Two common pressure techniques found in contemporary string writing are overpressure and scratch tone and these will be fully explained and demonstrated in the following pages.
The term “overpressure” is actually something of a misnomer in the sense that it only partially describes the function of bow pressure in creating the range of distortion in the tone quality. As I described in the bow techniques page, “normal” tone production occurs with a appropriate balance of three factors: bow speed, pressure, and contact point. When one or more of these parameters is taken to an extreme, we get sounds that are not “normal.”
So, for example, if we keep the bow speed and pressure constant while moving the contact point towards the bridge, we get the sound we call ponticello because the bow speed overwhelms the range of the string’s oscillation at that contact point, and the bow hair “slips” more than it “sticks.” So it follows that we can achieve a similar sound even when away from the bridge — by increasing the bow speed and decreasing the pressure.
We are accustomed to thinking of sul tasto as being the opposite of sul ponticello, and in terms of contact point this is true. In terms of sound production, however, it is overpressure that is the opposite of ponticello, because we manipulate those three sound-production factors to cause more “sticking” than “slipping.” The string gets stuck underneath the bow hair, interrupting the wave of the moving string and distorting the sound. So, actually, an “overpressure” sound can be achieved without even exerting extra pressure, but rather by slowing the bow speed and moving away from the bridge.
Quite often overpressure is notated simply with the words “Overpressure”, “Heavy Pressure”, or sometimes more accurately, “Too-Slow Bow.” Sometimes it is abbreviated “O.P.” Transitions from normal pressure to overpressure are often indicated with dotted lines or arrows.
A more elegant solution is employed by Finnish composer Kaija Saariaho. She uses black wedge shapes to show how much relative pressure should be applied, with the thinnest part of the wedge being normal or near-normal pressure, and the thickest part of the wedge showing the greatest pressure, leaving almost no tone.
This notation is preferred because its intuitive symbology is quicker to read than text, not to mention the fact that the arrows (or dotted lines, as the case may be) could be confused with arrows for ponticello/tasto notation.
Scratch Tone is what it sounds like: the obliteration of pitch so that all that’s left is a guttural, scratchy sound effect. Although notation for this is by no means standardized, it is sometimes indicated with an “x” inside the note-head, like this:
Note that this technique is usually notated on open-string pitches to show which string to play on, although no pitch is expected.