Extractor /ducting noise and intake size

i have a 6" TT extractor its pulls at about 540m3/hr or something like that, what intake should i buy, i am looking to keep the negative pressure so needs to be less and im not sure what is sufficient, i have 5 cheese in a 1.2 x 1.2 x 1.8m tent

Noise
also the noise of my extractor is quite bad and im not sure wether its the air running through the ducting or what? if i buy acoustic ducting will this deminish the noise enough to not be annoying? i have my extractor inside an old carbon filter sleeve inside a box, compact abit, its also hanging from ceiling to avoid vibration, thanks for any help

i have a 6" rvk m8 . acoustic ducting would help i have a silencer on my set up and recently bought a variac speed control .as i grow in shed and noise was a problem so on a night when lights are out i turn fan down and can hardly here it running. no humming noise out of fan when turned down with the variac speed controler unlike the sms controler. i had and took back to shop. i paid arond £55 .for the variac. a lot of the noise is just the air in the ducting.