First overtone help

[MStaples]

I have a marimba bar I'm making that hopes to be a 3G# bar someday.  Unfortunately, partially through the sanding process I find that the bar reads flat 3A# (which I think is fine for this point in the process) for the fundamental and flat 5F# for the first overtone (not so good I think).  I assume that sanding the first overtone below its target note is not a different problem than sanding the fundamental too low and only a very small amount can be done to raise its pitch (by carving an end corner of the bar).  Is there another solution?

Thanks!

Michael S

[Jim McCarthy]

Hi Michael
I’m afraid it’s not good news from me!
I certainly don’t know of any way to raise the pitch of one mode without raising them all – I’m 99% sure there isn’t one. CERTAINLY there is no way at all, to raise the pitch of any mode at all, in a SIGNIFICANT way without actually making the bar shorter, which would destroy your nodal positions etc. Once you lower past a semitone or even ½ a semitone it CAN be a matter of throw it out and start again.

As you are pretty close to your target pitch, it might be worth having a go though, although that's a pretty big difference with the fundamental. You are correct saying that the overtones are similar to the fundamental with the "go too far" problem - but you might be able to manage it. It might not work too, but you have nothing to lose - at any rate the experiment will really teach you a lot about what works with your timber etc.

You would have to start by raising the whole bar to the point where the overtone is at least a little bit above the target. Then remove timber from the bar arch RIGHT IN THE VERY CENTER. The closer to the exact middle you can get the more it will effect the fundamental without changing the 1st overtone. I think you will find the overtone is still lowered too much anyway, but it's worth a shot.

The real trick is to work as hard as possible in the very early stages of shaping the bar, to get the 2 octave relationship of the overtone correct. This usually means starting your sanding more in the middle. Once you get the relationship correct - hopefully BEFORE the final target is achieved!.... then you sand a bit and test... sand a bit and test - etc working all the time to lower the tones together whilst keeping the relationship correct. The tricky thing is that you can easily lower the 1st overtone more than the fundamental by sanding closer to the nodes away from the bar middle - BUT - the reverse is not so much true. Or at least the isolated effect is not so great. Sanding right in the middle does tend to lower BOTH tones. You can get the fundamental moving a BIT more than the overtone, but it's more difficult than the other way around.

Hope this helps you out Michael.
Jim