My basic answer is : It depends on how empty the head and how hard you hit them!!

re. The computer/engineer thingy - well I'm employed as a computer analyst/programmer and reckon I'm pretty competant!!! but music theory etc is all Greek to me! I have to draw a little ring-a-round any time I want to change a song's key!

re. Bad male/female singers we get a fair proportion of both! The only common denominator is their singular lack of tonal appreciation. As a singer only hears what they are singing thru the cranial bone structure I guess some peoples bones adjust the sound worse than others - it's got to be the only reason they think they sound ok:}}