Official Luthiers Forum!

remind me... is there a general rule when shooting for a particular tone that you add/remove mass to raise/lower pitch?

is it localized to the area you tap in?

thanks,
adam

_________________
"Everyone wants to BE something, but nobody wants to BECOME something" - William Cumpiano

Mass and stiffness are 2 different things..
You Remove Stiffness or Add mass to lower frequencies... but the effect seems to have different behaviors depending on what you do.

Raising mass with the same Stiffness = lower frequency..... but it also acts like a "Low pass" filter -- that the added mass cuts high frequencies faster than it cuts bass.... so it makes things sound deeper and more "Pure".. less overtones.

Increasing stiffness with the same mass seems to act like a "High pass" filter -- that the stiffness makes it hard for the low energy bass frequencies to move the top.

Carving braces removes stiffness much faster than it removes mass... so it tends to lower frequencies.

Try this out with some poster putty or a quarter and some sticky tape... This adds lots of mass and no stiffness...

Good luck

John

Thanks John, that's good info.

I was carving braces last night, and i noticed when i tapped the board after carving my first one, there was a pronounced "BONG" that sounded really nice to my ears. Got me to thinking more about what are the cause and effects, what happens if i carve too much, basically how am I going to influence this?

At this point, as I am on my first, I am not going to try to get too deep into that, rather just do what I can to keep the tone pleasant wtihout going to far, and learning to recognize when I may be on the verge of "too far"

If I do go too far, how do i add some back? is that possible?

_________________
"Everyone wants to BE something, but nobody wants to BECOME something" - William Cumpiano

Half of any art is knowing when to stop.

There have been discussions on the list before about the 'cube rule': the stiffness of a brace varies as the cube of the height, and the stiffness of a plate as the cube of the thickness. The mass, though, goes directly as the height/thickness. Thus, making a brace twice as tall makes it eight times as stiff, but only twice as heavy, and the same goes for doubling the thickness of a top.

What this means in terms of frequencies of tap tones is less obvious than it might seem. There are a lot of ways for a top to vibrate; we sometimes call these ways 'modes' of vibration . Some of them involve a given brace bending, and some of them don't. The same goes for the motion in different spots: a brace might be on a spot that moves a lot, or it might be on a stationary 'node' line. If a brace is bending a lot, then it's stiffness is probably helping to keep the frequency of that mode of vibration high, and if it's moving a lot, but not bending much, then the mass of the brace is keeping the pitch of that mode down. If it's neither bending nor moving it's not effecting that mode to speak of, but it might have a major impact on another mode. On the whole, as you make the braces taller and th etop thicker, the pitches of the modes goes up, and it's those modes of vibration that are the 'tap tones'.

I think that the clarity of the sound you get when you tap is probably more important than the actual pitch of the mode, although that tells you something too. The clarity seems to be related to a sort of balance between the braces and the top, so that, if you go too far in shaving braces,you might be able to retrieve the situation by thinning the top. You have to be careful with this, of course; you need a certain amount of strength there too, and that's what the pitch is telling you, in an indirect way. you might be better off, if things get out of balance and floppy, to just add in some more bracing.

I find most of the stuff written about tap tones to be a bit confusing: maybe it's because my hearing is so lousy, but I can never seem to hear what these guys are hearing. I use a more technical method for getting that information, and a bit more. In the end, we all have to find the way that works best for us. The neat thing is that all of these methods seem to work well for somebody.

I found a talk that Dana Bourgeois gave at last year's GAL to be extremely helpful in voicing my tops.

Here's an article that captures Dana's approach.

http://www.pantheonguitars.com/voicing.htm