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I'm thinking of making and more decorative top structure for my next build and I'm wondering what everyone's thoughts are as regards to the sound hole size/shape, and has anyone ever played around with rosette designs that aren't necessarily restricted to the soundholr perimeter? I've seen a Harry Fleishman guitar with an eagle feather design where the feather (abalone or pearl) overlays the soundhole a little bit.
I know on some novelty ukes, the soundholes are cut out to resemble seals or dolphins, but I'm thinkning of an M-size guiatr.

I did some loudspeaker ( bass reflex) design years ago. And for ported speakers, the important thing is the area of the opening, not the shape of the opening.

My guess is that for guitars, it's probably the same thing: as long as you keep the soundhole area the same, it probably won't change the sound very much, if at all.

Phil

What Phil said, you can make it larger and it will sound louder too!

I don't know as much about this subject as I would like, but I'll toss in what little experience I have on the subject.

I built two prototypes a couple of years ago -- classicals. The first one had a 24-fret neck fingerboard, and because of this I didn't use the traditionally placed soundhole, which normally intersects the 19th fret. Instead, I installed twin soundholes, above and below the fretboard. They had the same area as a single soundhole has, however. The second prototype had a single soundhole of traditional size and placement.

While both guitars sounded fine, the one with the single soundhole had much more of a traditional classical sound.

In fairness, however, I must also admit to other variables. The first prototype not only had twin soundholes, but had a redwood top and a lattice bracing pattern. The second had a spruce top and a traditional 7-fan bracing pattern. I'm sure that the wood and bracing pattern choices had an effect on the overall sound of the two instruments. But to what degree, I can't say. It has always been my gut feeling, however, that the soundhole placement had more of an effect on the guitars' resultant sound than the other variables.

Just my oh-two.

Best,

Michael

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Live to Play, Play to Live

Do sound "ports" have a tonal affect?

Where's Al Caruth when you need him? IIRC, he has done some testing on this and has come up with the same conclusion as Phil.

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Joe Breault
Merrimack, NH
Perpetual novice

IMO sound ports have a tonal affect at the sound port but almost none at the sound hole so over all I would say no. The tone or overall sound emitted by the sound port seems to me to be more treble dominate but that is fine by me because the tone the player misses the most when playing a non ported guitar is the higher tones. at least it is for me.

[QUOTE=Serge Poirier] What Phil said, you can make it larger and it will sound louder too![/QUOTE]

Bigger sound holes bring out the trebles more
Smaller sound holes bring out the bass more.

Within reason of course....

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

Thanks Brock, i stand educated!

Art, On a related note, if you don't mind.
Lets say you carve a pierced rose on a flattop, how would you clamp a fixed bridge for gluing?

Wade

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Wade
Nashua, NH
http://www.wadefx.com

You'll find a lot of good poop on this topic here and here.

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D..

Wade, I bet colin knows the answer for that as he carves the pierced rose on his lutes.

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Joe Breault
Merrimack, NH
Perpetual novice

You got that right Joe,
I have just had a pm back and forth with Colin this morning.
I thought I’d put the question out there to see what others experience has been.
Some of the big issues with making a flattop with a pierced rosette come from the fact that there is no more access to the inside once the box is closed.
I can see why you don’t see too many doing this.
I suppose you could make a tail door.
Also, Art,
Going back to your question, I have made a few odd shaped sound holes and I don’t think they made much of a difference with the sound. One thing for sure, they don’t need to be perfectly round. I’d be interested to see what you come up with.

Wade

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Wade
Nashua, NH
http://www.wadefx.com

I'm figuring that access for gluing/repairs/etc. could be through a trap door in the end block. I'm also thinking of a variable compensated saddle/bridge idea I've been chewing on for awhile.
Thanks for all the input, I'll keep eveyone posted...vpelleri38947.7496180556

So Wade, what did Collin say?

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Joe Breault
Merrimack, NH
Perpetual novice

Been busy...

Soundhole size effects the pitch of the 'main air' resonance, the lowest one that can put out any sound. It will be lower the smaller the hole. It will also be less powerful, and have a lower 'Q' value as the hole size is decreased. One would expect the lower pitch to make the guitar 'bassier', but at some point I suspect it's possible that the loss of power would overbalance that, and make the bass weaker. Haven't done that experiment yet.

I believe part of the extra 'brightness' of a larger soundhole has to do with the higher Q value of the resonance: it's more sharply defined, and that usually sounds brighter. A larger hole should also have a lower 'upper cuttoff' frequency, and radiate high end sound more effectively up to that pitch. The range for normal soundhole sizes would seem to be from 3-5 kHz. Again, I need to do the experiment to be sure...

Moving the hole toward the neck edge of the body will drop the Helmholtz pitch fast, assuming the size remains the same. As far as I can tell, there's not much benefit to going below, say, F# on a guitar in normal tuning.

The more edge the hole has in relation to the area the greater the drag as the air moves through it. This cuts down the activity of the Helmholtz mode, without effecting the peak frequency much. Roses, whether wood or paper, go a long way to killing the bass.

Iirc, a slot of 'reasonable' width has an effective diameter equal to 1/3 its length: a 3" long slot 1/4" wide should act a lot like a 1" diameter hole, but with more drag. The slot may 'hear' internal air resonances that a smaller round hole would not. Michael's two holes may have been hearing more of a croswise mode in the upper bout than the central normal hole did.

Side ports also hear stuff the main hole doesn't, and the sound level generally increases at those pitches. OTOH, the port also bleeds off pressure from the 'main air' mode, and this seems to drop the overall output of the guitar very slightly. My measurements indicate that the total power output with a port may increase, but only by 1-2%, which is hardly worth the effort.

The main hole and top radiate low frequency sound equally in all directions; since they are so much smaller than a wave length of these low tones they act like 'point sources'. The player can hear them about as well as anybody. As you go up in pitch the highs tend to go out more toward the front of the guitar, and the player only gets reflections. In a large or 'dead' or noisy room, like a restaurant gig, you can't count on hearing much above the open G string pitch. A port pointed at the player gives you that high end sound.

The closer the port is to the main hole location, and the smaller it is, the less it is likely to alter the timbre of the guitar. On guitars with ports I cut down the main soundhole size. It's hard to say how small to make it, though, since the farther the port is from the main hole, and the larger it is, the more it effects the Helmholtz mode pitch.   

Thanks Al.
I don’t know what we would do without you!
So.. if I make a pierced rosette at the usual place for a “normal” flattop, would the extra obstruction of the hole cause a drop in the “Q value”?
Would it make sense to make the piercing slightly larger to compensate?

Joe,
For his Lutes, Colin glues his bridge on the top before gluing the top on.

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Wade
Nashua, NH
http://www.wadefx.com

Ah, I was going to suggest that. I would imagine the only other alternative would be to use a tailpiece and flaoting bridge.

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Joe Breault
Merrimack, NH
Perpetual novice

I suspect that the pitch of the Helmholtz mode will be pretty well predicted by the diameter of the rose. If there's a lot of wood in it, it may act smaller than the diameter suggests, if you see what I mean, but for the most part I think it's the diameter that would determine the pitch.

The Q value will be in large part a function of how 'draggy' the rose is. I'd be willing to bet there's no good way to predict that exactly on the basis of things like line width and the ratio of line:space area. I think you just have to measure it. It's probably likely that a few big areas seperated by wide lines would have a higher Q than a lot of little areas with narrow lines, for example, but there are so many variables, such as the frequency you measure at, that I should just shut up.

Very enlightening Al. Thank you so much for sharing your insights.