Closed box T(1,1)2 less than 200 Hz - Edge thinning

[AKEric]

I just closed the box of my second OM sized falcate (fourth guitar overall). This one is a sitka bear claw top and IER back and sides.

On the two I have built, the T(1,1)2 have both come in at less than 200 Hz at this phase.

#1 = 100-193-235 for T 1 2 & 3 (14 fret, sitka braces with 6.5 mm max brace height)
#2 = 103-196-235 (12 fret with cut-a-way, sitka braces with 6.1 mm max brace height)

At least I have some consistency.

Per the books, I would prefer the T2 to be over 200 Hz so as to thin the sound board’s edges.

I love the concept of thinning the edges and couldn’t resist on the first one, so I disregarded wisdom and thinned them a bit anyway. T2 dropped from 193 to 188 Hz. The finished guitar came in at 94-171-214 Hz and sounds wonderful, but I was striving for 95-180-226. Obviously thinning the edges was a mistake because of the excessive T2 drop.

I chose to reduce the brace height on #2 because #1’s bridge rotation was only 1.3 degrees, and its top is both lighter and less dense than #2.

I will not thin the edges on this one, but on future builds I’d like to do so.

So, is getting the T2 higher than 200 Hz at this phase a function of the tops mass? Is mine a bit too heavy? Or is it a function of the bracing stiffness, and would it help to lower the max brace height some more? Which would also help reduce the mass.

Thanks for any help.

Eric

[johnparchem]

Thinning the edges is a good way to Lower T(1,1)2
Lowering the braces is a real good way to Lower T(1,1)2
Having A thinner top is a good way to Lower T(1,1)2

The tops resonance is based on both stiffness and mass. In all the above three cases you are lowering the stiffness more than you are lowering mass.

So the best way to have a higher top resonance is to have stiffer braces (taller) as that is the most efficient way of raising stiffness relative to mass.

[AKEric]

Thank you John. That's as clear as it can be made. Somehow I had the relationships backwards in my mind, and am probably putting too much emphasis on bridge rotation. Trevor had indicated to me earlier that it is just one of many considerations, and probably not the most critical one. I'll see where this one's resonances end up when finished, and on future ones I'll probably make the brace height a bit taller and see what happens.

[johnparchem]

I think both of your guitars that came in around 195 Hz had a good chance of hitting 180 Hz once finished. It would have depended on the mass of the bridge and how much the bridge lowers the top's resonance. On the steel strings I have been seeing <-10 Hz adding the bridge and another <-5 adding the strings. So what you were doing was really close and better yet consistent before thinning the edge.