Re: Ruck type sound ports

On Jun 8, 9:06 pm, Richard Spross <rcspr...@xxxxxxxxxxx> wrote:
On Mon, 08 Jun 2009 12:44:56 -0700, alcarruth wrote:
Most people don't put ports in the lower bout, so I don't talk about
them very often. They can be interesting.

It turns out that the normal guitar has two 'Helmholtz' type air
resonant modes. Generally you only get one of these from an air cavity
with a single opening, but the normal soundhole position is close to the
waist, so the lower bout can act somewhat independantly of the rest of
the box in this regard. From what I can tell you need to have the normal
soundhole in the usual place, a fairly well defined waist, and some
flexibility in the box to see this. It often doesn't show up on steel
string Dreadnoughts, for example, probably because of the shallow waist..
Most people who study guitar acoustics don't talk about this one, for a
variety of reasons. It took me a long time to convince myself that it
was there when I first saw it, but since then I've seen it show up in a
very complete computer model, as well as my own measurements, and
corroborated it in another way that I'll explain below.

This second A-0 type mode usually seems to come in at around 250 Hz,
around the open B string pitch. The sound coming off the lower bout of
the top is out of phase with the sound coming from the hole on this one,
so they tend to cancel out. Since this is just a bit higher in pitch
than the 'main top' mode, which is one of the biggest sound producers,
the A-0-2 mode can act as a 'cuttoff' for the main top peak in the
spectrum. That is, when the A-0-2 is close in pitch to the main top
mode, the air flow through the hole cancels some of the sound off the
top, and makes the spectral peak of the top mode narrower.

Meyers found out many years ago that guitars with tall, narrow main top
spectral peaks tended to have a 'sharp' or even 'harsh' timbre. I had
this problem on my first two archtop classical gutiars, and one of the
things I tried to remedy that was to put a port in the lower bout on one
of them, next to the tailblock, to raise the pitch of the A-0-2 mode. It
worked to some extent; the air mode pitch went up, the 'main top' peak
in the output spectrum became a little broader, and the sound was a bit
less 'harsh'. It wasn't enough to make the overall timbre acceptible,
but at least it proved the theory.

Alan Carruth / Luthier
http://www.alcarruthluthier.com

Well you are well studied and have been working far longer at this than I..
I find the information interesting, but way above my head. ( Sorry )
However it is cool to know that some one of your experience has fooled
around with a lower bout placement.

Unfortunately I'll have to wait until tomorrow night to hear my guitar
played for me, since my Monday night student had to reschedule.

Later,
Richard Spross

Richard,
Is the guitar you put ports in one that you made? If so how would
you describe it in terms of projection before the surgery?

Tashi
.

Relevant Pages

  • Re: Ruck type sound ports
    ... Generally you only get one of these from an air ... around the open B string pitch. ... The sound coming off the lower bout ... Unfortunately I'll have to wait until tomorrow night to hear my guitar ...
    (rec.music.classical.guitar)
  • 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 ...
    (rec.music.classical.guitar)
  • Re: Ruck type sound ports
    ... Most people don't put ports in the lower bout, ... around the open B string pitch. ... the top is out of phase with the sound coming from the hole on this ... and makes the spectral peak of the top mode narrower. ...
    (rec.music.classical.guitar)
  • Re: Soundboard Tuning Torres - La Leona
    ... Usually the pitch you hear when you tap on the bridge is the 'main ... air' resonance. ... the 'main air' mode on a guitar will always be ... couple more effectively with the 'main air' resonance, ...
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    ... purpose of a speaker is to move air in a controlled fashion, ... is the question of the virtual sound stage. ... violin and not a clarinet. ... often chosen for efficient speaker design. ...
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