Official Luthiers Forum!

Looks really nice! What kind of finish did you use?

Looks great Miguel.
I like the rosette and the way the tornavoz merged with the edges of the sound hole.

What's the interesting patch at the neck fingerboard top about?

I am curious about the tornavoz. Can you point me to a resource that explains its use?

What a cool guitar. All the woods look great together and I really like the rosette design and how the angle at the end of the fretboard is reflected in the headstock. What's your impression of how the Tornavoz affects the sound?

[In the case of a normal guitar, if you made the box rigid but left the hole open you'd end up with a Helmholtz resonance somewhere around 125 Hz. With a backless box that has some sort of edge stiffener, the 'main top' resonance would probably be in the range of 160-170 or so. When the box is assembled the two have to work together. The top moving changes the air pressure in the box, and pumps air through the hole, and air moving in and out, changes the pressure on the top, altering it's pitch. You end up with a 'main air' resonance somewhere near 100 Hz, and a 'main top' closer to 200.

Putting is a tornavoz drops the pitch that the Helmholtz resonance would come in at in a 'rigid' box. Also, the added drag of the tube reduces the amount of air that can be moved, and spreads out the resonant pitch. On the assembled guitar you end up with the 'main air' pitch quite a bit lower in frequency than normal, and reduced a lot in strength.

___________________



If you look at Helmholtz' original work, you'll see that there are two variables in determining the pitch of a given size bottle: the diameter of the neck, and it's length. A longer neck works like a smaller diameter, yielding a lower pitch for the Helmholtz mode.

Although most guitars don't have a physical 'neck, there is, nonetheless, some 'length' to the hole. In Helmholtz' work this is called an 'end correction'. It's sort of ad hoc; he basically admits that his model is not exactly right and introduces a fudge factor that makes the equation work pretty well most of the time.

The idea is that the air in the neck acts as a rigid piston of a certain mass, and the air in the bottle is a massless spring. The end correction just allows you to add some of the mass of the air outside of the bottle to the piston, since that has to be moved too, effectively making the neck longer.

Drag in the neck doesn't so much slow things down as just waste energy. You get the same thing when you add drag in the hole in the form of a carved 'rose' or a 'feedback buster'. Those don't alter the Helmholtz pitch, but they do drop the peak height in the spectrum, sometimes spectacularly. Smaler holes have relatively more drag, of course; there's more 'edge' for a given 'area', and it's edges that cause most of the drag.

A tornavoz both makes a longer neck and adds drag.

Thanks Sen! That "interesting" patch is a cover up for an "interesting" accident when a pointed tool was left to fall upon the soundboard after i had just finished cutting the binding rebates. Should have tried to fix it there, but i just installed the bindings and the pull from the rope worsened it, so i ended up gluing a patch of spruce there - which still looked bad, so there went the tulipwood "lid" - now i kinda like it that way!