Characterization and modeling of the Erhu

The erhu is a two-string bowed instrument characterized in Chinese organology as a silk instrument (owing to the traditional string material), and by Hornbostel and Sachs as a spike fiddle. The overall length of the instrument is about 80 cm, and the vibrational length of the strings is around half of that. The soundbox is a tube (hexagonal, octagonal, or cylindrical) of about 13 cm length and 9 cm diameter. One end of the tube is covered by a membrane of tensioned python skin and the other is mostly open, usually with a grille. The two strings are stretched over a small bridge positioned centrally on the membrane. In spite of the great difference in soundbox structure between the erhu and a Western violin, the radiated sounds of the two instruments have a lot in common with each other (and with the human voice). Vibrational and acoustic measurements have been made on several erhus: cavity modes, surface velocities and radiated sound. A straightforward coupled-oscillator model has been found to reproduce many of the measurements, and also to allow mechanical characterization of the membrane in situ.