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

There’s 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 guy you see on the packages feels right at home. Those little pieces of foam that help prevent the contents of the packages from being broken or damaged during transport. You will want to get a couple bags of these little ones. Enough to fill the enclosure you plan to measure.

When the enclosure is done 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, plug the speaker mounting holes so they won’t 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″ tall x 6″ wide x 6″ deep. Those are the inside measurements, not counting the walls of the box. That way, if you could freeze the air inside the box and get it out, you’d have a 6x6x6 inch cube of frozen air.

Once you have your measuring box, start scooping the peanuts out of the fiberglass box and placing them in the measuring box. Fill it up. Write or mark somewhere how many times the measuring box needs to be filled. Let’s say you drink 5 times, so the airspace in your box is 2 and ½ cubic feet. ft. Each 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 isn’t much of a threat. In your more expensive high-end systems that use a lot of power or need to be exactly matched to the manufacturer’s specifications, then you can’t have extra space.

The best way to remedy this is to introduce something that displaces the extra air inside the box. For example, we’ll say 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. So it would essentially fit inside your measuring box of peanuts. Then, I would find a spot within the enclosure, glue it down, and screw it down.

If your box is made entirely of fiberglass, you can’t bolt 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 do have to be careful when using litter in a box, the bag needs to be VERY tightly sealed so it doesn’t leak and send litter particles down the inside of your box. This can ruin subwoofers very quickly if sand leaks into the box.

If you want you can fill it with soil 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 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. I then tie the top with a utility wire. I do this about 5 times, layering 5 different layers of the bag material on top of 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 over the tie down. 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 pitch bags, make sure that whatever method you use to enclose the sand inside is suitable and won’t leak. Make sure that no glass splinters protrude inside the box and could puncture the bag.

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