High precision measurements of acoustic impedance: Applications to the voice and respiratory tract, musical instruments and their players

The acoustic impedance spectrum of the vocal tract and trachea is important in speech and singing. The operation of musical wind instruments depends on the impedance spectra of their bores, and sometimes also on the impedance spectra of the player’s vocal tract. Here, we describe two measurement techniques. The three microphone technique, calibrated with three non-resonant loads, achieves large dynamic range measurements of the frequencies, magnitudes, and bandwidths of resonant duct maxima and minima. The impedance minima are particularly important in flute family instruments and in the voice, where resonances correspond to minima at the lips. These measurements can also be used to estimate the duct geometry and acoustic properties at the distant end, which may be otherwise inaccessible. The capillary or current source technique has been used to measure impedance maxima inside the mouths of wind players during performance, e.g., where saxophonists tune their tracts to the note played, and for “ecological” measurements during speech and singing that give estimates of resonant frequencies that are rather better than those from formant estimation at high pitch. This technique has recently been updated to run in real-time and made available as open source software, with modest equipment requirements.