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Re: Slide guitar - acoustics question
Dear Nina and List,
by damping (touching) a vibrating string in the middle of its length you get
two vibrating half-strings.
Now if you slide the damping finger, one half get shorter producing a rising
gliss and the other half gets longer producing a falling gliss.
The falling gliss is much softer, because a string section that gets longer
increases in mass and thus dramatically decreases in vibration amplitude.
You might like to have your students find the solutions themselves by
various double damping experiments.
Neuroscience of Music
S-671 95 Klässbol
web site: http://w1.570.telia.com/~u57011259/index.htm
----- Original Message -----
From: "Cornelia Fales" <cfales@xxxxxxxxxxx>
Sent: Wednesday, March 21, 2007 11:14 PM
Subject: Slide guitar - acoustics question
Can anyone explain this slide guitar technique?
1) Lightly damp a single guitar string at, say, half its length to get
the harmonic pitch an octave above the open string pitch (ie, interfering
with the fundamental and odd modes of vibration),
2) Replace your damping finger with a steel slide at the same damping
pressure, and slide it up the neck to some point along the neck.
The result is the expected ascending glissando from the beginning harmonic
pitch to the pitch at the end of the glide, but also a fainter descending
glissando that moves at the same speed and distance as the stronger
ascending glissando. The downward glissando is easily audible with
practice - more so if the rising harmonics are filtered out - and is also
clearly visible on a spectrogram.
My students and their teacher have two questions: 1) if the slide really
maintains the same pressure as the original damping finger, then
theoretically its movement doesn't change the length of the string or the
fundamental, but must instead be interfering with different vibration
modes as it slides upward. So how does it produce a (chromatic)
2) what accounts for the descending glissando?
Thanks for any insights.