Violin tone - physics behind bow speed, pressure, contact point

January 21, 2019, 5:04 PM · Very recently I realized on experimenting the extremes of violin tone (venturing outside "normal" tone boundaries into guitar like distortion and "creaking door sounds" or, on the other side, whistling sounds from skidding over the strings) that most accounts I was aware of explaining the fundamental parameters of violin tone were somehow incomplete. Both bow pressure and contact point variation can be used to obtain these extreme tones. Do these approaches (varying bow pressure or changing sound point) actually differ in HOW they allow us to change timbres, or is the underlying mechanism the same?

This might seem to be a wild theoretical rambling only relevant to people experimenting with extreme tones, yet the the answer to this question actually has consequences for normal tone production. If we take into account feedback systems for parameters that influence tone, we can very readily understand that bow speed is easily perceived by the player, and so is bow pressure (how much we "push" into the string). Contact point is harder to determine. Depending on playing style, you may be able to obtain visual feedback on the contact point, yet this feedback is 1) slow (our visual processing is slower than both touch and auditory processing) and 2) a lot of the time we cannot afford to be looking at the contact point constantly (eg. when sight-reading). This means the main way to obtain feedback on the contact point is through hearing. Yet most contact point exercises focus on looking at the contact point, and not so much in describing what to look for in the sound.

Searching for a resource with detailed explanations on the parameters of violin tone and their (physical) influence on sound, I found the work Violin Pedagogy and the Physics of the Bowed String by Alexander McLeod, a PhD thesis available at the library of University of Toronto.

Needless to say, I vividly recommend the reading of this work, which explains in detail the physics behind the main parameters of violin tone production (bow speed, bow pressure, contact point) and also some minor parameters (bow tilt, left hand finger pressure). In doing so, it concludes that works by major pedagogues, although correct in principle, very often overlooked or even sometimes blundered some aspects of tone production - simply due to being unaware of the detailed physical mechanisms behind tone production, which have actually only recently been well described.

Besides educating me well beyond my initial inquiry, I did in fact conclude that the mechanism by which bow pressure and contact point alter violin tone (in particular, its spectral content or "timbre") is pretty much the same. This sheds some light on why there are conflicting schools on such simple matters as on how often to change contact point and to what extent. In fact, bow pressure and contact point, as it pertains to timbre (excluding the no small matter of contact points near the bridge allowing for greater volume) are pretty much interchangeable. This means that the impact of contact point on timbre cannot be evaluated independently from the pressure applied. If you apply more pressure, you will (provided you don't crush the tone) obtain a timbral modification close in quality to a move of contact point towards the bridge, and vice-versa.

Practical impacts? In order to determine, by hearing, where your contact point stands, you must first be aware of the bow pressure you are exerting. The feedback mechanism for the player, as it regards contact point, and excluding the unreliable visual feedback, consists of the timbre you hear plus the bow pressure you are exerting.

As regards teaching and personal practice, I posit this information is much more actionable than visual feedback of the contact point. As we all know, if we and our students learn the mechanics of the contact point through visual feedback, the contact point will be neglected as soon as the eyes are off the violin. An exercise I have been trialing runs thus as follows: Maintaining the contact point (with visual feedback), increase pressure and note the timbre change. Now return to normal bow pressure and, without visual feedback and maintaining the same bow pressure, move the contact point until you obtained the same timbre as you did by increasing pressure.

This kind of exercise allows the student to build experience on aurally determining the contact point, and also establishes the relationship between timbre, pressure and contact point, and how these two mechanisms are somewhat interchangeable, and thus allow the students to choose very diverse bow strategies to achieve similar results.

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