The low down on the double bass: looking for the effects of torsional modes

The action of the bow produces torsional oscillations in a string, as well as the normal transverse motion [1]. The torsional modes have frequencies several times higher than, and not harmonically related to, the transverse modes. Via the strongly non-linear bow-string interaction, the torsional modes are driven at a harmonic of the translational modes [2]. The torsional mode frequencies for wound steel bass strings decreased weakly with increasing tension, while those of twisted gut increased weakly. When inexperienced string players bowed notes, the sound spectra of the relatively long starting transients contained strong components near the first torsional resonance. In another experiment, eight professional bassists both played and listened to notes on steel strings of two brands prepared with different degrees of inharmonicity between torsional and translational modes. As both players and listeners, their responses were highly concordant. They showed no clear preference for harmonically related modes. They did, however, show a clear preference for one of the brands. Perhaps because of the relatively small quality factors (~1520) of the torsional resonances and the high frequencies at which they occur, it appears that any effects on playing caused by the coupling of inharmonic torsional modes are smaller than those due to other parameters controlled by string makers.