All the bow techniques covered in the following sections explore and exploit the relationship between bow speed, bow pressure, and the contact point.
We know from traditional technique that we can manipulate these three factors to bring out a variety of tone colors, and that we cannot go “too far” with any of these parameters without squeaking, scratching, or crunching. The following diagram, created by John Schelleng in the 1960′s (and somewhat simplified here), graphically demonstrates this idea.
As the diagram shows (and as we know from experience), it is easier to create a normal tone when closer to the fingerboard, and as we get closer to the bridge, we must play with more bow pressure.
Now look at the modified diagram below. Here you can see that when we step outside the bounds of a “normal” approach to the string, we can create some interesting sounds.
This shows that we can get a ponticello sound more easily when playing near the bridge, but that some version of that sound quality can be accomplished at other contact points. The reverse is true for “overpressure”; it is more easily achieved nearer the fingerboard, but is possible with a lower contact point. We also see that playing tasto requires relatively light bow pressure if we want to maintain consistency of tone.
These diagrams, however, assume that the speed of the bow remains constant, effectively disregarding one of the three dimensions of sound production. Adding that important variable back into the equation greatly expands the palette of sounds available to us.