Fitting a pipe does decrease volume.
Past week I have been driving with a std. Airbox with disconnected CAF (accoustic valve) and the sound/noise was pretty insane. Incar sound on the highway was too much for me when accelerating at 100km/h and upwards.
Last weekend I removed the CAF (+accoustic valve) and fitted a 75mm CAF directly to the airbox, this silenced it quite good. @30mph it sounds good when accelerating and @60mph is just ok incar.
Directed the CAF to the perforated part infront of the passenger side front wheel, below the bumper.