Laryngeal Air Sacs
Humans do not have the pharyngeal air sacs that are present in many non-human primates. This may reflect an adaptation enhancing the robustness of speech communication. Speech is produced by filtering the source of acoustic energy generated at the larynx by the process of phonation in English vowels and “voiced” consonants such as [m], or by turbulent air flow at the larynx (the sound [h]), or at constrictions formed by the tongue such as the sound [s].The airway above the larynx – the supralaryngeal vocal tract (SVT) acts as a filter, allowing maximum energy to pass through it at formant frequencies that are determined by the SVT’ shape and length. Formant frequencies are arguably the primary acoustic features that differentiate the phonetic elements that specify words. Pharyngeal air sacs would absorb energy at specific frequencies, interfering with the complex perceptual processes involved in recovering the formant frequency pattern from the flow of speech. Some of the war gases used in World War One enlarged the vestigial pharyngeal air sacs present in humans. Observations of the gassed soldiers noted that their speech often was distorted and difficult to comprehend. However, if recordings of their speech were made, they have been lost. Computer modeling studies of the acoustic effects of adding pharyngeal air sacs to a human SVT would address this question.
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