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Thanks for a great article.
If I'm not mistaken, the double-slipping motion described in the article actually does correspond to an acceptable sound in Western classical violin playing. It is the sul ponticello sound, which is called for by Schumann in his D minor sonata, by Beethoven in his quartet op. 131, and may also have been called for as a special effect by Baroque composers. It's ghostly quality can be accounted for by its weakening of the fundamental and emphasis on upper partials. I wonder, in fact, if there may also be triple-slipping and quadruple-slipping, etc., corresponding to higher partials? The sound gets its name, of course, from playing close to the bridge where it's easy to be outside of the Helmholtz triangle and difficult to be inside of it. However, as the article shows, one doesn't need to be close to the bridge in order to be outside of the triangle and, therefore, produce the sul ponticello sound.
Sul ponticello has an interesting relation to standard violin playing. If one plays sul ponticello and then increases the bow pressure markedly, also with a slight increase in speed, one can literally POP inside the triangle. A popping sound occurs as the string rapidly reorganizes itself from double-slip motion into Helmholtz motion--I assume that the pop is a quick burst of chaotic noise as the string reorganizes itself. My students know these as "ponticello pops" and I believe that they are the explanation for well-articulated slurred string crossings (crossing from one string to another without stopping the bow). The initial sound on the new string is briefly sul ponticello, since the weight of the bow is still mostly on the old string upon initial contact with the new string. This initial sul ponticello is drowned out by the still-full sound from the old string. As the weight of the bow transfers to the new string, the initially double-slip motion pops into Helmholtz motion. This explains how a string crossing, which involves no actual finger attack on the finger board, can produce the same popping sound as a finger attack.
Please let me know if any of this needs correcting. Thank you!

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