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Old Jan 20th 2015, 12:20 PM   #1
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Why does touching a guitar string make it sound different to pressing down on it?

I assume it has something to do with nodal planes etc., but I'm not entirely sure what the exact physics behind producing these harmonics is. Thanks!
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Old Jan 20th 2015, 12:32 PM   #2
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I'm not sure.

I just tried it with an electric guitar. When I pressed down, the pickup was acting as a fret. So I got a higher tone than what I got when I just touched it. But I don't know if this is the scenario you're asking about.
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Old Dec 21st 2015, 03:18 PM   #3
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Guitar strings

This is a standard technique with any string instrument. (For a practical demonstration pull up on your computer a performance of Sibelius' Violin Concerto.)
Pressing down on a string adjusts the free length which is able to vibrate. With a guitar you press against a fret and so each note is fixed. For a viol the string is simply trapped between a finger-tip and a smooth finger board (which is why listening to somebody attempting to learn to play a violin is such a painful experience!).

If instead of pressing firmly on a string you just lightly touch it then the effective length of the strng is changed but the string retains the ability to vibrate beyond the finger. Thus the string is divided but free. If - and only if - you divide the string accurately then the two parts agree to a good old English compromise and settle on a suitable harmonic mode.

This demands a certain minimum quality from the instrument and a heck of a lot of practice on the part of the player.

Ken Green

Last edited by kengreen; Dec 21st 2015 at 03:21 PM.
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