Is poly fill really necessary for a lil sealed enclosure?
IF indeed it is necessary, is there a formula or rule for how much to use?
thanks!

 

It's only necessary if your enclosure is too small.

Rough equation is 1 pound/sq. ft. will make the enclosure seem ~25% larger.

 

This is from Jl Audio.

While it is always a pretty good idea to stay away from perfect cubes, they don't necessarily have to be avoided like The Plague. Due to the very small dimensions and generally low cutoff frequencies used in most mobile subwoofer systems, there is little chance of generating standing waves in the enclosure (standing waves can cause nasty response fluctuations). For a standing wave to exist, the distance between parallel boundaries must be 1/2 the wavelength of the frequency at which the standing wave exists. Considering that sub-bass waves vary from 56.4 feet (20 Hz) to 11.28 feet (100 Hz), the generation of a standing wave is going to be virtually impossible.

Any standing waves that might be generated by upper order harmonics (caused by distortion) in the enclosure can be readily absorbed with the addition of damping material such as polyfill (available at your local cloth store--it is used to stuff pillows and quilts) or they can be broken up with strategically placed bracing within the enclosure.