An acoustical study of sound production in biphonic singing, Xöömij

A theory that the high melody pitch of biphonic singing, Xöömij, is produced by the pipe resonance of the rear cavity in the vocal tract is proposed. The front cavity resonance is not critical to the production of the melody pitch. This theory is derived from acoustic investigations on several three-dimensional shapes of a Xöömij singer’s vocal tract measured by magnetic resonance imaging. Four different shapes of the vocal tract are examined, with which the melody pitches of F6, G6, A6, and C7 are sung, along with the F3 drone of a specific pressed voice. The second formant frequency calculated from each tract shape is close to the melody pitch within an error of 36 cents. Sounds are synthesized by convolving a glottal source waveform provided by the Rosenberg model with transfer functions calculated from the vocal tract shapes. Two pitches are found to be successfully perceived when the synthesized sounds are listened to. In a frequency range below 2 kHz, their spectra have a strong resemblance to those of the sounds actually sung. The synthesized sounds, however, fail to replicate the harmonic clustering at 4–5 kHz observed in the actual sounds. This is speculated to originate from the glottal source specific to the “pressed” timbre of the drone.