Very likely to be one of the most advanced airfilters on the market. Doppler has designed a system utilising the so-called venturi effect. What does that mean? The idea is that by reducing the effective flow-area in precisely defined places, one reduces the suction needed to start an airflow. Just think of different straws, one with too small a flow area (diameter of say half a millimeter) you will find it difficult to suck your drink through, with a straw of a diameter of 5 cm again you will notice that suction poses problems. Flow areas too small as well as those too large require loads of suction forces, and that's something our 70cc engines can't really supply by the truckload. Doppler has set out to solve this problem, their filter comes with a second insert to adjust it to the use with any carb between 12mm and 21. And though it has not been mentioned yet: the venturi design also helps to keep the noise down as much as possible.