Hi, I have read up on Helmholtz resonators as I too have a bad drone at cruising rpm 188-2100 ish! I have seen the pictures of what you have made and I’m going to give it a go on my 360 duster! Could I just ask you what size tubing you used? My pipes are 2.5” so I was thinking 2” for the resonator? Calculating show I need to make total length in the region of 25”.