Warbling in steadily bowed viol strings

The timbre of a note played on a bowed string instrument depends on a number of performance parameters, including bow pressure, bow speed, and distance of the bowing point from the bridge. When these parameters are held constant during the bowing of a note played without vibrato the radiated sound usually has a stable timbre, with a frequency spectrum which is unchanging apart from small random fluctuations. For certain combinations of instrument and bow this timbral stability is disrupted, leading to a periodic modulation of the timbre which can be described as a “warble.” The warbling effect was first noticed on a large bass viol, but it can also be found on other bowed string instruments including the cello. Unlike the well known “wolf” phenomenon, the warbling effect is not associated with a particular played pitch on the instrument, but occurs over a wide range of pitches. Experimental studies on bass viols have shown that the warble is a modulation of upper harmonics of the played note, with a modulation frequency of a few hertz. Possible mechanisms which could explain the generation of this modulation are discussed.