Re: guitar refinish

alcarruth wrote:
David asked:
"Would you say that the soundhole is like the speaker in an
enclosure in that most of the
sound is radiated from there?"

If you're comparing the guitar to a ported bass reflex speaker
enclosure, then the soundhole is the port, and the lower bout area of
the top is the speaker, more or less. At low frequencies, near what we
call the 'main air' resonance, the soundhole puts out most of the
power. At that pitch the sound output from the the hole and the top is
out of phase, but the because of the compliance of the walls of the
box, particularly the back, there is enough flow through the hole to
put out some sound at a distance. Near the 'main top' resonant pitch,
about an octave higher on most guitars (roughly, the open G string
usually) the top and the hole are in phase, and there is generally a
much bigger peak in the spectrum.

Oh boy, thanks for that simple answer to the fundamental operating modes of the guitar that has puzzled me for a while. I'm guessing that a great guitar will handle the point where the in-phase/out-of-phase modes are of equal power output better than a poor one i.e., there will a smooth transition without a drop or peak in power output.


" Have you done any experiments with varying the soundhole radius to
control the coupling of the main and top resonances? ".

Repeat after me: "There are NO independant variables on the
guitar" :o)


"There are NO independent variables on the guitar"
"There are NO independent variables on the guitar"
"There are NO independent variables on the guitar"
"There are NO independent variables on the guitar"

:-)

Varying the soundhole radius changes the pitch of the Helmholtz
resonance directly, and so, indirectly, the pitch of the 'main air'
resonance of the assembled box. I have not checked to see whether
there will be a larger or smaller pressure change in the box with a
smaller hole: I can see how it could go either way. There's an
experiment I'll have to do. I do know that when you make the hole
smaller there is more loss in the resonance: there is more 'edge'
relative to the area of the hole, and that makes more drag in the air
flow. Lute roses add a ton of drag, and cut down on the 'main air'
peak height a lot.

BTW; when thinking about the length of the port represented by the
soundhole, you don't just take the thickness of the top. That's pretty
much negligable anyway. However, as the air is moving in and out of
the hole it takes a little time for it to turn the corners: it doesn't
just flow out like water from the end of a hose, but tries to go
around and fill in the space around the outside of the hole. You could
think of it as a sort of 'virtual trumpet bell', flaring out on both
sides. The exact shape of the flare will depend on a lot of things,
including the speed of the flow, and this flow is slow enough to make
quite a radius. As far as the air moving in and out that 'virtual'
shape is real: flow doesn't cross stream lines, and it adds to the
length of the port. There are 'end corrections' in the equations
Helmholtz came up with to more or less account for this, but they are
approximate.

Putting in any sort of port extension, such as a tournavoz, drops the
main air pitch like a rock, and the amplitude as well, at least in the
measurments I've made. You have to be careful in how much you do. A
short extension can drop the pitch of the 'main air' mode without
costing too much power, and the 'bassiness' of the tone would improve.
However, at some point, you start to lose power and the bass response
suffers. The resulting sound can be interesting, but may not be to
everyone's taste.

Alan Carruth / Luthier



Thanks for this info Mr. Carruth, it's going to take me a while to sort this out. :-)

david
.

Relevant Pages

  • Re: Ruck type sound ports
    ... It turns out that the normal guitar has two 'Helmholtz' type air ... the top is out of phase with the sound coming from the hole on this one, ... Since this is just a bit higher in pitch ...
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  • Re: guitar refinish
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    (rec.music.classical.guitar)
  • Re: guitar refinish
    ... "Would you say that the soundhole is like the speaker in an ... At that pitch the sound output from the the hole and the top is ... resonance directly, and so, indirectly, the pitch of the 'main air' ...
    (rec.music.classical.guitar)
  • Re: Acoustic design and sound
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