Speaker
Description
Musical instruments of the wind family present bore geometries that are usually approximated by acousticians as analytical functions such as cylinders, cones, exponential or Bessel horns. However, a closer look at actual instruments shows that the makers use adjustments of the bore profile to control the intonation of the instruments with a rather fine precision, typically of the order of 1/20th to 1/50th of the bore diameter. Drilling such bores in the workshop is one of the challenges of wind instrument making, whether it involves drilling in wooden instruments or shaping metal instruments on a mandrel. The negative bore concept was first introduced at ISMA 2019. The main idea was to replace the bore with non-ideal geometry with a pipe of ideal profile into which a needle (called "negative core") is inserted, provided that the diameter of the needle is calculated such that the resulting flow cross-section of the pipe is similar at every point along the pipe's axis to that of the original bore. However, this process increases the wall surface, degrading the acoustic equivalence between the two pipes. By studying this effect, the aim is to determine the negative's geometry giving similar acoustic properties that the original bore.The presentation will include a theoretical analysis of the problem in terms of the influence of the viscous and thermal effects on the acoustic wave propagation, together with experimental data, both showing the dependence on the hydraulic perimeter of the pipe including its negative core.