The following post is a bit to get through, but I think will be worth the time to read to help you fully appreciate frequency limits:
However, to quickly address your question, yes 72 Hz will become 36864 Hz as we are doubling this number until we arrive at a frequency that is over the 20000 Hz.
Some assumptions I had to make. You have the octave harmonic type specified and when you say upper frequency limit of 20000 Hz, that it was actually the < 20000 that was used which sets the lower limit for the frequency limits.
It also turns out that if you have the harmonic type set to 8x, that this also works as 36864 Hz is the first frequency we get to over 20000 Hz when multiplying by 8 three times.
For more details, please check the link: