Read Scientific Research on Voice Science for Free!
The first and most important step: grab a cup of coffee.
Google scholar is my go-to.
If I find an interesting study I plug it into google and see if I can find a full version for free. "Study name PDF" usually works well.
I look at the studies which have cited a relevant study. I also look at the references the study lists to find more studies!
If I find a study which has different terminology from other studies, I plug that terminology into google scholar. "singing registers" and "laryngeal mechanism" provide two very different search results.
If I plug a study into google and find an article sourcing the study, I read that article and look it it's citations.
How to read a study quickly:
1. Read the abstract first; see if it's even relevant.
2. Read the introduction because it usually has sourced information you can't really find other places.
3. Read how they conduct the study; you can ignore a lot of the fancy words unless you really want to get into it. Take note of any flaws in the research like subjective opinions of registration and stuff like that.
4. Skip to the conclusion. Read the entire conclusion. If you have any specific questions you can refer to the data itself.. but a lot of times the data is just science-mathy talk that isn't easy to understand. The conclusion is the meat.
Rinse and repeat. Google scholar likes very specific search terms like "laryngeal mechanism fundamental frequency." Typing in "singing register and pitch" usually doesn't work very well.
Warning.. this process takes many hours. You might come across 15 inconclusive, subjective studies which ~seem~ accurate.. but aren't quite definitive. Sometimes you need to speculate, sometimes you need to see if an expert has an opinion to base your ideas on, and sometimes you just have to throw out the study and start from scratch.
Be non-specific in your search terms "how does an opera singer sing high notes?" will not work. "sing pitch register" is far more likely to yield a result.
Use different search terms. "Sciency" words tend to work well. If "sing pitch register" isn't working try "voice larynx register" or try "vocal fold register" or try "voice laryngeal mechanism." Try everything.
Visit the website or read articles written by voice scientists or educated people referencing studies. Doctoral work can be helpful to find studies as well.
Other research search engines:
Journal of Voice