MJL21193 said:
A piece of wool felt or cotton felt placed next to a panel or a mass of loosely tufted polyfill have very little, if any, direct affect on the panel (and what they do directlt affect are only at higher frequencies). It is improtant to differentiate between airspace damping -- primarily intended to damp out reflection, standing waves, and higher harmonic pipe resonances -- from panel damping, which is intended to directly damp a panel. They are not the same beast at all.
dave
MJL21193 said:
Yes... you will get cancellation (& reinforcement) of the waveform but only at frequencies smaller than the cabinet dimensions and very much dependent on frequency & where you are considering the waveforms, These waveforms are actually mostly non-desructive and largely pass thru one another.
At low frquencies (wavelenghs longer than any cabinet dimension) the air acts as a lump and the 2 (or more) drivers act on the air as would a single larger driver (a woofer does not care what side of the box you mount the drivers on.
carpet tightly bonded to a panel should be effective at panel damping, As to standing waves it will have a very limited bandwidth related to its thickness and the material it is made out of. (here i am imagining your boxes lined with the orange shag carpet from the 60s/70s
)dave
MJL21193 said:
No. What i am saying is that panel damping pushes the panels in extactly the opposite direction of what i am trying to achieve... it lowers panel resonance frequency by adding mass without increasing stiffness making it more likely that panel resonances will be excited and need to have the damping material. I didn't say it wouldn't work, it is just not as elegant (or as cost effective -- especially if you have to ship the box or lift it)
dave
planet10 said:
So, I guess what you are saying is that even though you are damping your panels, you like to say you're not.
The difference between the panel damping you do and one that is honest about it's intention is effectiveness. When the objective is to reduce panel vibrations, it can be addressed efficiently.
Take that same tuning fork and just touch it to the polyfil ever so slightly. Something magic happens.
MJL21193 said:There you have it: 20 pairs of speakers on the scrap heap because the designer didn't understand the limitations of solid wood, even here in the "sahara" like climate of Canada.
It's probably the first time in my life that ANYONE accused BC of being too dry
Actually, it just shows that you need to know what you're doing. A friend exports solid wood doors to Canada with minimal problems. It just requires getting the wood to the appropriate level of water content and properly sealing it, as you mentioned before. I believe the real limitation with wood in speakers is like I mentioned earlier, that it transmits sound quite well. I'm unsure how well damping it from vibration will counter the ability of sound to travel along the grain structure.
johninCR said:
And this gentleman has got it in one. The moisture content. Having never been to Canada, the moisture content required was not told to me. Here in the UK, we kiln dry to 10-12%. Provided it stays over here, there isn't a problem. If I'd been told, it could have been kilned to the right content.
planet10 said:
carpet tightly bonded to a panel should be effective at panel damping, As to standing waves it will have a very limited bandwidth related to its thickness and the material it is made out of. (here i am imagining your boxes lined with the orange shag carpet from the 60s/70s
dave
Not at all. The orange shag is on my floor
, not in my boxes. I have commercial carpet tiles with a rubberized plastic backing(not the cheap foam rubber). As you pointed out earlier, low frequency sounds have long wavelengths. There are not many materials of the thickness required to absorb these waves that will fit in a modest sized box. So, when I say standing waves, I'm talking about the waves that can be interupted by this material.
Here's a pic of both the shag and the damping material:
Attachments
MJL21193 said:
Not at all. The orange shag is on my floor
As you pointed out earlier, low frequency sounds have long wavelengths. There are not many materials of the thickness required to absorb these waves that will fit in a modest sized box. So, when I say standing waves, I'm talking about the waves that can be interupted by this material.
Here's a pic of both the shag and the damping material:
Gotta be fair to you. Just looked at your pic. That's a nice well built cabinet.
Ricky said:
And this gentleman has got it in one. The moisture content. Having never been to Canada, the moisture content required was not told to me. Here in the UK, we kiln dry to 10-12%. Provided it stays over here, there isn't a problem. If I'd been told, it could have been kilned to the right content.
With all due respect: ever hear of equalibrium moisture content? It's the point where wood is most stable. The key to solid wood stability is to maintain EMC by completely sealing the wood, inside the box and out. If you start with wood that is not at EMC, it will dry out and shrink, regardless of the sealing process. The best solution for this problem(and the very thing that started this thread), is to not use solid wood.
MJL21193 said:
It is the rubber backing that acts as a panel damper. Commercial carpet fibres will have little air space damping and will have little affect on standing waves. And in a subwoofer of that size, it is unlikely that any box dimensions are large enuff to get excited by the subs bandwidth... what XO frequency are you using?
dave
MJL21193 said:"Gotta be fair to you. Just looked at your pic. That's a nice well built cabinet."
Ricky: Sorry for my last harsh sounding post, you obviously are a learned gentleman and scholar. Thanks.
Don't worry about it. The points you make are very valid and, if I'd used my brain, I should've asked the moisture content at the time. We live and learn.
Whilst on the subject of hardwoods, I had a discussion with Lawrie Fincham of the original KEF and there are also issues that hardwood is in fact porous at certain frequencies.
Ricky.
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