Fig. 4. Vocal tract resonances emphasize sound frequencies near 2 and 5.5 kHz. (A) Vocal tract resonances estimated as the difference between a spectrum obtained from the speaker mounted by itself (black line at zero) and spectra recorded after inserting the port of the speaker into the trachea, setting beak gape at 5 mm, and orienting each subject towards the microphone (0°, colored lines). (B) Vocal tract resonances estimated from recordings obtained at a distance of 15 cm from horizontally oriented subjects. The normal condition represents a trachea length that was deemed as natural. The trachea was then shortened or lengthened by gently pushing or pulling on the cut end of the trachea. (C) Vocal tract resonances estimated as in B after removing 5 mm or 8 mm segments from the exposed portion of the trachea. Results for subjects 390 and BB are similar. Spectra in (A) are noisy (rippled) because these recordings were obtained at a distance of 90 cm. Spectra in (B) and (C) are less noisy because these recordings were obtained at a distance of 15 cm.