Fig. 1. Synthesized examples illustrating four types of biphonation and two-voice phenomena observed in mockingbird songs. (A) The spectrogram (top) and spectrum (bottom) illustrate the spectral properties typical of both `type A' biphonation and two-voice phenomena, consisting of two independent frequencies, f<sub>0</sub> and g<sub>0</sub>. (B) `Type B' biphonation consists of a fundamental frequency and sidebands. In this synthesized example, f₀ is a 1 kHz tone. The second frequency is a 250 Hz modulation frequency, m₀, which appears spectrographically as sound energy 250 Hz above and below f₀. (C) A synthesized example of `dual biphonation' similar to the type observed in mockingbird vocalizations. Each side of the syrinx produces `type B' biphonation simultaneously. In this example, two unrelated fundamental frequencies (f₀ and g₀), originate from opposite sides of the syrinx. The two `voices' are each modulated by an unrelated, lower modulating frequency (250 Hz, m<sub>0</sub>). (D) A synthesized example of `dual biphonation' similar to C, but in this case m₀ is also frequency modulated, resulting in sidebands that are not parallel to f₀ or g₀. BP, biphonation; 2VC, two-voice phenomena; SB, sidebands; 2SB, dual biphonation (two f₀ values, each with sidebands).