Some physical mechanisms underlying the pipe voicing art
Samuel A. Elder · The Journal of the Acoustical Society of America · 1979
This paper examines underlying mechanisms for changes in the flue pipe spectra caused by selective changes in mouth cutup, pipe length, languid position, etc. The physics of the problem involves the mechanics of jet waves, and the subtleties of their interaction with sound waves. Special interest centers at lower lip, where jet waves originate, and at upper lip, where sound is produced by collision of jet and pipe fluid. The necessities of feedback amplification require jet and sound motions to act in a reciprocal way so that, for example, delay in impulse transmission along the jet is exactly matched by acoustic transmission delay in pipe. Actually, jet waves and their associated sound waves behave in remarkably self-consistent and complimentary ways. Just as sound waves travel up and down pipe to form discrete modes, jet waves also appear in discrete pairs, which travel up and downstream. One of the principal acoustic factors in voicing is the natural “stretching” of pipe mode frequencies, which tends to give pipes voiced to oscillate above resonance a different character from those voiced to oscillate below. An analogous property of jet is thickness Strouhal number, or nondimensional frequency. Since cutoff occurs at a critical Strouhal number, harmonic development on the jet is not possible unless S < 0.05 Sc, High and low Strouhal number jets, therefore, exhibit opposing spectral tendencies. Thus voicing is seen as adjustment toward optimum complimentarity between jet and resonant cavity, consistent with a desired spectral output.