Stevens & Keyser (2010)* hypothesized that the human speech production system is structured in such a way that the sounds that it can generate and the articulatory attributes that produce these sounds define a set of quantal states. This paper explores some aspects of the quantal theory: thresholds in aerodynamic parameters and their consequences on the acoustic output and changes of state due to fluctuations in articulatory parameters also affecting the acoustic output. Stevens & Keyser (2010) state that quantal relations defining the distinctive features appear to arise from two physical principles. The first physical principle involves acoustic coupling between resonating vocal-tract airways. In this case the quantal aspect is a consequence of the movement of zeros in the vocal-tract transfer functions. The second physical principle involves an aerodynamically based articulator parameter that creates airflows and acoustic sources, and these interact with compliant vocal tract surfaces.

The paper discusses the implication of aerodynamic thresholds of intraoral pressure (Po), oral (Oaf) and nasal airflow (Naf) to account for acoustic realizations that do not necessarily imply changes in articulatory settings but that can be accounted for as quantal changes. This is illustrated by the predictable acoustic variations of voiceless fricatives in Guarani that when nasalized turn into another fricative with a flatter spectrum ([x̃] > [h]) and are identified at another place of articulation. This implies thus no articulatory change in place of articulation but only a change in the noise spectrum. In a similar way the realizations of bilabial trills [ʙ] critically depend of the high value of Po that has to be maintained for a period of time to allow the realization of the Bernoulli effect on the lips. Compliance of the vocal tract walls also plays a crucial role in maintaining a high Po. In Khoisan languages the loss of alveolar clicks [!] turning into voiceless velar stops [k] implying both a change in mode and place of articulation are considered as changes of state and also account for quantal changes. In this case the weakening of the articulatory setting of the alveolar click triggers a change in the intensity of the burst noise that becomes similar to that of a voiceless velar stop [k]. These examples will be discussed in the presentation.