When working with fiberglass enclosures, I am sure you may notice that the internal volume is not as easy to measure as your regular square enclosure. There are twists, turns, and falls throughout a glass case, so we can’t just pull out a tape measure and get started.
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 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 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, cover the speaker mounting holes so that 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 inch cube of frozen 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 the measuring box needs to be filled. Let’s say you take 5 times, so the airspace in your box is 2½ cubic feet. ft.Each time you fill it in, that represents 6 square inches of airspace.
So what happens if the box is too big? If a cabinet is too big it loses its tuning and can ultimately damage the subwoofer. In 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 can’t have the 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 cu. in. 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 enclosure, glue it 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 just small bags of sand. 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 through the inside of your box. This can ruin subwoofers very quickly if they leak sand into the box.
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. Just measure the amount required to make up for the extra air inside the box, put it in your bag, seal it and put it in the box. I usually use sturdy garbage bags, cut some squares, and make a little sandbag. Then I tie the top with a utility wire. I do this about 5 times, forming 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 slowly working your way over the tie. You still need to use some kind of tie on the bag, this just helps to ensure it doesn’t spill.
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 that no glass splinters protrude inside the box that could pierce the bag.
Eventually, if you make a lot of these boxes, you will be able to “observe” them and know more or less what the airspace is. This will also help when building the boxes, you will be able to make boxes from scratch that are practically dead in the airspace you wanted. Just make sure at first that you aim to be too big, because small is much more difficult to remedy than large.
