Delay-induced low-frequency modulation of the voice during sustained phonation.

An important property of negative feedback systems is the tendency to oscillate when feedback is delayed. This paper evaluated this phenomenon in a sustained phonation task, where subjects prolonged a vowel with 0-600 ms delays in auditory feedback. This resulted in a delay-dependent vocal wow: from 0.4 to 1 Hz fluctuations in fundamental frequency and intensity that increased in period and amplitude as the delay increased. A similar modulation in low-frequency oscillations was not observed in the first two formant frequencies, although some subjects did display increased variability. Results suggest that delayed auditory feedback enhances an existing periodic fluctuation in the voice, with a more complex, possibly indirect, influence on supraglottal articulation. These findings have important implications for understanding how speech may be affected by artificially applied or disease-based delays in sensory feedback.