It does a similar sort of thing - asymmetric offset propellors shunt the frequency of what gets emitted way down. It still emits a similar amount of energy, but

It does a similar sort of thing - asymmetric offset propellors shunt the frequency of what gets emitted way down. It still emits a similar amount of energy, but the direction of emission changes, and with the lower frequency, there's a lower perceived sound. Toroidal props actually have less sound coming off them, and at a lower frequency. I've got some ideas about how you'd potentially then have a greater spread of frequency by creating an offset toroidal, but I suspect the efficiency goes down, and I've got better things to do with my time than model sound profiles of weird, hard to manufacture (other than printing) prop geometries. Related note - the offset bit is similar to how car tyres work by having patterns in the tyre which spread the frequency response generated by each, so they don't have a multiplicative effect. (I can get really boring on applications of acoustic design but I'll cut off there)