The closest speaking space was suggested by:
The space between the teeth during casual repetition of the sound “s”. It is considered the closest relationship of the occlusal surfaces and incisal edges of the mandibular teeth to the maxillary teeth during function and rapid speech.
This phonetic method is one of the several techniques to determine vertical dimension of occlusion in dentate and edentate patients. This method was proposed by Silverman.
Pound
McGrane
Neswonger
Silverman