Physical modelling of a harp from Central Africa

Central Africa harps are string instruments, often anthropomorphic, serving an essential cultural role. Compared to pedal harps, their body is small in size with a soundboard mainly made of animal skin and a neck made of a wood beam. In this paper, a physical model is developed as a tool for assessing the specificities of these musical instruments, from a vibro-acoustic perspective. The modeling strategy is based on the modal Udwadia-Kalaba formalism which is a multibody substructuring technique. Input modal parameters of the body and the strings are experimentally identified and the main steps of the estimation procedures are detailed. The reliability of the physical model is investigated via the comparison between simulated and experimental data for several plucking configurations. Different hypotheses are assessed such as the string/neck coupling which proves to strongly influence the dynamic response of the body when there is a coincidence between string and neck modal frequencies. The inclusion of geometrical nonlinearities proves to be of major importance, even for a weak plucking, as it allows qualitative representation of double frequency terms in the simulations. Overall, physical simulations of the soundboard motion are in good agreement with measurements indicating characteristic features of the instrument are captured.

Effects of Twisting and Surface Finish on the Mechanical Properties of Natural Gut Harp Strings

Natural gut harp strings are made from twisted bundles of gut strips, which are dried, ground, and varnished. The effects of varying the twist angle and surface finishing on the mechanical properties of gut harp strings have been explored. Strings were tested over a range of twist angles from 23.5∘ to 58.3∘, and with all four combinations of ground or unground and varnished or unvarnished surface finishing. The principal effects of varying the degree of twisting were that the breaking strength and tensile Young's modulus both fell as the twist angle was increased. String makers must therefore make a compromise between sound quality and string strength and durability. Leaving the string unvarnished dramatically increased the sensitivity to changes in humidity, which, in turn, affected the thermal tuning sensitivity and creep behaviour. Grinding the string surface prior to varnishing had no significant effect on the behaviour, but did make some difference to the thermal tuning sensitivity if the string was left unvarnished. Increasing the humidity frequently triggered episodes of additional string creep. There appeared to be a threshold effect, with the additional creep triggered when the string linear density exceeded its previous maximum. When the string was not creeping, there appeared to be a strong coupling mechanism between changes in the linear density and complementary changes in the string tension, such that there was almost no net effect on the string frequency. This behaviour was independent of the twist angle and the surface finishing, suggesting that whatever the coupling mechanism was, it was not dependent on the twisted structure of the string.

A necessary condition for double-decay envelopes in stringed instruments

Measurements of body vibration characteristics of five different stringed musical instruments have been used to address the question of whether and when they might be expected to produce transient response featuring a "double decay" sound profile. The phenomenon has been well documented and studied in the context of the piano but has not been systematically studied for other instruments. The results show considerable variation among instruments. The piano is indeed predicted to show double decays over most of its range. In the tested guitar, by contrast, double decays are likely to be confined to a few notes near strong body resonances. Other instruments fall between these extremes. The lute and the mandolin, both normally strung with double strings, should both exhibit double decays over much of their playing range, especially towards the higher end. The banjo is single-strung but is also predicted to show strong double decays, especially for higher notes in its range.