Natural Science Forum / Physics / Acoustics / March 2006

For an aoustic guitar, sound from the loudspeaker drives the flat body
of the guitar, likely the sound also penetrates into the cavity through
the holes. I think it is unlikely for coustic guitars that strings
driven by the moving air are the primary feedback element since since
their area is so small in comparison to that of the acoustic guitar body

There will be a difference between acoustic guitars with body pickups,
and the bodyless guitars often used as instruments which have only
magnetic reluctance pickups. In that regard, the likeyhood that the
strings do the sound intercepting is much higher. It requires a closer
approach to the loudspeaker to cause the exitation.

Random sound in the air will start the string vibrating an infinitesimal
amount. This is amplified, etc.

Thought: Place the guitar and speaker in various sound fields; traffic
noise; by the furnace; in a room with persons chattering. The "takeoff"
of sound in each case will have a timbre that is colored by the
environment. There should be some weird effects available there!

    Angelo Campanella

Aine,

guitar perhaps) who's function it is to create feedback with a guitar amp. <

That will be tough unless you have a way to fret down the strings. Most
electric guitar feedback - the "good" kind - includes the string vibration
in the feedback loop. That's why feedback follows the notes you play (or
their harmonics), and why you can influence the pitch and apply vibrato to
the feedback by bending the strings. Some old electric guitars can feed back
through the pickup windings, but it sounds terrible (too high pitched) and
is uncontrollable and unmusical.

A better approach to what you're asking for is to record each of the
feedback sounds you want, then play them on cue from a sampler.

--Ethan

for a solid-body electric guitar with decent (humbucking) pickups (e.g.
the classic Les Paul), the strings have to vibrate for a signal to be
induced and applied to the guitar amp input.  so the mechanical motion
of the strings is a necessary link to closing the feedback loop (and
that is a very good thing).

that's because the mechanical motion of the strings is in the feedback
loop and you just stated why it's a good thing (user control).

in the olden days, there was this product called a "Talkbox" which was
a driver with its horn replaced by a flexible (and usually clear) tube
about 2 cm in diameter that would go from the output orifice of the
driver, straped up along the guitarist's mic stand and then right up
alongside the microphone.  the guitarist (Peter Frampton was sorta
famous for using this) would hit a footswitch that would connect the
driver directly to the guitar amp output and then with this high
intensity sound squirting into his mouth (in lieu of vocal cords) would
mouth words thus shaping the guitar with formants from his mouth.

a more high-tech version of this today would be to use a (channel)
vocoder and instead of lip-syncing the words, the guitarist would
actually sing them and the vocoder would imprint the spectral envelope
of the sung audio onto the guitar audio.  i don't understand any other
scheme (in principle, the algorithm wouldn't have to be a vocoder) for
applying parameters derived from one sound onto another.

another item from the olden days was some hard-core rock guitarist (i
can't remember who, but i wonder if it was Ted Nugent) had a small
loudspeaker (driven by the guitar amp) mounted right in the solid body
of his guitar underneath the strings for a real hard-core feedback that
didn't require him to stand any particular place.  i've always thought
a more elegant way to do this was to mount a driver onto a stand at a
little above waist level (unless you're Jimmy Page and you like playing
the guitar with it hanging down around your knees) and the driver horn
replaced by a 5 cm diameter tube, maybe 20 cm long.  the guitarist
could walk right up to it and get a lot of acoustical coupling for
feedback and control it better by adjusting his distance (as they do
normally with a regular amp and speaker) as well as by fretting or
bending strings.  never saw anyone do that, but i always thought it
would be the best way to do it.

if you are outputting some other prerecorded music from such a
loudspeaker and coupling that to the solid body electric guitar, then
it really isn't feedback because there is no loop.  your guitar is
acting as an electro-acoustic processing device, kinda like the Talkbox
and Peter Frampton's mouth was an electro-acoustic processing device
(although different).

r b-j