Frequency, Amplitude, and Waveform Modulation in the Vocal Vibrato
Paul B. Oncley · The Journal of the Acoustical Society of America · 1971
Various observers have noted that the fluctuation of pitch in the normal vocal vibrato is accompanied by a fluctuation of loudness and of timbre. Although this coupling is an implicit outgrowth of the formant principle, the point does not appear to have been explicitly pointed out in the literature, and seems to have been missed by some. Consider any single partial lying on the slope of a resonance peak, such as a vowel formant. If the slope is positive, an increase in frequency leads to an increase in intensity; if negative, the converse is true. Since the voice has many partials randomly located with respect to the formants, the loudness may be expected to vary with pitch, but with sometimes a positive and sometimes a negative phase relation. The overtone structure will also vary though the vibrato cycle. This timbre vibrato is clearly shown in a motion picture made some years ago at Bell Telephone Laboratories. Several other interesting points concerning the practical utility of the vibrato in singing are also mentioned.