There are measurable changes to intensity, pitch accuracy, formant frequencies, and phonation as measured by electroglottograph
The twist, however, was that each singer was "wired" through a lightweight headset microphone and portable electroglottograph device (EGG).
The instrument I used to take these measures was a two channel electroglottograph.
Glottal closure patterns related to voice registration were assessed with Voce Vista Professional software from signals generated by an electroglottograph
(EGG) model EG2-PC, manufactured by Glottal Enterprises.
It outlines the science of human voice production in his theory, in terms of acoustic waves; vocal anatomy and physiology and measuring voice organs; the basic experimental facts of the human voice collected by microphones and voice signals, electroglottographs
and voice data, and in-vivo pressure measurements; and the physics of voice production, particularly that involved in vowels.