Where noise transfer must be cut
 
Acoustic Insulated Flexible Ducting Ø 10cm: acoustic foil duct is perforated along its entire length with micro perforations to enhance acoustic performance. 
Special types of flex like insulated and acoustically lined can be used wherever the loss of heat needs to be minimised, or where noise transfer must be cut.
Tip: Insulated foil flex is compressed into the packaging; it's inadvisable to open it until you are ready to use it.