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Does box ornamentation effect resonance?

Hi guys,

I'm starting a new build, and was thinking of going to town and add brass corners and grommets. The question I have is, does the clutter glued, screwed or bolted to the lid of a box cause a noticeable change in the box's volume?

Also, I get the idea of a sound hole allowing the sound to escape from the box, but any cutting to the box's lid would cause the quality of the resonance to dip. Is there an optimum sound hole size to get the best of both worlds?

(I've added a photo of the box I'll be using, just for a point of reference.)

Cheers

Hedley

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Permalink Reply by BUGGY (C) on August 20, 2012 at 12:06pm

http://www.youtube.com/watch?v=Ajn0TmhvwCU&feature=related

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Permalink Reply by Vinylhed on August 20, 2012 at 12:20pm

I love this guy! That's a feller who knows what he's talking about.

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Permalink Reply by BUGGY (C) on August 20, 2012 at 12:17pm

http://www.youtube.com/watch?v=MAbvClxqbfo&feature=related

It's all in the wood! maybe we should get him to build a CBG!

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Permalink Reply by BUGGY (C) on August 20, 2012 at 12:20pm

http://www.youtube.com/watch?v=-UU1WRlYG5o&feature=related

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Permalink Reply by Vinylhed on August 20, 2012 at 12:29pm

Thanks for posting those vids, Bug. I'll give them a watch and glean what my simple mind can from them.

In many ways, the builders of classical guitars have it easy. Hundreds of years of developments, and they only tweak bits here and there to their liking. The design is, within reason, fairly set in stone, scale lengths and size are uniform. The cigar box guitar is more unique. The demands a specific box puts on the construction of the guitar will nearly always change.

I'm not sure where I'm going with this...I'm certainly not belittling the craftsmanship of guitar makers, I'm in awe of their skills.

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Permalink Reply by RTZGUITARS on August 20, 2012 at 3:43pm

Where does it end.

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Permalink Reply by BUGGY (C) on August 20, 2012 at 8:46pm

http://www.phys.unsw.edu.au/music/guitaracoustics/construction.html

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Permalink Reply by BUGGY (C) on August 20, 2012 at 8:48pm

Found it!!

A guitar sound hole is designed around this principle http://www.phys.unsw.edu.au/jw/Helmholtz.html

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Permalink Reply by BUGGY (C) on August 20, 2012 at 9:03pm

I believe this expalians it and perhaps why a smaller holes works on a CBG:

Its bit like blowing over an empty beer bottle why that works..This also helops explain to me why the tiny holes on my resonator Toilet Seat Guitar works... Think of the top a spring and air compression and the holes is the restrictor. In theory

More air can be compressed by the sound board, if it cant escape too quickly, producing more spring and sound waves.

Might be worth trying the cardboard test below.I note this is why some tunings sound better for the woods to get the woods G spot frequency! This also relates to the links last night of classical builders about tuning of the wood etc.

This is the science behind it... now how do we use it?

Helmholtz resonances and guitars

* I said above that the air in the body of a guitar acts almost like a Helmholtz oscillator. This case is complicated because the body can swell a little when the air pressure rises inside – and also because the air 'in' the sound hole of the guitar has a geometry that is less easily visualised than that in the neck of a bottle. Indeed, in the case of the guitar body, the length of the plug of the air is approximately equal to the two 'end effects' at the end of a 'pipe' which is only a couple of mm thick. The end effects, however, are related to and of similar size to the radius of the hole, so the mass of air is substantial. The length of the end effect of a cylindrical pipe that opens onto an infinite, plane baffle is 0.85 times the radius of the pipe. Although the soundboard of a guitar is not infinite, one would expect a similar end effect, and so the effective length of the 'plug' of air would be about 1.7 times the radius of the hole. (Some makers increase this by fixing a short tube below the soundhole, with equal radius.)

A couple of people have written asking how big the sound hole should be for a given instrument. Well, we can use the equation above to start to answer that question. However, the swelling of the body is important. This makes the 'spring' of the air rather softer, and so lowers the frequency. The purely Helmholtz resonance can be investigated by keeping the body volume constant. When measuring this, a common practice is to bury the guitar in sand, to impede the swelling or 'breathing' of the body. However, guitars are not usually played in this situation. So the Helmholtz calculation will give an overestimate of the frequency of resonance for a real, flexible body.

Let's assume a circular sound hole with radius r, so S = πr², and L = 1.7r as explained above. When we substitute into the equation for the Helmholtz frequency, using c = 340 m/s, we get:

Notice that we are using standard SI units: we have used the speed of sound in metres and seconds, so the volume must be in cubic metres and the frequency in Hertz, to give an answer in metres.

It is more complicated when the tone holes are not circular, because the end effect is not equal to that of a circle with the same area. PhD student and luthier John McLennanis writing up a report of some measurements about this, which we'll post here soon.

On guitar and violin family instruments, the Helmholtz (plus body) resonance is often near or a little below the frequency of the second lowest string, around D on a violin or G-A on a guitar. You can reduce or shift the Helmholtz frequency substantially by covering all or part of the hole with a suitably shaped pieced of stiff cardboard. If you then play a note near the resonance and then slide the card so it alternately covers and reveals the hole, you'll clearly hear the effect of the resonance.

Is the 0.85r effect reasonable? Ra Inta, who did a PhD on guitar acoustics in our lab, suggests an interesting demonstration: Damp the strings on your guitar so they don't vibrate (e.g. a handkerchief between strings and fingerboard). Hold the palm of one hand above the soundhole, and close to it. With a finger of your other hand, strike the soundboard a sharp blow near the soundhole and close to the 1st string. You will feel a pulse of air on the palm of your hand. The blow of your finger pushes the soundboard in and squeezes some air out of the body. Now move your hand gradually further away from the hole, and continue tapping with the finger. When do you cease to feel the movement of the air? This will give you a rough estimate of the length of the 'end effect' in the case of the sound hole.