Thank you, Andrés, that is good information. Fortunately in the meantime there was another 'yodel' thread posted which helped me to express what I'm interested in.
What happens when your voice 'breaks' or you make it break? Some people refer to that as 'head voice' but this seems to be wrong. Is it what is called 'falsetto' or is that still something different? Does anybody know what is happening there, in either case? My own hypothesis is that the vocal cords are made vibrating in a higher harmonic mode (I suppose it's called the first harmonic but I'm not sure). Is that correct? You know there's second possibility: One can also obtain the first harmonic by filtering the basic frequency away, provided the original sound contains enough harmonics.
These are very interesting questions to a singer as well as to a physicist and maybe also to a physician (and I'm at least kind of a physicist in my other life), and better understanding (also understanding each other better) might be very useful to all of them.
Please help them all.