Replies to This Discussion

Permalink Reply by Sean Laffey on May 27, 2013 at 10:21pm

In stringed instrument making its best to remember that the sound comes form the top not the hole.It probably doesn't matter and is pretty subjective. In the more professional  stringed instruments, ie the ones made from tone woods, the hole can be dispensed with altogether, Godin make guitars like that and  you can even buy a plastic plug to cover the sound hole in a guitar. 

Tone woods are thin and have the grain running in tight parallel lines, a good tone wood will have  a least 14 grains to the inch, spruce and cedar are two of the commonest. Go away from the parallel grain then tone suffers, cheaper ukuleles sound pretty flat because they often have mahogany or koa  sound boards, these give a warmer bass and counteract the high G string treble, but if you want a brighter sound at a reasonable price then spruce is a great way to go. Plywood sound boards are dead because (often) the grains in plywood are at right angle to each other.

There  are formula based on the Helmholz equation to work out the best size of sound hole for the volume ( space not sound) of the instrument.  In reality the size and position  of a sound hole is usually a structural compromise to accommodate bracing of the top.

CBGs are made  from non-tone woods, the woods are usually quite  thick, often acoustically dead and the board lengths  are very short making the top very stiff, so the  strings have a hard time driving the sound board (ie the top). Floating bridges might work better as more of the energy  gets into the top of the guitar ( one reason why mandolins and fiddles generally  have floating bridges). For floating bridges the lighter the bridge the better, check out banjo bridges for an idea.

To get the most  resonance out of the dull stiff tops that are CBGs you might be best to place the sound  hole at the top left corner, ie on the bass side if you  are right handed. That would give the maximum length of soundboard  vibration and you'd have intrinsic strength near the sound hole because  of the corners of the box (reducing the need for too much sound sapping bracing). It would also be near your ear, so you'd get a better personal appreciation of the sound, but it might not add much to the forward projection of the instrument 

Good article here : http://www.tejagerken.com/Article_Folder/soundholes.html

Permalink Reply by Sean Laffey on May 27, 2013 at 10:51pm

What happened to my edit? Anyway here is an addendum: The sound of the guitar is affected by factors such as

The fundamental tone of the top, attack, sustain,decay,resonance, harmonic overtones, harmonic damping,volume, balance across string gauges... enough to keep you experimenting for years

Permalink Reply by Don Tucker on May 28, 2013 at 1:56am

IMHO sound holes are pretty much cosmetic. I have left the sound holes out of several instruments until after they're finished and strung so I can play them first. Putting the holes in made only a very subtle difference at best (at least to my old ears) Placement also doesn't seem to make much difference to me. I've put sound holes on the edge or the end at times and again hear little difference. I should also point out I only build neck thru's. Maybe with a different type of construction it makes more of a difference.

Permalink Reply by Michael Recchione on May 28, 2013 at 4:07am

I think this notion that the sound comes only from the top and not from the hole has become almost an urban legend, but it's not actually true.  There's a complementary urban legend, also unsubstantiated, that making the sound hole's diameter equal to 1/4 of the diameter of a sphere with the same volume of the soundbox results in the loudest instrument, all other things being equal.  (I've searched all over the place, and there's no reference to any scholarly work to justify that.  Other folks on MIMF have also searched - it's just not in any of the primary sources...)

However, it's known that, as you increase the size of the soundhole from 0, i.e. no sound hole, to whatever, the instrument gets louder with increasing soundhole size, up to a certain point, and then starts to get quieter.  So, for each instrument, there is an optimum size for the soundhole.  The Helmholtz resonance (which is typically the lowest resonance of the instrument), also rises in frequency as you increase the soundhole size.  How important is that?  I've heard that you usually want the Helmholtz resonance to be between the fundamentals of the lowest two strings, closer to the second, but not exactly at any string's fundamental (to avoid booming at that note). 

As you said, I'm not sure how much of a part tuning the Helmholtz resonance would play in determining the sound of a CBG.  But my own experience is that having some kind of soundhole is better (louder and fuller sounding) than having none at all.  (My first build didn't have a soundhole, ostensibly to avoid feedback when plugged in, and it sounds great plugged in.  But really horrible not plugged in...)

You're right about the box - but there are "real" instruments that use small, stiff soundboxes, and that sound good acoustically.  (Examples:  The Greek tzouras and baglamas, which were typically carved from a single piece of wood, also the Puerto Rican Cuatro, also traditionally carved from a single piece of wood ...)  The key is getting the top right. 

One way to improve the CBG top is to thin it at the edges, or taper it from the middle to the edges, so that it moves more like a speaker cone.  I've also experimented with thinning the whole top, and then using bracing.  But this is all off-topic for hole placement...  I think upper bout, towards the neck, is a good place if you're using a through the body neck design.  Otherwise, center also is good.  I've used smaller soundholes on both bouts, closer to the neck, with good results.  The idea of a soundport on the side facing the player is also intriguing, but I haven't tried it yet...

Permalink Reply by Sean Laffey on May 28, 2013 at 11:16am

The best test would be a before and after on a CGB that has a sound hole cut into it after it's first set up. Try recording the sound and looking at it visually on pro-tools  or one of the excellent cheaper alternatives, like the free Audacity.You should be able to measure any differences.

See some physics here:  http://www.bsharp.org/physics/guitar

and here http://ffden-2.phys.uaf.edu/211.web.stuff/billington/body.html

The latter is well worth looking at, it shows  resonance patterns on a guitar top,  notice how they avoid the sound hole ad how  they mover upwards as the pitch rises. The higher pitches also have more complex resonance patterns.

Permalink Reply by Michael Recchione on May 28, 2013 at 12:24pm

Thanks for the references.  The pictures are really cool.  The standing wave patterns in the holographic images are interesting, but it's not really surprising that the dark spots avoid the sound hole - there's nothing there to move, so an image of the movement there is going to show nothing moving... :) 

Permalink Reply by Sean Laffey on May 28, 2013 at 12:56pm

Sure, and the  wave patterns do form very close to  the sound holes, so they have an effect, they channel the sound .

Pix of my latest  build here.  I went for a rosette design, just because I fancied doing one, not for any other reason.

ww.cigarboxnation.com/photo/dulcimer-1-7?xg_source=activity

Permalink Reply by Michael Recchione on May 28, 2013 at 1:08pm

By the way, there's a great book by Roger Siminoff, "The Luthier's Handbook", where he looks at this stuff.  He builds some basic test setups to look at resonances, etc.  Some of the techniques are old, pre-computer.  I was thinking about creating one (maybe this should be it's own thread or something) using a powered speaker, like a PC speaker, with the cone removed, and attached to a top at the candidate bridge position.  I would then use Audition or Audacity to create test signals, e.g.  white noise, a frequency sweep, tones at various frequencies, and bursts.  Using Audacity or Audition, I'd play the test signal out to the guitar while recording with a mic placed a fixed distance from the guitar. 

Things to look at would be the power spectrum of the recorded signal, also the total power in response to a fixed stimulus.  You could use this setup to evaluate different bridge positions, to try to find the best size for the soundhole (starting with none, then making a small one, and enlarging it gradually while testing the results), and also to evaluate bracing schemes and thicknesses.  Siminoff uses a similar setup, except he excites the box with a motor that thumps it, and looks at the output on a SPL meter, i.e. pre-computer technology...

I've been so busy with work that I haven't had time to mess with any of this for a month now, but when things slow down...

Permalink Reply by Sean Laffey on May 28, 2013 at 1:14pm

Know what you mean.. I am up to my neck in the day job and involved in all sorts of community projects here in Ireland. Building CGBs  is just so relaxing, I love the discipline of glue, epoxy and varnish, you set it up and walk away for a day.. so refreshing in a " must get it done this minute" world

Permalink Reply by wormil on May 28, 2013 at 5:27am

You want the sound hole at a nodal point, a point where the top is not vibrating. You can scatter salt across the top and vibrate it to identify the nodal points or simply guess as they will normally be in the center and corners. 

Beyond that, I've read a corner on the bass side will emphasize bass and vice versa. Honestly I don't have enough building experience to agree or disagree but my gut feeling is that is any effect will be minor. I believe the size of the soundhole will have more influence on bass/treble than position.

Permalink Reply by Michael Recchione on May 28, 2013 at 5:49am

The nodal points vary in position with pitch - they're not the same at all frequencies.  To really oversimplify, consider the top as a string.  Consider the lowest frequency component (a half-wave). In that case, the nodes are at the edges where the top meets the sides.  Then, in the case of the first harmonic of that frequency, the nodes are at the middle and the two ends.  Then there's the F3 mode, where the nodes are at the ends, and 1/3 of the way from the ends, and the middle is an anti-node.  And so on.  Depending on the frequency, the middle can be a node or an anti-node.

Complicate this further by the fact that the top is a plate, not a string, and generally not a uniform plate, so the modes of vibration are a lot more complex.  As you said, the corners (or any place the top is not free to vibrate, so wherever it's attached to the sides) is a node.  But nobody ever puts the soundhole there on an instrument.  Similarly, I've never seen an instrument with a soundhole in the center of the top.  So I'm having my doubts about the "put it at a node" concept...

I've been thinking about this stuff enough recently so that I'm just about motivated to put together a test setup to try to verify some of these urban legends once and for all...  Or not. :)

Permalink Reply by wormil on May 28, 2013 at 6:15am

You are taking "center" and "not vibrating" too literally, I suspect to make a point for argument. There may be no exact suitable point for a soundhole where there are zero vibrations. Compromises are made for both aesthetics and practicality. 

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