[Peter J]

I now have a video on YouTube that compares the resonant properties of three materials: 13ply 3/4" plywood, 1" thick Richlite, and 3/4" MDF. Each panel is suspended by a wire and tapped with a hammer. Check it out.
http://www.youtube.com/watch?v=SkSxH40x5SA



Wayne

Wayne, Im curious. The MDF you show looks to be much larger particle size than what I'm accustomed to. MDF I've used is like consolidated dust. The stuff you call MDF resembles particle board and although I've never tested like this, I'd assume has different resonant qualities.
Maybe a regional thing?

[WGH]

The size thing - I was willing to cut down the particleboard scrap to match the Richlite sample but the 13ply is still a usable scrap and I didn't want to turn it into 2 unusable scraps on a whim, by the noise it makes you get the general idea. The tone of the Richlite was what I was after, the other tones are a bonus.

The MDF or Particleboard thing - it does look strange though I usually buy MDF, it still could be an old particleboard scrap. Once again you get the general idea.

A well made speaker cabinet would never have such a large unbraced area. The stock VSA VR-2 side panels were close which is why I added 4 H-shaped walnut braces per cabinet with Richlite in between each each brace, the change was dramatic with deeper better defined bass and improved focus.

VR-2 braces in ash


Wayne

[richidoo]

Link to Dave's chart

Looks like a reasonable hypothesis. So with resonance raised by bracing to make for easier damping, do you ever add panel damping, or do you just live with the lower energy resonance?  At higher freqs the internal waves will be somewhat absorbed by acoustic damping, but the external waves are still audible. Thanks

[planet10]

The plywood is inherently self-damping because of all the plys -- the interface between each ply has a change of impedance which damps things. Not as inherently well damped as a constrained layer, but with none of the construction issues. The only panel damping is that supplied by the 1/2" recycled cotton felt that inherently lines the walls.

Test builds are examined with a mechanics stethescope, and if anything significant can be heard thru the panels, more bracing is applied.

A too resonant test build will be discarded or have an additional layer of material applied. Sometimes dissimilar.

dave

[WGH]

Here is a fun little video I made a while back, another tap test on two VR-2 cabinets - one with bracing and Richlite and one without.
Please turn your head sideways while watching. (Professional woodworker - amateur videoidiot)
http://www.youtube.com/watch?v=l_nWFKVQi-Q



Dave, I'm still wadding through the big diyaudio thread you linked to, good stuff, I like your ideas.

Wayne

[richidoo]

Thanks Dave, that makes sense.

[BPoletti]

The plywood is inherently self-damping because of all the plys -- the interface between each ply has a change of impedance which damps things. Not as inherently well damped as a constrained layer, but with none of the construction issues. The only panel damping is that supplied by the 1/2" recycled cotton felt that inherently lines the walls.

Test builds are examined with a mechanics stethescope, and if anything significant can be heard thru the panels, more bracing is applied.

A too resonant test build will be discarded or have an additional layer of material applied. Sometimes dissimilar.

dave

Dave,

Where can we get that 1/2" recycled cotton felt? 

Bill

[planet10]

Bob (at CSS) sells it in multiples of 4ft^2, i supply it with flat-paks & optional with drivers, or you can get the full 200ft rolls (800 ft^2) from distributors of UltraTouch. It may be a special order thou.

dave

[BPoletti]

Thank you kindly.

[richidoo]

Sorry, what is CSS?   Link?   thanks

I have used the denim bat in 1" and 4" thickness, from Sensible Sound Solutions. It is powerful absorber.

[planet10]

Sorry, what is CSS?

http://creativesound.ca/

dave

[richidoo]

Thanks Dave.

[WGH]

Here are a couple of products that may be useful for cabinet construction. Since Dave likes the self damping properties of plywood then the 3/4" 13-ply Russian plywood would be a good choice (top).

The bottom sample is 3/4" clear pine plywood with a lumber core. I paid $38.61 per sheet.



Lately I have been making my own 1/8" veneer and using Baltic birch or Russian plywood with West epoxy for my door panels. I think my next speakers will have similar construction.



Wayne

[planet10]

Wayne,

that last material should be very good. The thickness of the veneer either side (and the expoxy) will create a high impedance mismatch and increase damping.

dave

[face]

http://www.skiingninja.com/Articles.asp?ID=157