The aerodynamic voicing constraint

Transglottal pressure drop must exceed 2 cm H2o

  1.  long voiced stops tend to become voiceless
  2. Dorsal consonants are more likely to be voiceless. Bilabial consonants are more likely to be voiced
  3. Passive expansion of the vocal tract during voiced stops. Voicing lasts longer with bilabial stops-more vocal tract wall to expand.

Aerodynamic constraint for frication:

Trans-oral pressure drop must be large enough to produce a Reynold’s number above 1700

Intervocalic voiced stops have a greater tendency than voiceless ones to spirantize (become approximants).

Glides and high vowels have a greater tendency to devoice than comparable lower vowels

Epenthetic stops.  “Early” closure leads to unintended stops (ohala Speech timing as a tool in phonology)

Vibratory movement of vocal folds under egressive and ingressive airflow. Greater lateral movement under egressive flow.