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Slide guitar - acoustics question

Dear Nina
the slide DOES change the vibrating length of the string - and also divides the string into two sections - as the slide moves one section lengthens and the other shortens.
With an acoustic guitar, the section of the string between the slide and the nut at the top of the neck produces a quieter sound because it does not cause the bridge to transmit vibration to the guitar body. If you have an electric guitar with two pickups at different positions both on, and move the slide between the two pickups so that each detects the vibration of one section of the string, you will more clearly hear the two complementary glides.due to one section shortening and the other lengthening..
Experience slide players often damp the strings with the fretting hand just behind the slide to prevent the unintended pitches from sounding.


guitarist's finger the difference between the slide and the finger is that At 04:20 22/03/2007, you wrote:

Date:    Wed, 21 Mar 2007 17:14:17 -0500
From:    Cornelia Fales <cfales@xxxxxxxxxxx>
Subject: Slide guitar - acoustics question

Hello List,

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.

Nina Fales

Dr Andrew Faulkner Principal Research Fellow Dept Phonetics and Linguistics UCL (University College London) Wolfson House 4 Stephenson Way LONDON NW1 2HE tel 44 (0)20 7679 7408 (direct) Internal tel 27408 fax 44 (0) 20 7679 5107 e-mail a.faulkner@xxxxxxxxx WWW: http://www.phon.ucl.ac.uk/home/andyf/home.htm