View Full Version : Top Perimeter Stiffness

Liam Ryan
09-19-2011, 12:16 AM
I could have derailed the other thread more than i already have but though it's probably a big enough topic to receive its own thread.

A good example of how important the perimeter of the soundboard is, is to take a uke body and tap it. Now route away the channels for the binding and tap it again. You will notice that the tap energy goes down to almost nothing because the rim is not sufficient enough to sustain the vibration. Now, the idea of adding metel to the rim of the top is a little different than cutting away the edge completely, but it shows you how influential the rim is and by adding a material that basically absorbs vibration could very well reduce the quality of the sound of the uke.

I know what you mean about the change in response after the binding channels are routed, although I notice that when the channel is wide enough to expose the kerfs in the linings, the response really dies, as opposed to when part of the sides stays intact. I'm thinking that this could be due to the significant decoupling of the top from the sides with kerf exposure causing greater decoupling and therefore more energy loss. With the kerf exposed, the air mass inside would become less resonant aswell? yes/no?

I can see how CF bindings would make a nice, stiff edge that stops energy bleed across into the ribs.

How do we know whether a material is one that absorbs energy or reflects it? Do you just tap it and see?

Also, is a material that absorbs energy also called "high damping" and "low impedance"? I've seen these terms alot in reference to instrument construction but never received an explanation that really stuck.

BTW, Eric I hope you don't think I'm having a go at you, your comments have really got my grey matter stirring is all. Hopefully Rick and the other pros can chime in too.

Pete Howlett
09-19-2011, 03:05 AM
Oooh er... a can of worms inviting all sorts of speculation. Not a debate I want to be invloved in.

Rick Turner
09-19-2011, 05:04 AM
I'm not so sure about the opening premise here, but that said, I do favor a stiff rim around the top. I think it's possible to have the rim be stiff, have the kerfing be stiff and reflective, to put in purfling, and not have a problem. I've never noticed Martin D-45's suffering in the tone department compared to D-28s, for instance. And I don't hear a difference among our bound and purfled ukes compared to those merely bound or those with no binding at all that I could attribute solely to the edge treatment.

I have made a couple of ukes with a side doubler...an inner lamination of rosewood about 3/4" tall around the rim that was then topped with kerfing. This added mass and stiffness to the rim, and those ukes sound great. They also got binding and purfling, but that's too small a sample for me to make sweeping statements other than that I do think the stiffened upper sides thing works.

It's also one of those things that adds more labor than shows to the end user. I know from experience that it's hard to sell instruments on engineering, especially when it doesn't show. We all sell to a relatively unsophisticated client base who are more into eye candy than ear candy. Maybe that's one more good reason to put in side ports...as a viewing window into the inner workings of the instruments!

09-20-2011, 09:01 AM
... is a material that absorbs energy also called "high damping" and "low impedance"?

Just a comment. Mechanical damping and mechanical impedance are separate characteristics for a material. Impedance is resistance to deflection. A popular way to describe the difference may be the wheel suspension on your car, the spring coil represents the impedance, the oil filled shock absorber represents the damping.

09-21-2011, 09:04 AM
When you cut the binding channel and expose the kerfed linings, you're adding several dozen sound ports....Your resonant air chamber is now leaking like a sieve so the tone changes. Try it with solid linings and see if you get the same effect
I'd say what ever effect the binding has on the tone is probably not noticeable. There are some great sounding ukes out there with plastic bindings, wood bindings, or no bindings so I wouldn't worry about it.