Enclosure Volume Question - Dampening/Lining

guangui

Member
2013-02-12 12:21 pm
I am sure this question has been posted before, but not sure what is the correct answer for this particular liner that I use to dampen the inner walls of an enclosure.

I like using 3/8" wool felt used mostly for upholstery jobs to line and dampen my speaker enclosures. Not sure what is the felt grade, but pretty sure it is basically the same for F3, F5, F13, etc. I use this one in particular, because I can just go to the upholstery supplies store close to my home and buy 36" wide rolls by the yard for around $3.75 per yard. So far it has worked great for my applications. I also buy 1 cu.ft. of poly-stuffing for around $3.00.

My question is, do I need to add the volume lost by the felt being used to the enclosure? Or, is it neglectable?
 
Volume is largely unaffected. In fact heavy fibreglass stuffing makes the volume effectively bigger. I am a great fan of picking up stuff at the builder's supply store. Nothing special about loudspeakers, IMO. You might as well use felt, carpet, rubber mats or air conditioning engineer's BAF wadding. Even pillowcases and old jumpers have something going for them.

Panel damping should be stuck to the cabinet walls for best effect. Wadding and loose stuffing, which you might call volume damping, does little at low frequencies, but damps the voices more.

Some good stuff here:
http://www.diyaudio.com/forums/multi-way/223174-interesting-read-i-found-lossy-cabinet-designs-harbeth.html#post3234256
Tips and ideas Copyright 2012-14
Cabinet-damping

Not rocket-science. Generally speaking, you don't use a lot of volume stuffing with reflex, the air needs to flow. Closed box, add as much as you like! :D
 
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guangui

Member
2013-02-12 12:21 pm
Thanks Steve for you quick reply! Will neglect it then, such a difference when I use damping and lining. A couple of years ago I got a pair of used PSB Alpha bookshelf for my sister. When they arrived they had the tweeters blown, so I ordered new tweeters to change them. When I opened them I was amazed at the great work done by PSB with damping, they just threw in, and I mean literally throw inside, a piece of 1" thick fiberglass insulation of the same width and depth of the enclosure. The insulation even had the foil on one side. I asked my cousin, who used to be a PSB dealer, and he said that it was how they came from the factory. I have a pair of PSB B25's, and I'm afraid of opening them and being disappointed as to what I will find inside being used to dampen.
 
Ah, it is shocking how little attention commercial designs pay to damping! You are right that throwing in a bit of white fluff is as far as most companies go. :D

Whilst Sony take the white fluff approach too, they creditably put little short corner battens to strengthen cabinet edges at crucial points, like near the woofer. It's a question of money and cost I suppose.

You often notice how old cabinets have come unglued at the joins, due to vibration. I usually reglue them and add battens. And I am quite keen on panel damping light structures.

The real idea is that below a certain level if you get damping right, resonances, even in light cabinets, are essentially inaudible. Mostly the cabinet should be the same everywhere, and strengthening one panel just drives the vibration to the weaker ones. Harbeth are good at this. They even find a loosely screwed front panel with a rubber gasket has some benefit: "A cracked Bell doesn't ring!"
 
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eriksquires

Member
2013-05-10 4:11 pm
It is possible to make an enclosure work like a bigger enclosure with the soft fluffy stuff. To dampen the walls though, about the only stuff I know that really works is Sonic Barrier, especially the multi-layer stuff. On my center channel, using the Dayton MTM 0.75 cabinet I used it quite effectively to reduce resonances in the front chamber (towards the drivers) where I could feel the most vibrations occurring. Still, it doesn't make miracles. On cheaper MDF enclosures it didn't seem to do much.

I understand Selah uses it too.

Best,

Erik
 

guangui

Member
2013-02-12 12:21 pm
It is possible to make an enclosure work like a bigger enclosure with the soft fluffy stuff. To dampen the walls though, about the only stuff I know that really works is Sonic Barrier, especially the multi-layer stuff. On my center channel, using the Dayton MTM 0.75 cabinet I used it quite effectively to reduce resonances in the front chamber (towards the drivers) where I could feel the most vibrations occurring. Still, it doesn't make miracles. On cheaper MDF enclosures it didn't seem to do much.

I understand Selah uses it too.

Best,

Erik

I have had good results with 3/8" thk. wool felt.
 
If it's a sealed-box design, you can increase the effective volume by about 20% by stuffing it with fibreglass etc. The reason is that the stuffing converts what would have been adiabatic expansion/contraction to isothermal.

This is because the stuffing material has thermal mass, so that the air in the enclosure is kept at a fairly constant temperature (as opposed to heating up and cooling down during contraction and expansion).
 

guangui

Member
2013-02-12 12:21 pm
The enclosures I've built so far are ported or t-line, none have been sealed, though working on an 18" sealed sub design for my HT. I go mostly ported as I don't like to rely on subwoofers, and since most of my DIY's are bookshelf or small t-line; a space issue. Using wool felt allows the lining not to be intrusive with the port. One of the designs I mildly stuffed with poly-stuffing and sound got better, bass somewhat deeper, and there was no issue with port interference. I guess that if you go heavy on the poly-stuffing or other damping material that takes a lot of volume inside the enclosure port interference would definitely become an issue.