to polyfill or not to polyfill

[cotjones 10+ year member]

what exactly are all the effects of polyfill in a sealed box? also what is the BEST material to use?

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[SethPhillips 5,000+ posts]

polly fill makes the driver think that its in a bigger box. the polly fill slow's down the sound wave...ive never sceen it in a ported box.. and really havent noticed a diffrence in sound.??

 

[TeoTorriatte 10+ year member]

Polyfill is generally used in sealed enclosures to make the woofer act as if it were in a larger enclosure (see #2 below) -- its effectiveness is greatly dependent upon how undersized the box is (see #3 below). Some will say to use it to prevent standing waves, but this only applies to home audio where woofers need to produce higher frequencies (see #1 below).

My generic polyfill post:

1) Standing waves will not be an issue in car sub enclosures unless the dimensions of the enclosure are very large... like in excess of 7 ft with frq's below 80hz. 

speed of sound / frequency = wavelength -- standing wave possible at 1/2 wavelength

1130 ft/sec / 80 = ~14 ft wavelength

 

2) Fill material does not actually slow down the wave, but partially changes the thermodynamic state of the system from adiabatic to isothermal, thus giving the effect of a larger enclosure. RC describes it better than I ever could hope to in this old Carsound thread... http://www.audiogroupforum.com/csfo...631&postcount=6

 

3) From my own experience (measuring and listening), adding polyfill to sealed enclosures does make a measurable difference, but not always an audible difference. I found that the higher the unstuffed Qtc, the more difference the polyfill made. If your unstuffed Qtc is already at ~.8, you aren't going to notice much of a difference by adding polyfill.

 

4) There is no replacement for displacement... this applies with box volume as well. Don't expect polyfill to be an adequate replacement for real airspace... if you have the space, use it.

 

With all that said, I always throw ~.5 lb/cu ft in my sealed enclosures just out of habit... it surely doesn't hurt anything to have it in there. If you are dealing with a grossly undersized cabinet, going up to 1 lb/cu ft would be a good idea.

 

[newusername 10+ year member]

http://www.soundsolutionsaudio.com/forum/index.php?showtopic=7168

 

[Curel 10+ year member]

I always add some loose polyfill to all my enclosures. Always seems to help. On the other hand I've had bad experiences with too much polyfill.

 

[phantom240 5,000+ posts]

We added polyfill to my friend's $90 sub system. $40 prefab ported box at the shop where I work, with a $45 street edge 10. At first, the lows sucked a fat one, but got moderately loud at certain ranges. After a pound of polyfill, the lows were DEFINATELY more present, and got even louder in the other ranges. All in all, a superb sub for $95 and a pound of polyfill.

 

[BJ The Man 10+ year member]

well actually i just finished putting polyfill in my sealed box. I started with a little and kept adding more till i was out of it. It does make it louder and the lower tones are more present but its not too significant

 

[mrogowski 5,000+ posts]

Punk nailed it. Not much more to say there...

For fill types, I have always used fibreglass, as it usually gives a decent density and is consistant. A bit messy but if you're only doing it once, who cares?

BTW, it also works for bass reflex as well... //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

Best,

Mark