The hard plastic flexible hose is used for spot coolers and air conditioning ducting. But most have ability to change their diameter only 10-15 percents.This is a type producted by ''KANAFLEX' whitch is available plus minus about 50 percent .The product name is 'Kanaflex hard plastic flexible duct 'NS'.
I tried to replace the 4"dia. dropdown hose with 5" type of this connected to the bandsaw and the planer. The left picture shows when increasing the diameter to 6" and the right decreasing to 4".
I runned the dust collector and measured over 700cfm at the 6" main duct. With the 4" flexible hose These were around 430cfm ,so much improved!
The retail price per meter is abot $24 here in Japan,that is cheaper than metal reducer and 5" flexible hose and also effective to reduce pressure loss by their long transition.