When to use poly-fill
  
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Subwoofer manufacturers provide consumers with many specifications, one of which is enclosure volume. Sometimes they give a range, others, just a number. You'll need to compare this requirement to the enclosure you plan to use. So, how do you know if you will need poly-fill? We'll get to that in a moment.

Let's start with an explanation.  When you look up the volume on a subwoofer, what exactly are you looking up? This is the recommended box volume that the manufacturer says to use with the subwoofer. The company designed the subwoofer to produce optimum sound for the average consumer in this size of box. Is this the only volume that should ever be used for the sub? Will it hurt it to use a different volume? In many cases, no. This recommendation is generally not set in stone.

Now, say you have a box with a volume that is less than what your sub calls for, you can compensate for this lost volume by using a material called poly-fill. This is widely available. Other materials will work for this purpose as well. Certain types of pillow stuffing will work, as will fiberglass insulation. You can compensate for about 20% of the total volume with this method.

There are a few things to remember when using poly-fill. To get the maximum benefit from it, use about 1.5 pounds of stuffing for every cubic foot of airspace. Be sure to keep the fill away from the basket and pole piece vent of the subwoofer. One way to accomplish this would be to use a brace behind the subwoofer. There are other benefits to using poly-fill as well. It tends to smooth the frequency response of the subwoofer.

When using a vented enclosure, using the stuffing technique is not recommended. Though, you may choose to line the walls of the enclosure with a material. This may reduce unwanted noise, and help the higher frequency response of the subwoofer. If you do this, be sure to secure the material to the walls. A good way to prevent bits and pieces of the substance from flying out of the vent is to attach a mesh grille to the opening inside the box.

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