On the cutoff frequency of clarinet-like instruments. Geometrical versus acoustical regularity

TL;DR

This paper investigates the cutoff frequency of clarinet-like instruments, analyzing how irregular hole placement affects acoustical regularity and the implications for the instrument's acoustic properties.

Contribution

It introduces a concept of acoustical regularity based on equal eigenfrequencies of T-shaped cells and explores how real clarinet holes can be divided into such regular cells.

Findings

The cutoff frequency depends on overall geometry, not fingering.

Real clarinet holes can be divided into acoustically regular cells with equal eigenfrequencies.

Spacing and radii of holes scale together, maintaining constant eigenfrequencies.

Abstract

A characteristic of woodwind instruments is the cutoff frequency of their tone-hole lattice. Benade proposed a practical definition using the measurement of the input impedance, for which at least two frequency bands appear. The first one is a stop band, while the second one is a pass band. The value of this frequency, which is a global quantity, depends on the whole geometry of the instrument, but is rather independent of the fingering. This seems to justify the consideration of a woodwind with several open holes as a periodic lattice. However the holes on a clarinet are very irregular. The paper investigates the question of the acoustical regularity: an acoustically regular lattice of tone holes is defined as a lattice built with T-shaped cells of equal eigenfrequencies. Then the paper discusses the possibility of division of a real lattice into cells of equal eigenfrequencies. It is shown that it is not straightforward but possible, explaining the apparent paradox of Benade's theory. When considering the open holes from the input of the instrument to its output, the spacings between holes are enlarged together with their radii: this explains the relative constancy of the eigenfrequencies.