OMG! I love the science of it - two frequencies, is there a formula or some calculation where FreqA+FreqB=C and FreqA-FreqB=D and you hear C and D over the frequency of B? Or just wing it. I can't imagine the resonance chambers as being from a stock formula.
Regardless, a brilliant design and it is loud. I'm going to print a number of them so my wife and some of my female friends have a robust assult/rape whistle. And some of my ham radio friends have one in their go kits. Many thanks for going through 29 iterations, it is nothing short of excellent.