Brenner

The "beats" that people hear are a synchopating rhythm that is created when two sound sources are very close to each other in frequency. What's happening is that the sound waves from each source are adding and cancelling, but with slightly different frequencies. The resulting frequency is the sum both sources.

This resultant frequency slows down and stops as the frequencies from both sources become matched. It's much less apparent with the APC props because the front and rear props have mismatched diameters. You can also get rid it when flying carbon fiber props if you trim the rear prop so that it's about 12mm to 15mm smaller in diameter than the front prop. This reduces the load on the rear prop so the planetary gearset equalizes the power loading by spinning the rear prop faster than the front prop. This raises the frequency of the synchopating rhythm enough so that it's no longer noticeable.

Making the rear prop smaller than the front prop helps in another regard as well. It keeps the tips of the rear prop away from the vortices that are shed from the tips of the front prop, which also reduces sound.

Then in addition, rounding the tips so that the round is oriented in the direction of rotation reduces the size of the vortices that are shed from the tips, which reduces sound even more.