Can you carve top variably to emphasize bass vs treble?

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Can you carve top variably to emphasize bass vs treble?

Postby Brian Evans » Mon Mar 25, 2019 7:04 am

A guy on the F'Book claims that D'Angelico carved and braced his tops differently on the bass side vs the treble side so that different frequencies were emphasized. The idea being that even a non-cutaway archtop was "handed" and should not be made left handed if intended to be right handed. I think this is bunk, that he carved and braced his tops normally, and that this whole idea doesn't work anyway. But I have to ask - any ideas on this? Stiffer for trebles and floppy for bass, vice versa, or the whole top responds to all frequencies so do a good job everywhere?
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Re: Can you carve top variably to emphasize bass vs treble?

Postby Alan Carruth » Mon Mar 25, 2019 11:32 am

As you say, the whole notion that there's a 'bass side' and 'treble side' to the top in terms of sound production is bogus. Basically, the whole top moves as a unit at lower frequencies, and it breaks up into smaller vibrating areas as you go up in pitch. Just exactly how that works depends a lot on on the wood of the top and on how it's carved and braced.

By comparison with flat tops, archtop guitars are very much 'treble balanced'. The carved arch adds a lot of stiffness moving the 'main top' resonant mode upward in pitch. The generally thicker top also tends to delay the breakup into smaller vibrating areas until a higher pitch is reached. Add to this the fact that the F-holes produce a higher 'main air' resonant frequency than is common on flat tops (often as much as a third higher) and the entire basis of the sound is shifted toward the treble end of things.

With all of that said, it's possible to affect the treble-to-bass balance a fair amount. The obvious place to start is by using smaller F-holes. From what I've seen 'parallel' bracing seems to produce a more 'bass balanced' timbre than X-bracing. It's likely that the top graduation scheme also can make a difference.

Back when I was taking voice lessons (from a fellow who also had a masters in jazz guitar from Berklee) he said that the way to improve your bass notes is to work on the trebles. Because of the way hearing works the energy of the high harmonics tends to be added to the fundamental of the note in your mind, so getting a clearer and stronger upper register can actually enhance the perception of bass tonality. This seems to me to depend more on how well the top is graduated, and how carefully the braces are balanced with the top, than the overall scheme of the top thickness and brace profile. Obviously, using a lower arch and thinner top graduations will probably help to emphasize the bass a bit, along with the afore mentioned smaller holes, and a deeper rib might help as well. Archtops seem to be even more a system than flat top guitars: you really can't change just one thing.
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Re: Can you carve top variably to emphasize bass vs treble?

Postby Mark Wybierala » Mon Mar 25, 2019 11:38 am

Tone happens
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Re: Can you carve top variably to emphasize bass vs treble?

Postby John Clifford » Fri Mar 29, 2019 5:55 pm

I think it's clear, as Alan said, that there's no such thing as a "bass side" and "treble side" to the top. However, the same is not necessarily true for the bridge. I haven't personally experimented with this, but I think you could carve a bridge to have less mass on the treble side, with a reasonable expectation that it would enhance treble response on that side. Also, the sound hole shape and placement makes a big difference. My guitars with a sort of pear-shaped sound hole in the upper bout have a much deeper bass resonance than the ones with slit-type sound holes in the lower bout. I can't prove this either, but I think leaving the top as flexible as possible also enhances bass response. I've shortened my X braces from what Benedetto recommends and tapered the vertical dimension considerably, so that they are pretty much providing structural support only under the central flatter area of the arch around the bridge, the idea being that the more curved peripheral areas are self supporting and should be as flexible as possible. Seems to work for me.
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Re: Can you carve top variably to emphasize bass vs treble?

Postby Brian Evans » Fri Mar 29, 2019 8:25 pm

John, how interesting that your guitars with a upper bout sound hole have deep bass response, because I have the same experience. I also try to minimize the X braces on that guitar with a taper. We share some basic ideas, I think.
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Re: Can you carve top variably to emphasize bass vs treble?

Postby John Clifford » Fri Mar 29, 2019 9:09 pm

It also seems to me that cutting big holes in the lower bout must interfere to some extent with the monopole and cross dipole modes of vibration of the top. I'm wondering if anyone has studied this? It's kind of hard to produce chladni patterns on an archtop.
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Re: Can you carve top variably to emphasize bass vs treble?

Postby Alan Carruth » Sat Mar 30, 2019 6:00 pm

Moving the sound hole into the upper corner lowers the pitch of the 'main air' resonance, all else equal. Look up Allens' article on air resonance in American Lutherie #1 or the first 'Big Red Book' for more information.
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