Day in and day out, I see people asking questions about subwoofer enclosure designs and for good reason too. It is so important to have a properly spec’d enclosure designed and built as these will yield the best possible results with any speaker. The trick to it is getting every last detail of the enclosure design process 100% spot on in order to yield maximum results from your bass system and while the calculation steps are simple and straight forward, where the biggest confusion comes in is understanding what parts of the box it is that we need to be working with so I’ve put together this little article to help you better understand the basics of enclosure calculation.
Understanding Enclosure volume.
Before we dig knee deep into enclosure design, there’s some things I need you to understand first as this will make all the difference when designing an enclosure. The first order of business relates to the difference between gross and net volume but before we get to that I’d. While this may sound simple, believe it or not but there are tons of guys who get this wrong and I don’t entirely blame them as it’s easy to misunderstand the difference.
Gross external volume also known as total volume, is the entire volume of the enclosure including the port volume, the volume the subwoofer consumes when it enters the enclosure and also the material thickness.
Gross internal volume is the overall volume of the inside of the enclosure. This is the amount of area the raw enclosure has without the port or the subwoofer being installed. A common mistake people tend to make is stop their enclosure calculations at this point when this is actually only the first part of your calculation process.
As much as people do use this method of calculating enclosure volume the reality of it is that it’s completely useless information.
Net Volume is what you actually need to know and this is where tons of confusion comes into play. Net Volume is the total internal volume of the enclosure that is left after you take away the volume which the port and subwoofer would consume once they are placed into the enclosure. To help you better understand this try and picture this. When installing a subwoofer into the enclosure, the only thing that sits about the hole you’ve cut is the surround and mounting ring. The basket and motor both sit inside the enclosure and if the enclosure was filled with water, a large amount of the water would have to be removed from the enclosure in order for there to be enough room for the woofer to fit. This is called the subwoofers displacement volume and because it consumes volume, that volume needs to be taken away from the gross internal volume of the enclosure. The same applies to the tuning vent (port). If you were to close the bottom end of the port (irrespective if it’s a slot port, round port or even a triangular port), you would need to remove water from the enclosure in order to get it to fit. In case you were of the opinion that the vent (port) volume doesn’t take up space because it’s essentially a hole, just remember that the area inside the port belongs to the port and not the enclosure.
Should you be like me who likes using braces to strengthen the enclosure on the inside, the volume of these braces would also need to be subtracted from the enclosure volume as would anything else you place inside it.
There is a simple calculation which most people use as industry standard. The formula requires you to convert all your sizes from millimetres to inches and in case you didn’t know, 25.4mm = 1-inch. So if you have an enclosure with the following volume 900 (W) x 500(H) x 350 (D), you need to divide each value by 25.4 which would equate to 35.43(inch) x 19.68 (inch) x 13.77 (inch) = 9601.30. You then divide your answer by 1728 which is a standard number that never changes (In case you forget what this number is, just remember 12 cubed or 12x12x12= 1728) in order to find out how many cubic feet you have available. You’ll then get an answer of 5.55 which is in cubic feet which is in fact almost large enough to house a pair of 15-inch woofers tuned to around 45Hz….or is it?
This is where majority of folk goes wrong as this is only the first of many steps required to know your actual enclosure volume. The answer we have there is in fact your Gross enclosure volume. What you need to work out is your gross internal volume which you’ll see gets cut down pretty quickly after the next calculation.
The one way of calculating the internal volume would be to take measurements of the inside of the box and then using the formula above, you will come out to the answer you’re looking for. A quick way of doing this would be as follows. Say you’re using a 18mm shutter ply (which is most commonly used in competition systems because of its excellent strength), you’d then multiply 18mm x 2 as you’ll have an 18mm piece on each side. That gives you 36mm which you’ll then subtract from all three measurements. That will leave you with the following internal sizes, 864 (W) x 464(H) x 314 (D) after which you’ll convert to inches and then do the calculation above which will result in a final volume of 34.01 x 18.26 x 12.36 = 7675.83/1728 = 4.44 cubic feet. That’s 1.1 cubic feet lost to material thickness alone.
Following these calculations, we still need to subtract the subwoofer displacement then further subtract the port volume and then we’re still not done yet.
Now that you know how to calculate internal and external volume, calculating the area consumed by a slot port should be very easy as the principal is exactly the same. Convert all your sizes to inches and then follow the steps above using the outside measurements of the port. Whatever answer you come out with, subtract it from the gross internal volume and you’ll be another step closer to your final answer. Should you be working with round ports, you’ll have to use the Pie r- square formula to calculate it’s volume and then subtract that from your internal enclosure volume.
The final part of your calculation would be the subwoofer displacement. Subwoofer displacement is always provided by the manufacturer of whatever speaker it is you’re using. Should they not provide you with this, it becomes a little challenging to calculate
Once you’ve worked though all of the above, you’ll quickly understand how an enclosure that started off large enough to house a pair of 15-inch subwoofers suddenly reduced to something that is just about enough to house a single 15-inch woofer.