Would you brace a top differently if it didn't have a hole?

Quickstep192 · #1 09-23-2016, 08:58 AM

I'm in the wild idea phase of a project to build a stage/recording guitar. In it's current incantation, it would have no hole in the soundboard.

Would you brace it differently given the absence of a hole in the top?

jrmyrnsm · #2 09-23-2016, 09:41 AM

I would try and find some pictures of the bracing on a Tom Bills Genesis guitar or a Batson guitar to see how they are doing it. Both companies have no soundhole on the top and just use a soundport on the side... Never played either but it seems like a pretty cool concept...

jrmyrnsm · #3 09-23-2016, 09:47 AM

jrmyrnsm · #4 09-23-2016, 09:56 AM

Heres another video with one of the Batson brothers discussing the bracing. Around 8:00 he starts showing some of the bracing for a soundboard he's doing...

https://youtu.be/NxxUf8h-Vws

Alan Carruth · #5 09-23-2016, 12:59 PM

As always, it depends on what you're trying to do with this build in terms of sound. The usual justifications for making a top without a hole are to get more area vibrating for more sound, or else to reduce the perceived issue of have a hole in a high-stress area, so that you can lighten up the structure for more sound. Either way it comes out to more sound.

Stress on a guitar top is actually less of an issue than stiffness: they don't usually fail by breaking on normal circumstances, but often do because they're not stiff enough to avoid collapsing over time. The sound hole area is generally weaker than the rest of the top, but still stronger than it needs to be unless you drop it in such a way as to hit the back of the neck hard. Then the the glue line between the Upper Transverse Brace and the top fails, and the top can break along either side of the fretboard, allowing the neck to shift inward. This can also happen when there's a poor glue line between the top and the UTB, or the wrong glue is used and it cold creeps. If you're worried about strength or stiffness in that area there are various ways to overcome that, such as an 'A' brace or buttresses from the block to the waist, but I digress.

As I said, stiffness is usually more of an issue than strength in the top, with the idea being to get just enough stiffness to avoid problems, with the least amount of weight. Increasing the span of the vibrating area of the top reduces the stiffness, all else equal, and you need to beef up the structure to get it back. Usually we think of this in terms of the width of the lower bout, but extending the vibrating area up toward the hole would count too.

On most steel strings the upper end of the working part of the soundboard is the UTB, and it would be difficult to extend it much further without major re-engineering. Thus leaving out the hole really doesn't doesn't change that span much. It adds a little bit of vibrating area, but not that much: the real sound producing part of the top is still going to be from just above the bridge down toward the tail block. This is because the stress below the bridge is primarily tension, which is relatively easy to deal with. Above the bridge you have to contend with compression which, combined with the torque load of the strings, wants to buckle the top inward. There's a good reason why the most successful and widely copied brace patterns concentrate the wood between the bridge and the sound hole, and even without the hole you'd would not really be able to change that much.

You also have to ask yourself why the sound hole has always been more or less in the same place on guitars for the past thousand years or so. Some experiments I've done suggest that there are complicated interactions between the air in the box, the hole location, the shape of the guitar, and the vibrations of the top, that can add to the 'tone color' of the guitar. This seems, for example, to be why Dreads sound a bit different from other guitars with more pronounced waists.

I made one guitar with a pair of smaller holes in the upper bout rather than the usual large central one. It worked OK, but had some tone issues because of the pair of holes. Were I to do that sort of thing again I might make a large 'side port' the size of the usual hole just above the waist. This, of course, changes the direction of the sound radiation from the box, and, I understand from folks who have done it, is not all that wonderful either in some respects.

In the end, if you want something that sounds like a guitar, the easiest way to get it is to stick with more or less the standard design. Moving the hole off the top is a major change, and could require major engineering to produce a sound that's as good as the usual one. It's not as though nobody has tried it already, and the fact that it's not part of the 'standard' suggests that it doesn't do what most folks want.

OTOH, if you're up for an experiment, why not? Just keep in mind that most experiments don't work. If you have a pretty good idea about how the guitar works, and your understanding suggests that moving the hole will help produce the tone you're after, then it's worth a try. Even so, I'd tend to stick with more or less standard top bracing, which, for me, is a double-X pattern. I'm not averse to experiments, but I've done enough of them that I don't expect too much in the way of 'improvement'.

Quickstep192 · #6 09-24-2016, 10:14 AM

Jeremy, are those your tops?. The bracing is very cool

Al, I always appreciate your perspective on experiments since I think you've tried most of them! To your point about what I'm after tone-wise, I'm thinking of a "plugged in" guitar for stage/recording. Use of pickups with no hole would hopefully eliminate the booming and feedback that happens when micing a guitar while still retaining an acoustic sound and look.

Alan Carruth · #7 09-24-2016, 01:02 PM

Yup; the biggest feedback problem comes from the 'main air' and 'main top' resonances, both of which are can be driven easily through the hole by sound from the room. Leaving out the hole is one way to fight that. In that case I'd just leave out the hole and do everything else the same. You could even do the rosette inlay, for looks, and just not cut the hole. That would require either plugging the pivot hole, or gluing a block to the top for the pivot which you could cut off later. Not a biggie either way.

'Feedback busters' of one sort or another can be pretty effective, too. If you're in a Renaissance mood cut in a lute type rose: it weakens the 'main air' resonance a lot by adding a bunch of drag to flow through the hole. So do the 'inverted wedding cake' parchment roses. The nice thing about this is that you do still have the Helmholtz mode, so there's some of the low-end reinforcement from that, along with the added complexity in the tone. But then, it's also more likely to feed back than one with no hole.

In the end, the rule is that if the room can hear the guitar, the guitar can 'hear' the room, and will feed back if there's enough gain in the loop. How much you need to do to avoid feedback ultimately depends on how much gain you'll need in your application.