In low cost subwoofers, a vent is used to assist the bass out capability of the subwoofer. The vent is typically tuned to operate at the lowest frequency that the subwoofer is designed to operate at. At this frequency, the vent takes over the task of moving air to produce bass, and the driver itself does little of the work. The downside of this is that the air velocity can became very large, particularly with small diameter vents. At these high velocities the air becomes turbulent and you can hear this as “chuffing” noises. Although this can be helped by adding flares to both ends of the vent, it’s difficult to eliminate entirely. If the vent diameter is increased the airflow can be reduced, but the vent has to be made longer to keep the same tuning frequency. At some point, the vent becomes too large to fit into the box!
The solution is to replace the vent with a passive radiator. This is a driver unit of the same size or larger than the main woofer, but it has had its motor structure removed. It becomes just a passive diaphragm driven by the air inside the cabinet. The mass is chosen to match the equivalent of the mass of air that was in the vent, but because of it’s large size the movement is much less than the air it replaces. As a result, there is no “chuffing” and it behaves much more linearly.