Wednesday, June 1, 2011

Violin Open E String Whistling Problem (Part 2)

When I designed the Kaplan Solutions non-whistling E-string (KS311W 4/4M), I spent hours trying to make an E string whistle so I could take some measurements. This after years of trying to achieve just the opposite!

Last summer at the VSA-Oberlin Acoustics Workshop, Aaron Boyd, concertmaster of the Tucson Symphony Orchestra, taught me a very reliable way to whistle an open E. Start the bow moving above the string first, then contact the E string. This works equally well down or up bow! I then remembered a talk by acoustics researcher Knut Guettler where he showed that quick and clean starts of (normal) bowed notes required just the right amount of bow acceleration. I finally realized why preventing an E string from whistling is so difficult.

The normal bowed string vibrates in a transverse (side-to-side) motion, producing a saw-tooth shaped waveform. This motion is called the Helmholtz motion after the great 18th century German physicist Hermann Helmholtz who discovered it. Only certain combinations of bow speed, acceleration and pressure produces a stable Helmholtz motion.

When the bow contacts the string while it is already moving (such as crossing to the open E string from the A string), conditions are favorable for the start of torsional vibrations, and unfavorable for the start of normal transverse vibrations. One rarely whistles an open E-string when starting a note with the bow already on the string, or after changing bow directions, or when playing repeated notes because in these situations the bow starts each note with zero velocity, which favors the start of normal transverse vibrations over the torsional.

Therefore, the way to prevent the whistling E using bowing technique is to stop or slow the bow before it contacts the E-string. In addition, an increase in bow pressure on the E will favor the transverse motion over the torsional, and that is typically what players try to do. However, this usually fails because the increase bow pressure is often accompanied by an increase in bow speed, which is the exact opposite of what is needed! Unfortunately, the bowing conditions required to prevent the whistling may be undesirable musically.

In part 1, I discussed how wound E-strings can solve the whistling problem. In the next part, I will explore some common beliefs about whistling E-strings, and what might be done to minimize the problem through instrument setup.

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