I have been leafing through English, French, German and Italian Techniques of Singing by Richard Miller and am reminded of the days when I poured over anatomical drawings and sonograms in the name of learning how to teach singing. I have long thought that this was all bogus, and now it occurs to me why this might be. No one bothers to connect it back to anything observable. OK, this is what all these machines are telling me. Now what out in reality does it relate to?
If I am discussing styles of singing in different voice schools, the bare minimum is to cite actual singers, singers available on commercial recordings, who sing that way. No one wants to name names.
A voice teacher's life consists of sitting at the piano with the student standing where the teacher can see them, listening to the sounds the student makes, watching what their body is doing, and instructing them how to change this to achieve a better result. What can I observe from my chair that is going to help me in this task? I'm not going to crunch them through machines to see how fast their vibrato is. I have to figure out all on my own what their problems are and how to fix them. No. These kinds of books serve to allow one academic to impress another academic and have little if anything to do with actual singing. I teach through my eyes and ears and ability to explain what I want. I need it to connect back to that before I will think it's useful.
Of course, when I was actually teaching, my students were usually totally off, and it was not at all difficult to think of things to change without getting too deep into the minutia of vocal technique. One sang into his nose. He was a professional singer, so how he got that far is anyone's guess. I tried to explain that the objective was to resonate in the mouth. This was amazingly hard to communicate, but eventually he got it.
Others simply knew nothing at all and needed to know about breathing and vowels and simple stuff like that. None were in advanced stages of preparing for a classical vocal career. That's the spot where the comments need to get extremely specific. "See. There at 1:22 John Doe is raising his larynx as he was taught in the French school." Without this the information is virtually useless.