In the first part of “SUBWOOFER ENCLOSURE DESIGN BASICS”, I covered the first few steps on how to manually calculate enclosure volume. I shared the industry standard formula that everyone uses and I also explained how to use it correctly. I spoke about internal enclosure volumes and how you can quickly calculate these values coupled to a few other calculations in order to get to your final net volume which is the most important value needed in order to accurately work out a proper enclosure spec for your drivers.
In this article, I’ll be showing you how to design your own enclosure using these calculations and a Nanograph. As a side note, I personally recommend you invest in a good enclosure design programme and learn how to use it correctly. This is merely a guide to help those getting started with enclosure design.
Getting started – For too long, people have built enclosures blindly. They’ve taken a box, added a hole and then they drop in a standard size and length vent. This programme will allow you to better match the correct diameter and length of port according to your enclosure volume and preferred tuning point which removes the guess work and shooting in the dark from enclosure builds.
To use this chart properly, you need to have an idea of what size port you’d like to use and then work out your net enclosure volume. Once you have that value, and say it happens to be 2 cubic feet, you make a dot one the first line where it says 2 cu.ft. The next line is your tuning point. Should you opt to tune your system to say 33Hz, you’d then make a dot at 33Hz in the next line.
Using a ruler, you’ll then need to draw a line from the first dot pass the second dot on the second line and carry on until your line meets the third vertical line. You’d then make a dot there as well. The next step is to move your ruler horizontally and then draw a line from the dot on the third row all the way to right of the page and you’re done. The final step is to read the graph.
Now you’ll notice a number of different lines intersecting your horizontal line, all of which indicates a specific vent (port) length. So say you wanted to use a single 4-inch port, you’d look at the point at which the 4-inch port intersects your line and at that point, you’d just move all the way down and the marking at the bottom will give you the required port length.
What you’ll notice is that the diagram only gives you one a size for a single port. Should you want to use two ports, divide your net area by however many woofers you have and then do this exercise. That will give you the port size and length for each woofer.
Should you use to use a slot port and not a round port, you’ll need to convert the area of the round port to the area of your slot port however do not do a like for like conversation as slot ports have sharp 90 degree corners which hampers airflow so a little trick to overcome this would be to make the port slightly larger than needed, but very little.
Another tip I’d like to share with you which I’ve made a rule of thumb with myself is to avoid having a port that’s shorter than the woofer length inside the box. Rather use a smaller port that extends pass the woofer inside the box or else you’ll general pick up horrible port noise which is another thing a programme like this won’t pick up before you build your enclosure.