Subwoofers are, by design, huge air pumps. For any speaker to make noise they need to vibrate. Higher frequencies vibrate faster while lower frequencies are slower. When you are dealing with Subwoofers you can often see the woofer moving and the move the cone moves, the more powerful the bass and more air it moves.
To aid in the sound reproduction these speakers are mounted in resonance boxes to contain and direct the sound waves and to further extend the frequency response you can add ports to the box and tweak how fast the speaker can move. Changing the diameter and length of the power can change the frequency and tune the driver to respond a certain way.
One of the drawbacks to ported speaker enclosures is the noise associated when speakers are operating outside their normal range and pushing too much air through the port. We call this “chuffing” and is when the velocity (or speed) of air passing through the port is above a certain threshold.
To solve this, you must increase the diameter of the port. This slows down the air velocity but, also requires that the port be longer to provide the same physical resistance to the driver.
Of course, now that the technical are out of the way, here is the problem. You can get port material from your local hardware store. I prefer ABS plastic pipes and they come in standard sizes. For instance 2” ID, 3” ID etc.. For the Double Sub Project a 2-inch port was too small and causing a massive amount of “chuff”. Moving up to a 3-inch port made the length of the port too long for the box so, I compromised by 3D Printing a custom diameter and matching that up with a custom flange with integrated flare.
The results were amazing.