I recently changed my ducting as part of my silencing program for my grow cab.
I'm now using Phonic Trap and it is awesome.
No glass fiber particles which is especially important for the air intake.
I bought the 5" as it is the same size as my S&P TD Silent 350's and I also bought the 8" to double layer and fit over my exhaust silencer.
One layer is really good two layers is awesome.
It is like having a silencer and outperforms glass fiber acoustic ducting.
It' not cheap at around £10 a meter but it's an investment, especially if stealth is your goal.
2 x TD silent fans in homemade acoustic boxes double phonic trap acoustic ducting and a 5" flexible exhaust silencer also covered in phonic trap and you could hear a gnat fart.
I'd say it is 99% silent and you'd have to know what to listen for and even then it is difficult hear it or pin it down, super super whisper quiet!
anyone else use phonic trap ducting?