Port noise can be heard when the air rushing is out of the box due to high amounts of bass production not being transferred smoothly out. This can cause sound such as air or a chuffing noise to be heard when the subwoofer produces low end bass around 30 HZ. If you are getting some crazy port noise with your enclosure, there are a couple of ways that can help reduce it slightly. Most car audio enthusiasts find that port noise gets louder when you go below the tuning of the actual port. If you have done the following and you still notice loud port noise, you will have to make a new box that has larger internal volume and a better structured port design.
1) Increase the length of the port. Sometimes what can happen is the port itself can be too short which creates the air to not effectively be forced out of the diameter If your box design is a slot vented box, you can add a piece of mdf to extend the current port length. Remember to use a free subwoofer box calculator software such as torres to help you work out what frequency the port will be tuned to when the length has been increased. If your box uses a aero PVC port design, you can simply remove the current one and replace it for a longer port.
2) Add polyfill into the box. This can trick the subwoofer into thinking it is placed into a bigger box. Sometimes internal volume can become too little for the port to work with. Stuffing this white fluffy stuffing into the box will help fix port noise. You can buy it cheaply from large DIY shops or extract it from an old pillow. Experiment with different amounts to see what kind of results you can gain to stop the chuffing noise.
3) Flare the ports. Smoothing out the edges of the port towards a more curved shape will help the air rush out more smoothly. You can flare an aeroport by placing it on a metal bowel, using a heat gun and twisting it until the plastic at the bottom transfers into a flared shape. For slot ported boxes, you could add some 90 degree angles in all of the four corners in the boxes to smooth out the air flow.
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