Correlations among subjective performance qualities and objective acoustical parameters of the didjeridu

Traditional didjeridus have a broad range of unknown bore geometries, and consequently have advantages for examining relationships between subjective and physical parameters. This study involved 7 experienced players assessing each of 38 traditionally made didjeridus in terms of 8 subjective parameters. Their assessments were then correlated with detailed measurements of acoustic input impedance spectra and geometry. There was a small, but useful, consistency in the rankings of the 7 players, although it was surprising that backpressure was the least consistent term, because this term is widely used by both didjeridu and brass players. A high degree of positive correlation existed between most of the parameters (clarity, resonance, loudness, vocals, overtones and overall quality). The other two parameters studied (backpressure and speed) were correlated with each other, although not significantly correlated with the other parameters. The parameters clarity, resonance and backpressure made the most important contributions to the overall quality of an instrument. In general there was a strong negative correlation with the characteristic impedance of the instrument for overtones, vocals, resonance, loudness and overall quality. The parameter speed was found to be positively correlated with the frequency of the first (or lowest) frequency resonance. Assessments of PVC pipes were consistent with these results. The overwhelming preference of the assessors was for instruments with a low magnitude of impedance across the spectrum, particularly in the 1 to 2 kHz region; a feature that allows time-varying resonances of a player's vocal tract to have a dominant influence over the spectral envelope of the sound.