The acoustic (or semi-acoustic) guitar is a resonating chamber which will amplify the sound. As a chamber, or enclosed space, it has a certain volume, or capacity. Try this experiment in a quiet place: Take two guitars not of the same type (which would then have the same approximate capacity). Hold the guitar so that you can hum into it. Hum a scale, as best you can, only do it continuously so that there are no steps or intervals. At some point in the scale you will hear your voice sound more alive - that is the guitar "talking back" to you. And the guitars will not do it at the same note (or pitch), but at a different frequency. You have hit the natural resonance of the instrument, be it a G# or Db or some other note. Any note you play on the guitar will inherently sound more alive. And when you amplify a guitar, it will resonate more at that frequency, or some harmonic of it. I believe that this is what is happening, that the top is oscillating more at a certain frequency (thus causing feedback) because it is now driving the pickup more, and then feeding back on itself to increase the oscillation.
When your friend put in the dowel rod, he found the "sweet spot" to dampen the oscillating tendency of the instrument. A guitar (or fiddle) top is a vibrating plate, but it doesn't vibrate evenly over all the surface. So until he found that magical spot, the top was oscillating (or vibrating) out of control enough to cause feedback. Once he found the place to dampen it, the oscillating stopped.
Alexander is "spot on" in many regards about vibrating frequencies, but I don't think the dowel rod, even though it does change the natural frequency, will necessarily affect the sound that much on a guitar (although it might!) A fiddle or other higher-pitched instrument would suffer greatly because of their "intense harmonics", but a guitar has an extremely broad and rich acoustic range without the intensity of the higher harmonics which would make it more noticeable.
I think that would solve the problem. Your friend might also talk with a luthier about gluing a small block of wood on that spot to dampen the vibration. The luthier would not use wood glue, but rather hide glue so that the block could be removed without difficulty or damage. And the luthier might have other suggestions also.
And I hope this furthers Alexander's answer.
Regarding f-holed arch top guitars, I don't believe they resonate in quite the same way as a flat-top, and that they are more heavily constructed and are also made for a more-solid "presence" rather than brilliance as an orchestral instrument. But then the only arch-top box I ever owned was an inexpensive Hohner many years ago, so I'm on much thinner ice here...
Bob
Re Toby's question on "feedback"
I'm going to venture an opinion of this from an engineering view point [
I am a retired member of that profession]
All "structures" have a natural vibrating frequency. They can be very
destructive in such things as buildings and bridges or just troublesome
in musical instruments. The most famous destructive example is the the
Tacoma Narrows Bridge [Washington State?] where the bridge self
distracted because the geographic area, prevailing wind speed, etc.,
combined to produce a frequency which was the same as that of the
bridge. It is quite dramatic to see [its all on film] six foot "I" beams
twist like a ribbon in the wind.
Musical instruments like other structures also have a natural frequency
at which the wood vibrates. An acoustic instrument, I think, would be
more susceptible to this than would say a solid body guitar because the
latter's natural frequency would tend to be out of range of much or
most of the musical frequency spectrum than would the former.
When the instruments natural frequency is amplified it "feeds back"
that frequency to the instrument causing it to vibrate more and more
and would self destruct like the Tacoma Narrows Bridge were there
sufficient sustained energy.
In recognition of this problem some amps are equipped with a "notch
filter" which allows you to find your instruments natural frequency and
then to suppress it. Usually this can be done without too adversely
affecting the overall sound. Another helpful feature in some amps is
phase reversal which doesn't suppress the frequency but causes them to
be unsynchronized.
Re your friend's modifying his instrument. Any modification will change
its natural frequency. That will eliminate the troublesome frequency
but not necessarily the problem. The natural frequency will shift to
another frequency which could be just as much of a problem. It would
also change the overall sound of the instrument, particularily a violin
which has so many intense harmonics all of which would be influenced by
this.
I hope this is helpful, Toby
Alexander Mac Donald
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