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Fig. 7. Unilateral biphonation and `dual biphonation' in mockingbird song. (A) Spectrogram of two syllables produced in sequence by mockingbird m108. The first syllable in the pair (a) is an example of unilateral biphonation. This syllable is produced with the left side only, but two independent frequencies are present, the fundamental frequency (f₀) at 1800 Hz, and a lower, modulating frequency (m₀) visible as sidebands 115 Hz above and below f₀. The bird adds phonation from the right side of the syrinx in second syllable (b), and a second fundamental (g₀) appears, also with sidebands 115 Hz above and below, indicating that g₀ is also modulated by m₀. This is a two-voice syllable in which each voice is biphonic. (B) Expanded view of the sound waveform at arrows a and b in spectrogram (A), showing the AM pattern on the waveforms at a rate of 115 Hz (period of one modulation cycle 11.5 ms) both during unilateral flow (a) and bilateral flow (b). In addition to the 115 Hz modulation pattern, the two-voiced sound exhibits a second pattern in the waveform, which is likely the result of beating between f₀ and g₀. A beat frequency is equal to the difference between f₀ and g₀, in this example, the second modulation rate in b is approximately equal to 550, which corresponds with the difference frequency between g₀ and f₀ (2695–2135 Hz). (C) Power spectra taken at arrows a and b in (A). Sound was filtered with a digital 800 Hz Hanning shape high pass filter. Peaks labeled correspond with fundamental frequencies as well as the sidebands resulting from the interaction of modulating frequency (m₀) with the carrier (f₀) in a, and with the two carrier frequencies (f₀ and g₀) in b. Abbreviations as in Fig. 3.

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