When working with fiberglass enclosures, I’m sure you may notice that the internal volume is not as easy to measure as your regular square enclosure. There are twists, turns, and drops throughout a glass case, so we can’t just pull out a tape measure and get started.

There is actually a pretty easy way to calculate the internal volume of a box, without using tape measures. Our weapon of choice in this will be packing peanuts. The type you see in the packages feels like home. Those little pieces of foam that help prevent the contents of packages from breaking or being damaged during transport. You’ll want to get a couple of bags of these little ones. Enough to fill the enclosure you plan to measure.

When the enclosure is finished and you want to check the airspace, go ahead and fill the entire box with these peanuts. All the way to the top. If necessary, cover the mounting holes of the speakers so that they do not fall out.

Now you want to get a cardboard box or make a small wooden box that has an internal volume of 6 cubic inches. That would be 6″ high x 6″ wide x 6″ deep. Those are the internal measurements, not counting the walls of the box. That way, if you could freeze the air inside the box and get it out, you would have a 6x6x6 inch cube of frozen air.

Once you have your measuring box, start removing the peanuts from the fiberglass box and placing them in the measuring box. Fill it to the top. Write or mark somewhere how many times the measuring box needs to be filled. Let’s say it takes 5 times, so your box’s airspace is 2 1/2 cubic feet. foot. Every time you fill it up, that’s 6 square inches of airspace.

So what happens if the box is too big? If a box is too big, it loses its tuning and can ultimately damage the subwoofer. On low power systems, this is not much of a threat. On your more expensive high-end systems that use a lot of power or need to fit exactly to the manufacturer’s specifications, then you may not have the extra space.

The best way to remedy this is to introduce something that displaces the extra air inside the box. For example, we will say that your box is 6 cu. in. of additional airspace. In this case, I would take some MDF and create a very small 6 X 6 X 6 box. It would then essentially fit inside your peanut measuring box. Then, you would find a spot inside the enclosure, glue it down, and screw it down.

If your box is made entirely of fiberglass, you can’t screw the small box to the box, so we have to use something else. I have used what I call “tuning bags”. These tuning bags are just small bags of sand. However, you must be careful when using sand in a box, the bag must be VERY well sealed so it doesn’t leak and send sand particles around the inside of your box. This can ruin subwoofers very quickly if sand seeps into the enclosure.

If you want you can fill it with dirt from your backyard, it doesn’t really matter what’s inside the bag as long as it’s similar to dirt, sand, flour, salt, sugar, something like that. Simply measure out the amount required to make up for the extra air inside the box, place it in your bag, seal it, and put it in the box. I usually use heavy duty garbage bags, cut a few squares and make a small sandbag. Then I tie the top off with a utility cord. I do this about 5 times, making 5 different layers of the bag material over the sand inside to make sure it doesn’t leak into the box. You can even seal the top of the bag material by melting it on itself and inching it over the tie. You still need to use some sort of tie down on the bag, this just helps ensure it doesn’t spill.

Regardless of how you end up creating these tuning bags, make sure whatever method you use to lock the sand inside is adequate and doesn’t leak out. Make sure there are no glass splinters protruding inside the box that could pierce the bag.

Eventually, if you make a lot of these boxes, you’ll be able to “watch” them and more or less know what the airspace is. This will also help when you build the boxes, you’ll be able to make boxes from scratch that are pretty much dead in the airspace you wanted. Just make sure at first that you’re aiming for too big, because small is much more difficult to remedy than big.