This is HN, so I think I can be pedantic without reservation here.
In Rober's video the drone is not any less loud, it's just that the power in each frequency is smoothed out. In that, the total power is the same, just now more evenly distributed. We humans then perceive this as more quiet. The drone is essentially more akin to a white noise machine than a machine that buzzes in certain frequency bands.
There is also a short posted below that which shows the drone in action. Seriously watch it, it's 10 seconds long and you can hear how quiet it is with no other sounds around.
In Rober's video the drone is not any less loud, it's just that the power in each frequency is smoothed out. In that, the total power is the same, just now more evenly distributed. We humans then perceive this as more quiet. The drone is essentially more akin to a white noise machine than a machine that buzzes in certain frequency bands.
For the curious, Tom Scott talks about the oddities of human hearing here : https://www.youtube.com/watch?v=Is_wu0VRIqQ
TLDR: Human hearing is more 'sensitive' to certain frequencies, perceiving them as louder despite being the same power.