The evolution of the syrinx: An acoustic theory
Fig 2
Sound production is more efficient when the sound source is in a syringeal position.
(A) PTP was consistently lower in the syrinx than in the larynx. (B) The syringeal sound source generated higher sound intensities (depicted as Amplitude, relative change in output voltage of microphone signal) than the larynx for most tracheal lengths. (C) The fluctuating fundamental frequency is evidence for the strong interaction between sound source and vocal tract filter. (D) Glottal efficiency of a laryngeal or syringeal sound source at different tracheal lengths. Between 40 and 80 cm tracheal length, the effect is most dramatic. The two vertical dashed lines indicate a tracheal length range (40 cm ± 10 cm), which is most likely to correspond with a sound source of 1 cm vocal fold length used in this study. This tracheal length corresponds to an avian archosaur of approximately 20 to 30 kg body mass [18]. Tracheal lengths much shorter or much longer than this range could correspond to a few extreme exceptions in which the trachea is very short or very long. Numerical data used in this figure is included in S1 Data. PTP, phonation threshold pressure; TE, tracheal elongation.