Design Your Own Subwoofer Box - Part 1: Box Design

This video is about how to design your own subwoofer box. The first step is to choose the subwoofer driver that you’d like to use. I’ve chosen The Infinity 1252W 12-inch sub. It's a top seller right now. So once you pick the driver, you want to find a specsheet for that driver. You’ll find the Thiele/Small parameters and there are three numbers that you’ll need. The first is called fs and here it is free air resonance, fs, for this driver. It's 24.23 Hz and I’ll write that down. The second number that you need is called Qts, total Q. Qts for this driver 0.48. I’ll write that down also. The last number You need is Vas, compliance volume. On this specsheet it’s shown in cubic feet and liters. I will write down the cubic feet. I prefer to work in cubic feet but the software will handle either one, you just need to keep track of which you've chosen, so 3.48 cubic feet. The next step in doing the design is to use some speaker design software and I like to use the Subwoofer Design Toolbox for this. So the first thing we need to do is enter those three numbers into the software. So fs was 24.23 Qts 0.48 Vas in cubic feet, (we could have chosen liters, we chose cubic feet) is 3.48 cubic feet Now the first thing we're going to do is see how this driver acts in a sealed box, and the software lets you choose sealed, ported and other types of boxes too. We’re going to start with sealed and I’m just going to choose a half cubic foot, 0.5 cubic foot box and see what happens. So update the plot. Now if you're not used to looking at these frequency response plots they show you the bass response. Think of it as the loudness of the bass versus frequency, so here at the higher frequencies we’re near zero dB, but as you start to get lower and lower, there's a point where it just starts to fall off and so for this speaker we’re -3 dB at about 49.6 Hz. Up here it shows it's called data tracking you can see where this cursor is so it looks like about 49.6 Hz. Well, our goal is 40 Hz or maybe even better. So let’s see if we can do a little bit better. I'm going to put a data description in here. We’ll call this sealed box and it’s 0.5 cubic feet. So the next thing I want to do I’d like to keep all these curves up here I’ll just add a data set and the next one I want to show will also be a sealed box. But let's make the box one cubic foot. So I’ll put the box volume of one. I’ll put 1.0 up here. I’ll update the plot, so now this plot shows the red line is the sealed .5 cubic foot, the green line is the sealed one cubic foot. Well you can see what happens here, we got an improvement. Now we’re -3 dB At about 41 Hz compared to about 50 Hz before, so that's a good improvement. Let’s try making it even bigger, let's try a 2 cubic foot sealed box. I’ll add a new data set, we’ll call that sealed 2.0, make the box volume two cubic feet, update the plot. Well now, something interesting happens which is, it’s a little bit better than one cubic feet, but not much. And so we've kind of hit the point with this speaker where making the box bigger doesn't really do much for us and so when that happens it means it's time to look at a ported box. So what I'm going to do is I’ll leave the sealed .5 and sealed 1 up there, but I’m going to now play a little bit with some ported boxes. Let's have a ported box with a volume of two cubic feet. Now with a ported box there's one more number that you need to add. With a sealed box, the only number you need to worry about is the volume of the box. With a ported box there's this other thing here called port frequency. That has to do with how you tune your port. Well, the software recommends a value to use and were going to take its recommendation. I’ll just double click here, it’ll put that value in there, we will update the plot and see what happens. So the yellow line is showing us a properly tuned port in a two cubic foot box. Well now we’re 30 Hz at the -3 dB frequency and we were not able to get that out of a sealed box. Only by going to ported were we able to squeeze that extra 10 Hz out of the system. This shows one of the rules of box design, which is you always have a small box that’s sealed, and a large box that's ported. I’ll just fix up the description here, this should say ported. Let’s add one more data set. let's try a ported four cubic feet. Well you can see a similar thing that happened before, which is we're starting to run out of steam. Now here we have -3 dB at about 23 Hz, so yes it's an improvement but you can't just keep increasing the box size forever and keep seeing those improvements. The design process is you usually start out with a sealed box, increase it until you get your design goal of 40 Hz and then if you can't get there, switch to ported and once again keep increasing the box size and hopefully you'll be able to hit your 35 or 40 Hz goal. In later installations of this, I’ll talk about how to use the port design tool, the enclosure design tool and the woofer selection tool. If all you're doing is a sealed box, you don't need the port design tool. The woofer selection tool helps you choose woofers, we just knew we wanted that Infinity and didn't have to mess with that here. But if you're shopping for woofers, you will want to use that tool. And the enclosure design tool helps you build the wood cabinet, to make sure that you've got the volume that you need.