‘Warming up’ a wind instrument: The time-dependent effects of exhaled air on the resonances of a trombone

To study the effect of ‘warming up’ a wind instrument, the acoustic impedance spectrum at the mouthpiece of a trombone was measured after different durations of playing. When an instrument filled with ambient air is played in a room at 26–27 °C, the resonance frequencies initially fall. This is attributed to $CO2$ in the breath initially increasing the density of air in the bore and more than compensating for increased temperature and humidity. Soon after, the resonance frequencies rise to near or slightly above the ambient value as the effects of temperature and humidity compensate for that of increased $CO2$⁠. The magnitudes and quality factors of impedance maxima decrease with increased playing time whereas the minima increase. Using the measured change in resonance frequency, it proved possible to separate the changes in impedance due to changes in density and changes in acoustic losses due to water condensing in the bore. When the room and instrument temperature exceed 37 °C, condensation is not expected and, experimentally, smaller decreases in magnitudes and quality factors of impedance maxima are observed. The substantial compensation of the pitch fall due to $CO2$ by the rise due to temperature and humidity is advantageous to wind players.