Re: Ruck type sound ports

On Tue, 09 Jun 2009 06:48:35 -0700, michaelthames1 wrote:

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

Yes,

It is the one pictured on my 'workbench'. It's been 'screwed over' for
years. It's projection was phenomenal. I have heard it in live large room
with high ceilings and it used to carry to the back of the room so well
people shopping in the back, ( btw with me at the front facing the door
about 15 feet ) confused the sound with that of a cd being broadcast.

Yet that was a quite space. In a similarly large space with air
conditioning, air exhaust fans going over the cooking area, it's sound
gets swallowed up, hence the need for amplification.

Now it remains to be seen if the openings affect the carrying power of
the guitar. The funny thing about it though is that close up it doesn't
sound loud at all, just ordinary from the listener's perspective.

My morning student though remarked that the guitar's harmonic resonance
was now way amplified and as well as it's sustain. It had a great sustain
before so maybe the improved harmonic resonance makes it more obvious.

As I remarked in the earlier paragraph, when placed beside a 'high' end
guitar or even an mid level guitar it would not impress thus my students
usually sound way better than I. Only if one compared it in the large
room situation would it show it's uniqueness.

Richard Spross
.

Relevant Pages

  • Re: guitar refinish
    ... At that pitch the sound output from the the hole and the top is ... thanks for that simple answer to the fundamental operating modes of the guitar that has puzzled me for a while. ... resonance directly, and so, indirectly, the pitch of the 'main air' ...
    (rec.music.classical.guitar)
  • Re: Ruck type sound ports
    ... Generally you only get one of these from an air cavity ... 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 ... and makes the spectral peak of the top mode narrower. ...
    (rec.music.classical.guitar)
  • Re: Acoustic design and sound
    ... reproduce the sound the artist and his or her instrument is producing. ... Taylor or a Take Mine than it is to produce a truly great acoustic guitar. ... Body depth certainly changes the 'main air' resonance. ...
    (rec.music.makers.guitar.acoustic)
  • 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: Acoustic design and sound
    ... *that* sound or the guitarist could just get a guitar designed for it like a ... but that isn't the object of recording. ... acoustic guitar. ... Body depth certainly changes the 'main air' resonance. ...
    (rec.music.makers.guitar.acoustic)
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