not be with sub enclosures) about standing waves simply make sure no two inner surfaces of the enclosure are completely parallel to each other, that way wave frequencies inside the enclosure can never build and form a standing wave.

Acoustic Absorbent material is of NO USE and might actually be a detriment in a sub only enclosure. I have built a 3 cu foot enclosure of equal dimensions HWD for a 15" sub driver in a very solid built box. The results were a very natural low frequency playback quality, crossing below 80Hz.
The need for Acoustic Absorbent material is relevent for a typical bass reflex design where the crossovers for a 3 way system are typically 700 & 5K Hz.

[Aucosticraft]

Originally Posted by Scott L

The need for Acoustic Absorbent material is relevent for a typical bass reflex design where the crossovers for a 3 way system are typically 700 & 5K Hz.

you mean Acoustic absorbent material is not needed when you are working with subwoofer that is crossing at 160Hz. did I get you right?