A 6th order bandpass box has two chambers with the driver mounted in the dividing partition between the chambers. Each chamber must be "tuned" to a specific resonant frequency. This is done by a combination of setting the chamber volume and the length and area of a tuned port that protrudes into the chamber. SoP uses one rearward firing port and two smaller downward firing ports. For Beastbox, I changed the arrangement somewhat so that there are two rearward firing ports which have the same area so that the back end of the box looks symmetrical. Also I figured that firing the sound out horizontally would be better than at the ground. The ports can be any shape but if you are building them out of wood a rectangular shape is most convenient. If you want to use circular ports you can buy ready made port tubes (search eBay) which you can cut to the required length. I couldn't find any big enough for Beastbox so I went for rectangular.
Before starting on the box design you need to choose a bass driver because the chamber volumes have to be tuned to the driver you intend to use. I used a Fane Sovereign 15-400
15 inch 400W driver. The 400W is probably a bit of overkill but I could get a more or less unused one cheaply. A better choice would have been a driver that uses a Neodymium magnet like on SoP because these are a lot lighter but they are harder to find in large sizes.
To design the 6th order bandpass box for Beastbox I used a program called WinISD
. I don't think this has been updated for a while and I couldn't get it to run under Windows 7 but it works fine in XP mode. If your driver doesn't already exist in WinISD's database you have to define the driver by inputting its Thiele-Small parameters. These will always be available on the manufacturer's website for any serious bass driver. Once you have the driver defined it's basically a process of trial and error adjusting the box and port dimensions to get the chamber volumes correct while keeping the overall size within the range that will fit on your chosen trailer.
One thing to bear in mind is that the chamber volumes are net
volumes - i.e. you have to subtract the volume of the batteries, amplifiers and anything else that takes up space inside the chambers (including the driver and
the volume of air enclosed by the ports). I found that a simple spreadsheet was helpful to keep track of the weights and volumes of the components and also to work out the chamber volumes for given external dimensions and port areas and lengths. The front chamber is 70 litres and is tuned to 60Hz and the rear chamber is 35 litres and tuned to 130Hz. The design gives a nearly 6dB boost at 77 Hz and over 8db at 150Hz.
Once you have the volumes and dimensions for the box and ports you can roughly work out where all the other components are going to fit. I used Google Sketchup to design the box and it was very helpful. The Sketchup files are attached.
The box is mostly constructed from 12mm birch plywood screwed and glued together. The port tubes are made from 6mm birch plywood. MDF would be another choice but I think it is more dense than plywood so it would make for a heavier box. Whatever you use, the main points to note are that the box must be rigid and there mustn't be any gaps. Rigidity is important because you want the panel resonances to be much higher than the bass frequencies that the box is producing. Basically if you rap on the side of the box it wants to sound "tight" rather than boomy. Gap-free construction is important because the bass driver generates large pressure fluctuations inside the box and if there are holes you will get whistling noises!
I cannot over emphasise how much it helps if you can get the panels accurately cut to size. I have a friend in a woodworking shop who cut mine for me and it makes assembling them much more straightforward if they are all the correct size with clean, square edges. I used 3.5 x 30 wood screws (No 6 x 1 1/4" for any Imperial fans out there) at 100mm intervals along each joint. Pre-drill pilot holes for the screws so that you don't split the panels. Make sure you use plenty of wood glue (PVA) on the joint, you can always wipe off the excess with a damp cloth. A couple of long clamps are very useful to hold the panels in place while drilling the edges and waiting for the glue to dry.
The front chamber of the enclosure has a removable cover which is held in place by the midrange/tweeter mount and some tie downs. This allows easy access to fiddle with the electronics and remove the batteries. The cover is sealed on to the chamber by a strip of neoprene draught excluder tape which runs all around the edge.