So much of what characterizes compositions of today is creative use of the non-standard timbres that string instruments can produce. In the section that follows we will explore the nexus of bow pressure and bow speed to generate some really colorful sounds on the cello. Two main areas of exploration are tone distortion (commonly known as overpressure) and non-pitched sounds such as scratch tone and air-noise.
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 an 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, a distorted sound can be achieved without 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” or “Heavy Pressure”. Sometimes it is abbreviated “OP”, “mOP” (molto Overpressure) or “pOP” (poco Overpressure). Transitions from normal pressure to and from 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 to text because its intuitive symbology is quicker to read, 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.
Fausto Romitelli, the late Italian composer, uses symbols that look like “double” down-bows and up-bows in his “Professor Bad Trip” trilogy.
German-American composer Hans Thomalla uses a different symbol for a similar technique in his 2010 string quartet “Albumblatt.” He refers to it as “too-slow bow,” which he distinguishes from overpressure by indicating that he does not want a “snoring sound”, but a slight distortion of the sound with the tone still shining through.
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 not at all standardized, it is sometimes indicated with an “x” note-head, like this:
Note that this technique is sometimes notated on open-string pitches to show which string to play on, although no discernible pitch is expected. If the notation indicates a pitch, that usually means to place the fingers in that area on the string.
Flautando, a performance indication dating back centuries, can be thought of as a pressure technique (or anti-pressure technique, if you like). An extreme version of molto flautando has been defined by some composers as “Rauschen,” or “air-noise”. This technique requires one or more fingers to be placed lightly on the string, and when accompanied by an extremely light bow pressure results in a soft, airy noise with just a hint of pitch. Hans Thomalla and other composers who write in the musique concrete genre use this technique to great effect.