If you want to try a HR start with

a 4" by 4" by 12" Plenum.

 46656 cubic inches = 1 cubic  meter


4" x 4" x 12"  = 192 cubic inches


192 / 46656 = 0.004115226 cubic meters

Plug the numbers into this calculator
http://www.vk2zay.net/calculators/helmholtz.php

 111.562 Hz or 6660 pulses per second

This is the fundamental freq.

Second order is 13,000 PPS is more like what it is seeing.

6660 from each rotor.

Paul Lamar

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