The idea that humans need the diverse micro ecology of earth in order to not become ill over the course of generations is pretty interesting.
Really pretty well-supported by current science, too. I teach chemistry at a community college, so maybe I’m an outlier, but I read a ton of current research about the importance of diversity in “gut biomes” and the damaging effects of monoculture on global ecology, etc.
It seems pretty clear that even if engineers could solve the physical and chemical issues with a generation ship, the limiting constraints are almost certainly going to be biological and ecological, and KS Robinson’s estimates for the upper limits seem pretty reasonable based on current knowledge
Profession can absolutely affect volume. Even without any hearing damage, any job that regularly requires that you project can become a habit.
I’m a chemistry professor at a community college, fairly well educated, and I flatter myself to say reasonably intelligent, but I still slip into what my wife calls my “teaching voice” in some social settings or even occasionally at home.