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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) glissando?
2) what accounts for the descending glissando?
Thanks for any insights.