f holes

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John Clifford
Posts: 180
Joined: Mon Dec 18, 2017 3:08 pm

f holes

Post by John Clifford »

I just watched a long, rambling but quite interesting (to me) video of a talk given by Ken Parker on innovations in guitar making. One thing that stood out to me was Ken's explanation of the reason violins have f-holes, and why they are not such a good idea for archtop guitars. His argument is that because a violin bridge transmits energy to the top primarily by rocking side-to-side (in the bowing direction), putting elongated holes in the top on either side of the bridge facilitates that movement by allowing the top to flex more on that axis. Makes perfect sense, right? Whereas the bridge works very differently on an archtop guitar. It moves more up and down than side-to-side, even though the initial pluck may be more sideways, because the string is not constrained and constantly re-energized by a bow moving sideways across it. So the guitar doesn't benefit from being partly decoupled from the sides by f-holes, and in fact loses some surface area for vibration, while also suffering from increased feedback problems when amplified. It is essentially a ported enclosure, so you should put the hole where it is most out of the way.

I had reached the same conclusion intuitively, and have been using rounded holes in the upper bout instead of f-holes on my guitars, but had not thought of the reason f-holes came into being in the first place.

The video is available here, if you're interested:

https://www.youtube.com/watch?time_cont ... Jkm_IhBi-M

Alan Carruth
Posts: 1012
Joined: Sun Jan 15, 2012 1:11 pm

Re: f holes

Post by Alan Carruth »

I'm not sure I agree with Ken on this.

Certainly the bridge on a violin rock sideways, driven that way by the bow pulling the string across the box rather than perpendicular to the top. That's why the sound post and bass bar combo was such a major innovation. Both had been used separately prior to about 1550, as supports for the top under the down pressure of the bridge, but neither was acoustically much help without the other. The post under the treble foot of the bridge 'nails' it in place, so that the rocking of the bridge acts as a bell crank, pushing the bass bar vertically to drive the type of top motion that produces sound. The active part of the top is narrow, and it it stiff and hard to drive compared with a guitar top, but when you have the continuous power of the bow instead of an intermittent pluck, it works fine. Making it too efficient risks 'wolf' notes; witness the 'cello.

I use the Chladni method of 'tuning' the top and back of arch tops, as well as violins. The procedure I follow is to 'tune' the 'free' (off the guitar) top and back so that they vibrate in similar ways at similar frequencies before cutting the holes and installing the braces. Cutting the holes typically throws everything out f line on the top, but putting in the brace(s) and trimming them properly gets it all back. This holds quite well for both fiddles and guitars. The brace(s) pretty much make up for the stiffness and mass lost to the holes.

I've made both round hole and F hole arch top guitars. There are differences. of course. On round hole ones the 'X' braces (I have not yet made one with 'parallel' braces; it's coming...) don't need to be as big, but you do need a cross patch above the hole. I usually make that low and wide; a 'patch' rather than a 'brace'. On a round hole with parallel braces I think I'd use two patches, above and below.

Gore suggests in his book that we don't really want the sides to be pushed into motion be the top anyway. He adds significant mass to the sides below the waist to 'keep the sound in the top' , which is, after all, the most effective sound producer. The top and back are coupled by air pressure changes in the box anyway, and those pretty much act out of phase with any energy being transmitted by the sides. O'm not as sure as he seems to be that this is a bad thing. It certainly can cut down on the power output of the instrument, at least at some frequencies, but may also add to the 'tone color', which is not a bad thing.

Round hole arch tops do sound more like round hole flat tops than F-hole arch tops do, of course. Most modern arch tops have pretty big holes, for one thing, and that shifts the the 'main air' mode pitch 'way up. Most flat tops tend to have that pitch around G on the low E, with the usual range being from F# (~92 Hz) to around G# (~104 Hz) or a bit higher. Round hole arch tops also tend to come in near that range, or a little higher. Arch tops often have their 'main air' pitch as high as A# (~116 Hz) and as high as C or C# (up to about 135 Hz). This probably accounts for most of the difference is the basic timbre, IMO.

'F' holes can also 'listen' to interior air resonances that the hole in the center can't, and that has to make a difference, although it's harder to pin down in some ways. By the same token, a round hole on the center line centered above the narrow part of the waist can also interact in interesting ways that can color the sound, which are out of reach of holes in other locations.

All of which is to say that there are reasons why these traditions persist, and that are not always obvious. There are, of course, lots of ways to make good guitars, but if you're trying to make something that sounds like, say, a 1920's Loar L-5, then the easiest way to do it is to start by copying the main features of the original. I can't argue with Ken's results; he makes a fine guitar (and gets a lot more for them than I do *) but I'm not sure I agree with his reasoning.


*
Which brings up what's been called 'the American question':"If you're so darn smart, why aren't you rich?" There is, of course, no simple answer to this. ;)

John Clifford
Posts: 180
Joined: Mon Dec 18, 2017 3:08 pm

Re: f holes

Post by John Clifford »

No doubt there are many ways to make a pleasant sounding instrument, and certainly many great sounding archtop guitars have been made with f holes. On the other hand, they clearly break up the lower bout of the top plate, which is the main sound pump. If the job of the guitar body is to move air, it would seem that more air can be moved by leaving the lower bout whole and moving the hole(s) somewhere else. But whether you like the resulting sound better or not is purely subjective. I just thought it was interesting that there is, at least arguably, a physical reason for putting slit-shaped holes in the tops of violins and other bowed instruments, whereas Lloyd Loar seems to have borrowed them for his guitar design more for aesthetic reasons than functional ones. I don't think the folks at Gibson spent a lot of time scientifically testing all possible sound hole shapes and positions for the L-5 before settling on f-holes.

The question I find myself asking as an American in the 21st century is: "If you're so darn rich, why aren't you smart?"

Brian Evans
Posts: 900
Joined: Sat Aug 30, 2014 8:26 am
Location: Tatamagouche, Nova Scotia

Re: f holes

Post by Brian Evans »

I'm a big fan of Ken Parker, and I built a tribute to his style of archtop. It has an upper bout sound port combining the top and side of the bout. It has a modified X brace, and a longitudal brace between the neck and tail blocks just under (not touching) the top to take up string loading separate from the top. I found the free air resonance was quite low, around low F on the sixth string, and this gave a very robust bass response. The light build of the top gave a quite balanced mid and treble response, so I was very happy with that build. I don't "tune" the top and back plates (curly sinker redwood and birdseye maple in this case) but I do try to build lightly, brace lightly, and get a musical drum tone out of the plates, so that they are quite resonant when tapped in free air.

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