When working with fiberglass enclosures, I am sure you can find that internal volume is not as easy to measure as your normal square enclosure. There are twists, turns, and falls throughout a glass box, so we can’t just pull out a tape measure and go for it.

Actually, there is a fairly 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 kind you see on packages that are sent to your house. 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 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 speaker mounting holes so they do not fall off.

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 “h X 6” w X 6 “d. Those are the internal measurements, not counting the walls of the box. That way, if you could freeze the air inside the box and pull it out, you’d have a 6x6x6 cube of a inch of icy air.

Once you have your measuring box, start taking the peanuts out of the fiberglass box and placing them in the measuring box. Fill it to the top. Write or mark somewhere how many times it takes to fill the measuring box. Let’s say you take 5 times, so the airspace in your box is 2 and ½ cubic feet. ft.Each time you fill it in, that represents 6 square inches of airspace.

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

The best way to remedy this is to insert something that displaces the extra air inside the box. For example, we will say that your box is 6 cubic feet. on. of additional airspace. In this case, you would take some MDF and create a very small 6 X 6 X 6 box. So, in essence, it would fit inside your peanut measuring box. Then I would find a place inside the cabinet, glue and screw it in.

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 simple sandbags. However, you need to be careful when using sand in a box, the bag needs to be VERY well sealed so it doesn’t leak and send sand particles flying through the inside of your box. This can ruin subwoofers very quickly if they lose sand in the box.

You can fill it with dirt from your backyard if you want, 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. Just measure the amount needed to make up for the extra air inside the box, put it in its bag, close it and put it in the box. I usually use sturdy garbage bags, cut a few squares, and make a small bag of sand. Then I tie the top with a service wire. 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 about an inch above the tie. You still need to use some sort of tie-down on the bag, this just helps to further ensure that it doesn’t leak.

Regardless of how you end up creating these tuning bags, make sure that whatever method you use to enclose the sand inside is adequate and doesn’t leak. Make sure the inside of the box does not contain glass splinters that could puncture the bag.

Eventually, if you make a lot of these boxes, you will be able to “observe” them and know practically what airspace is. This will also help when building the boxes, you can make boxes from scratch that are practically dead in the airspace you wanted. Just make sure at first you aim to be too big, because small is much more difficult to remedy than large.

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