Characteristics, mechanisms and perceivability of combination tones in violins

Combination tones generated by the inner ear have been widely investigated in the past, starting from the famous Tartini's "third tone". Much less attention has been instead dedicated to the combination tones generated by musical instruments. In this paper we have investigated the combination tones generated by a set of violins of different quality and age when playing a selected set of dyads. Combination tones were found in all the violins and the strongest of them occurred at frequency below the lower note of the dyad. Its amplitude was strongly dependent on violin and dyad played, and was greatest in two old Italian violins and decreased down to a minimum in a factory made violin. All these findings are well explained by the boosting action of A0, the main air resonance of the violin that correlates well with the strongest combination tone. A listening test, performed using selected dyads and violins, showed that the differences between dyads with and without combination tones were correctly recognized by a group of professional and amateur musicians, suggesting a possible musical significance of the main combination tone. Results investigating the possible source of violin non-linearity are also described.