Research in vocal vibrato has established that vocal tract filtering is primarily responsible for the amplitude modulation (AM) present in Western classical vibrato. Using electroglottography (EGG) and the EGG speed quotient, which is sensitive to fluctuations in the amplitude of vocal fold vibration, AM was detected at the laryngeal (source) level, in addition to the subsequent AM which results from vocal tract filtering. Seventeen classically-trained opera singers sang vowels in three pitch and loudness conditions. EGG and microphone measurements of FM and AM and their rates, extents, and periodicity were made. Airflow was also measured, and the samples were rated by voice professors for vibrato consistency, speed, and width. Physiologic and acoustic data revealed that AM from vocal tract filtering, or the resonance-harmonics interaction (RHI) described by Horii and associates, was present throughout the vibrato samples. Laryngeal-level AM was also present throughout, with soft conditions having the highest mean extents. Singers with lower degrees of laryngeal-level AM were also those rated highest for vibrato consistency. Vibrato rate increased as pitch increased, and, to a lesser extent, as intensity increased. These findings document, in addition to the AM resulting from the RHI, the concurrent presence of laryngeal-level AM in a group of singers representing a range of training and experience.

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