The vocal tract is bounded by hard and soft tissue structures. These
structures are either essentially immobile (such as the hard palate and
teeth), or are movable. The movable structures associated with
speech production are also referred to as articulators. The tongue, lips,
jaw, and velum are the primary articulators; movement of these articulators
appears to account for most of the variation in vocal tract shape associated
with speaking. However, additional structures are capable of motion as well.
For instance, the glottis can be moved up or down to shorten or lengthen
the vocal tract. A downward motion of the glottis while the oral and nasal
cavities are occluded can produce the negative pressure in the vocal tract
needed for the implosive sounds of some languages.