Articulator Biomechanics

• Articulation


  The process by which sounds, syllables, and words are formed when your tongue, jaw, teeth, lips, and palate alter the air stream coming from the vocal folds
• Vocal tract


  Tube like series of cavities beginning at vocal folds and ending at lips. The shapes of these cavities are determined by the articulators
• Articulators


  Structures which directly form portion of vocal tract wall, or are directly attached to wall. May be movable or non-movable
• Movable Articulators


  • Lips
  • Mandible
  • Tongue
  • Soft Palate
• Fixed Articulators


  • Teeth
  • Hard Palate
• Purposes of Articulators

  • To control airflow simultaneously with changing vocal tract shape in order to create sound stream
  • Primary Role: Alter shape of vocal tract, Change filter characteristics of the tract (shape control = acoustic phenomenon)
  • Secondary Role: Alter flow characteristics of the vocal tract, Create sources of vibrational energy (airflow control = aerodynamic phenomenon)
• Source-filter model

  Speech results from convolution of source sound (e.g. voicing, frication) and filter effects (vocal tract, articulation). Source and filter are independent, hence harmonics and formants are independent
• Tongue, oral cavity, and pharyngeal cavity shapes determine different vowel sounds
• Characteristics of Movable Articulators


  • Behavior affected by different active and passive forces, Interact with the cavity in which they "reside", Focus is on how the change in the shape of the articulators changes the shape of the cavity
• Biomechanical Properties of Lips


  • Small mass relative to forces available, Highly elastic with strong recoil forces, Very small damping, Fast twitch muscle
• Biomechanical Properties of Mandible


  • Large mass and inertia, Not applicable stiffness as it doesn't change shape, Very small damping, Large and fast muscle forces but trade-off in precision
• Mandible Interaction with other Articulators

  Affects position of other primary articulators, Influences overall size of oral cavity
• The mandible is not classified as a "primary articulator" as no speech sounds are formed directly by the jaw
• The medial (internal) pterygoid and lateral (external) pterygoid are the primary muscles acting on the mandible in speech
• Biomechanical Properties of Tongue


  • Low inertia, Capable of being moved and shaped by intrinsic and extrinsic musculature, Muscular hydrostat structure allows for quick and precise changes in shape
• Vowel and consonant articulation are tied to different tongue shapes and positions within the oral cavity
• Muscular Hydrostat

  • Muscle-filled, incompressible container (mostly liquid)
  • No internal (rigid) skeleton
  • Musculature arranged in different arrays that allow for changes in structure shape without changing volume
  • Selective contraction of different muscle fibers can both move the tongue, and change its shape without changing its volume
  • Muscle contractions acting to decrease cross sectional area may elongate the structure
• Muscular Hydrostats in other animals: Possible ranges of motion similar to octopus tentacle or elephant trunk
• Finite Element models of the Tongue


  1. Some investigators have attempted to model tongue shapes in terms of geometric components

  2. Can predict what muscle activation levels of which muscles will shape the tongue a certain way

  3. Each cell contains equations that capture biomechanical properties of tissue in that cell

  4. Active and passive properties

  5. Forces applied to or generated within one cell will affect surrounding cells
• Surgical Modifications of the Tongue

  • Mouth floor resection
  • Hemiglossectomy
• Velopharyngeal Mechanism

  • Valve that couples and decouples oral and nasal cavities
  • If valving action not timed appropriately, or is not achieved, "abnormal" speech output may result
• Velopharyngeal Mechanism

  • Sagittal velar elevation movements occur superiorly and posteriorly
  • Mass: Low Inertia — Negligible relative to muscle forces available
  • Stiffness: Variable — through uvulus activation
  • Damping: Very small — negligible effect
  • The velum can adjust to wide range of task demands
• Shape Change of the Velum


  1. Mainly superior/posterior and inferior/anterior

  2. Becomes "hooked" when moving upward/backward

  3. Top of the hook is the velar eminence

  4. Undersurface of hook is the velar dimple

  5. Velar knee or dimple due to insertion of the palatal levator

  6. Contraction lifts up the middle of the velum
• Other Contributions to VP Closure

  • Pharyngeal walls also play a role
  • Generally accepted that posterior PW movement not sufficient to be of significance during speech production
  • Various strategies for closing velopharyngeal port (VP) have been identified
  • Strategies may vary based on individual anatomy
  • May also change over time as anatomy changes
  • Unlike with the tongue, VP muscle activity during speech is often measured
• Role of the Palatal Levator

  • Palatal levator is primary muscle associated with velar elevation
  • Levator, glossopalatine, and pharyngopalatine form coordinative structure to position VP mechanism
• Role of the Palatal Tensor

  • Palatal tensor largely inactive during speech
  • More active on swallow and to open Eustachian tube

  
• Role of the Uvulus

  • Uvulus may act to increase stiffness of VP seal
  • As the Uvulus straightens out the velum, it helps to increase tightness of seal
• Role of the Pharyngopalatine

  • Pharyngopalatine may produce fine adjustments in velar height when velum elevated
  • May also be involved in pharyngeal adjustments
• Role of the Glossopalatine

  • Glossopalatine is natural antagonist to palatal levator
  • Palatal elevation to close the VP is normally driven by muscle activation
  • Palatal lowering to open the VP is often largely driven by gravity and the stiffness of the velum when no time constraints
  • Glossopalatine helps to lower palate when time constraints dictate fast lowering
  • Also activated during elevation, as part of the coordinative structure
• As with the tongue, some have attempted to model soft palate movement using Finite Element Modeling
• Task dynamics


  • Model describes physical characteristics of articulatory movements based on variables that could be control parameters
  • Mass > mass of articulator
  • Damping and spring stiffness > antagonistic muscle activations, skin and tissue characteristics
  • No direct one-to-one relation
• Task dynamics - articulatory "gestures"


  1. Velum opening

  2. Opening of tongue body

  3. Lip closure

  4. Opening of vocal folds
• Coordinative structures/functional synergies


  Muscles and articulators form coordinative structures to realize higher-level articulatory goals
• Speech production & perception: Speech is fluent, continuous, each segment has unique vocal tract shape but no discrete jumps
• Coarticulation

  • The phenomenon that the specific properties of articulator movements are context dependent as articulatory movements overlap in time and interact with one another
  • Acoustically, this manifests itself as the realizations of consecutive speech segments affecting each other mutually
  • The effect is non-directional
  • Perseveratory or carry-over coarticulation: Influences of a segment on a following segment
  • Anticipatory coarticulation: Influences of an upcoming segment on a preceding segment
  • Coarticulation is not limited to adjacent segments and can occur across syllables
• Consequences of coarticulation: Allophonic variation due to phonetic contexts, articulatory and acoustic continuity between consecutive sounds
• Lips are optimized to move quickly and with percision
• Jaw position is more about speed and strength than fine position control
• Phonetic effects of the lips

  • Bilabial closure
  • Labiodental articulation
  • Lip rounding
  • Lip protusion
• Upper and lower lips are often treated as a single articulator - coordinative structure