Donald Gray Miller audio-vocal theories are very interesting, I have got and use the same about resonances and vocal analysis.
He has elaborated a good book with a lot of clear explanations about singing resonances, Resonance in Singing, and an audio software for formant analysis and electroglottograph analysis in real time, Voce Vista. These remember me my method Tertium Auris, (The Third Ear).
I don’t use the EGG (electroglottography) for my vocal analisys, but it can be an useful tool for analyzing the vibratory characteristics of the vocal folds.
Voce Vista, with the permission of its owner, Ph.D. Donald Gray Miller:
VoceVista-Pro is a software (plus optional hardware) system designed primarily for analysis of the singing voice and providing feedback in singing instruction. It utilizes two non-invasive signals, microphone and electroglottograph (miniaturized EGG), allowing singers to examine their voice production unimpeded in any location where they can take a laptop computer. The two signals generate a variety of convenient displays. The two basic display types both incorporate a spectrogram with a cursor that can be placed at any point in its duration for detailed analysis of the moment. In the one type a second display shows the power spectrum at the selected points (here at F4 and G4 in the [duplicated] tenor octave scale to A4-flat):
In the other type the second display shows the EGG and microphone waveforms, with a delay correction for precise alignment of the two signals in time. Here the selected points are C4 and D4-flat on a scale to A4-flat by a mezzo):
The typical work of VoceVista is done when the user (usually a singing teacher or singer) can interpret the signals and connect them with the traditional goals of voice building: extending the useful range, joining the registers, improving vocal quality and “carrying power,” equalizing the vowels, optimizing vibrato and legato. The two examples display consecutive notes in scales that illustrate two types of register transitions. The first, a transition in resonance, shows a tenor going from F4, the last note in “chest,” to G4, the first note in the upper extension. The dominant resonance shifts from the first formant on the second harmonic to the second formant on the third harmonic. The second example, a transition in the pattern of vibration of the vocal folds, shows a mezzo going from C4 in “chest” to D4-flat in “head,” where the difference is evident in the EGG signal as a reduction in the closed quotient (CQ) of the vocal folds. Using the signals to understand and control these moves, along with many others, is the subject of the book Resonance in Singing: Voice Building through Acoustic Feedback, by D.G. Miller, which is available, together with the software program, at the website www.voiceinsideview.com. See also http://www.vocevista.com.