What Might the Trombone Teach Us About the Singing Voice?-A Tutorial Review

The trombone and the male voice cover similar frequency ranges and, at a physical level, the basic anatomies of the voice and the trombone show some qualitative similarity: both have two vibrating flaps of muscular tissue (the vocal folds and the trombonist's lips, respectively), and in each case, these are loaded acoustically by resonant ducts both upstream and downstream. There are also large differences. For example, the downstream ducts differ in length. The trombone usually operates with an oscillation frequency close to that of one of the downstream resonances; this is only occasionally true of the voice. Because the lips of a trombone player are much more readily accessible for experiments, they have yielded more detailed measurements of longitudinal and transverse motion, AC and DC pressures, and flow under varying acoustic loads. In normal operation, the downstream motion of the lips or vocal folds leads the lateral opening motion, resulting in a sweeping flow that leads the flow through the aperture. The relative timing of these flow components is related to the phases of the pressure across the tissues and the downstream acoustic load. Further, the work done on trombonists' lips due to their sweeping motion makes an important contribution to maintaining oscillation with both inertive and compliant acoustic loads. This probably explains why trombonists can "lip" the pitch smoothly from above to below a downstream resonance. Similar calculations on measurements of vocal fold motion show a similar work contribution and suggest that this sweeping motion is significant in powering this component of laryngeal motion. Comparing and contrasting the operation of the two "instruments" gives new perspectives on the basic science of the voice, with practical applications including the use of resonances. This could be helpful to voice scientists but also useful background knowledge for singers and singing teachers.