thats the thing about actually saying its quieter any propeller rotating at a reasonable speed will make a sound as it travels through the air,and its the pitch of that sound ,that makes it seem to our ears as quieter or louder i think that one of the main contributing factors ,along with the bigger props rotating at lower speeds relative to the lift produced ,is the way they spin in relation to each other ,because they are at different heights this allows them to almost overlap each other, and also the front props dont spin over the body of the drone as in other models and even the rears only do so a little bit ,all these small things i think are helping to produce less tip vortices which in turn reduce the sound that the props make as they spin