Ported box lining

This quote I pulled up from a Polk forum member Jstas sums it up beautifully:

"However, in a ported or band-pass enclosure, even if the box is built to perfect specs, polyfil can be a great help. It helps cancel the backwave which can lead to port resonances. That makes the port whistle and vibrate which colors to music. It can also slow down the air movement in the cabinet which can reduce port velocity which also hels to stop port noise like whistling. Lower port velocity also reduces turbulence at the mouth of the port. If the velocity is too high and there is too much turbulence, you develop a high pressure area around the mouth of the port. What this does is two things. First off, it can have the same effect as changing the length of your port tube which changes the tuning frequency of your port. The other thing it does is kind of complicated. Since the high pressure air is there and making the tube seem longer, it also moves with the wave of the air coming out of the port. The port doesn't move because it is a solid, not a gas. As that gas oscilates, it will pinch that high pressure area around the mouth of the port. Just like if you were to squeeze teh mouth of a deflating balloon. This causes a low frequency chattering that sounds like distortion. The biggest problem is that the low frequency chattering creates it's own backwave the travels back down the port to mess around in the cabinet. Not good and it can cause uneven pressure behind the driver cone which can lead to cone break up and more distortion.

However, because of the dampening ability of the polyfil, it will smooth out the response of the driver in a ported cabinet. If the box is too small, it will also help the box behave like a larger box. It also smooths out the response in a sealed box. A driver that is in a box that is too small will have a very peaky frequency response. The dampening effect of the polyfill slows down the air flow so that the pressure waves slow down too and the air pocket behind the driver doesn't pressurize as quickly. It allows the driver to move further in its moved before the pressure wave inside the cabinet tries to push the driver cone back out. That pressure wave prematurly stopping the movement of the cone and pushing it in the opposit direction is what truncates the frequency response."