Official Luthiers Forum!

I read Brock's comments.......lets just say we are traveling a similar path relative to building adjustments/styles.

Greg

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Gwaltney Guitars

Does Ervin say _why_ he thinks the dipoles have the effects they do? I'm just curious as to how his theories fit in with mine.

I agree with Paul---what a great thread. Thanks everyone who's contributing.

Of you folks who studied with Somogyi would someone please answer Al's question? Thanks.

We would also like to hear from folks, and there are MANY of you here..., who like to build guitars for blue grass. Typically, but not always, these guitars are dreads and this is an application where projection and being a ba*jo killer is important.

For you folks that build dreads to project tell us what your design includes? For example thoughts on top thickness, bracing, scallop or no, backs reflective or active, preferred tonewoods etc.

Thanks

If I remember correctly, I believe it has to do with how these dipoles work with the soundhole. The long dipole acts as a pump with the soundhole, whereas the cross dipole does not activate the soundhole pump.

Alan, How does that previous comment jibe with your model?

Barry Daniels asked:
"Alan, How does that previous comment jibe with your model? "

First, it's nice to see _somebody_ use 'jibe' instead of 'jive'.

Hesh wrote:
"The long dipole acts as a pump with the soundhole, whereas the cross dipole does not activate the soundhole pump."

That's basically it, I think, although it's more complicated, and I'd bet Ervin knows that even if he doesn't spend a lot of time drilling it into students.

The monopole mode of the top is the most effective sound producer on the guitar. Some researchers think that it makes most of the sound all the way up to about 1000 Hz.: even the 'tail' of the monopole up at, say, 500 Hz may put out more sound than the tripole mode at it's peak. Keep in mind that, in many respects, the 'bass reflex couple' with the 'main air' mode is just a way of capturing some energy from the top monople that would otherwise be wasted, and it won't be strong unless the monopole is strong. Also remember that all of the other resonances work by stealing energy from the top, except maybe the first corpus or 'neck' mode.

Usually the 'cross dipole' is the next mode up in pitch from the monopole. It's nowhere near as effective a radiator of sound, if only because of the 'phase cencellation' of having two areas going in opposite directions. If it's not too far up in pitch from the monopole mode it can steal a fair amount of energy from it, and becomes a 'cuttoff' of that mode peak in the output spectrum. It's been shown (by Meyers, IIRC) that guitars with a narrow 'main top' peak in the spectrum tend to have a 'sharp', 'cutting' or even 'harsh' tone, so a low-pitched cross dipole can contribute to the 'projecting' character of the sound. I note that Flamenco guitars often have much lower cross dipole pitches than Classicals.

Steel strings tend to have relatively high-pitched cross dipole modes, due to the X bracing. This helps to give them a 'fuller', 'rounder' sound, which is what you generally need with steel strings. Tightening up the X angle (as measured toward the soundhole, say) will tend to decrease the crosswise stiffness of the top, and lower the cross dipole mode pitch, thus adding a bit of 'cut'. You can get something of the same effect after the fact by thinning out the 'wings' of the top between the end of the bridge and the edge.

The 'long dipole' mode also suffers from the phase cancellation problem, although to a lesser extent. It does have the advantage, though, of being able to couple usefully with the 'lenghtwise sloshing' A-1 air resonant mode(s). It's interesting that the A-1 mode 'should' occur at about 350 Hz on a box the length of most guitar bodies, and that you often see the 'long dipole' on the top at somewhere near that pitch, even with bracing patterns as different as X bracing and fan bracing. Proximity in pitch increases the strength of the couple between the wood and the air. This air mode is not normally a strong radiator of sound, but when it couples with the long dipole, AND combined with the effect of the waist and 'normal' soundhole location, you can get two 'A-1-type' modes, one of which can radiate very strongly. Dreads often lack the second 'A-1' type mode, even when the top and air are reasonably close in pitch. It's also generally lacking on the guitars I've measured that have the hole up in the corner someplace. The physics seems to be fairly tricky.

For various reasons the 'long dipole/A-1' couple does seem to help in the 'enveloping' end of the spectrum, so thinning the top below the bridge to increase the amount of air the long dipole moves, and lower it's pitch, seems to help with that in many cases.

As always, YMMV.

You could do worse...

This might even be enveloping... Though "Ethyl" Merman would actually be a chemical...

Peace,
Sanaka

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...imagine there were no hypothetical situations...

Great post Al !

Can someone remind me.......which pattern is the long dipole?

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Formerly known as Adaboy.......

Rocking from the top to bottom of the instrument. Side-to-side is cross-dipole. The piston type motion of the top is monopole.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Thanks Brock!

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Formerly known as Adaboy.......

Thanks Sanaka - maybe I need to reconsider my earlier statement.....

OK we have not heard from the dread builders who want their guitars to be loud and clear. How do YOU build a banjo killer???

Thanks

I guess I keep getting my spelling mixed up: somebody gave me a handout years ago about 'Making Ethyl Palpitate', which is not the sort of chemistry they teach in High School, even if it's the sort most of us were thinking about.

Now that's funny! Good one Al!

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Al, if guitars with sound holes "up in the corner someplace" lack the second "A-1" type mode, then does placing a sound port in the upper bout toward the player (where it is often placed) possibly lessen or eliminate the second "A-1" mode?

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Formerly known as Adaboy.......

Darryl Young asked:
"does placing a sound port in the upper bout toward the player (where it is often placed) possibly lessen or eliminate the second "A-1" mode?"

A lot depends on what you mean by 'lessen' and 'eliminate'.

A port in an area of large pressure changes of a mode will vent pressure from it. This reduces the sound pressure of that mode in the box, and generally raises the pitch. You will, of course, hear that mode outside of the box in the room more strongly. you have to put ina pretty big hole to 'eliminate' a mode entirely, but, of course, once the pitch has gone 'way up and the activity 'way down, it's not very effective.

I've got numbers and diagrams for some of this in the 'ports' paper on my web site, and you can download it as a .pdf.