The Subwoofer DIY Page
Tri-Chamber Bandpass Systems
last updated: 06 June, 2018
The Subwoofer DIY Page

Tri-chamber bandpass systems are basically bandpass systems composed of three chambers and two or more drivers, with at least one of the chambers vented to the outside.  Tri-chamber bandpass enclosures are very popular in the car audio world, but much less so for home audio. 

There are many different types of tri-chamber bandpass systems.  This page looks at the design and construction of a 4th Order Tri-Chamber bandpass system, the most popular of these systems (probably because it's the easiest to build!).  Due to the popularity of this system in the car audio world, this page will cover the design and construction of a 4th order tri-chamber bandpass system for the car audio environment.

A tri-chamber bandpass system is nothing more than two individual bandpass systems bonded together so that either the sealed or the ported sections are shared.  In the case of the 4th order bandpass system, this sharing  can be done in one of the following configurations:


tribp1.gif (5426 bytes)

This is a less popular configuration, but actually the one I'd recommend, as each driver sees its own sealed enclosure.  The volume of each sealed sections is calculated as for a simple 4th order bandpass enclosure using one of the drivers.  The shared, vented section is the SUM of the vented sections calculated for a simple 4th order bandpass system using one driver.  Therefore, if your calculations are correct, the total net volume of the trichamber bandpass sub should be TWICE the total net volume for a 4th order bandpass system using one of the drivers. You can also use this system with the drivers facing into the sealed sections instead of the vented section, but you may have problems with the magnets not leaving enough space for the ports in the vented section.

A more popular design, similar to the above.  However the section in the middle is sealed, and the sections at the ends are vented.  The volume of the sealed section is the SUM of the sealed sections calculated for a simple 4th order bandpass system using one driver.  The volume of each vented section is calculated as for a simple 4th order bandpass enclosure using one driver. As for the previous type of trichamber bandpass sub, the total net volume of the trichamber bandpass sub should be TWICE that of the net volume required for a 4the order bandpass system using one of these drivers.

Choosing a Driver
When choosing an appropriate driver for your bandpass system, first set the design goals.   First of all, how low do you want to go?  For car audio use, setting 42 Hz as your lower cutoff point should produce very good results.  Go lower than this, and you may end up with boomy, or flabby bass.  Note that your upper cutoff frequency is also important - too low and you're going to experience a lot of problems integrating your sub with the the rest of your system.  The minimum upper cutoff point I recommend is 80 Hz.  Finally, there's the question of box size.  Too large, and there's no space left in the trunk!  Too small, and you're going to have a lot of difficulty porting the vented section of the enclosure to the correct frequency.  The T/S parameters of your chosen driver will affect all the above-mentioned issues. 

The following is an example that shows what you need to consider when designing and building your own tri-chamber bandpass enclosure.

Design Criteria
You wish to build an enclosure than sits behind the rear seat in your car. You want to use two 10 in. drivers, and you are prepared to give up at most 3 cu.ft. to your enclosure. You'd like the enclosure to fit snug between the two wheel wells, which are separated by 42 in.. You are going to be using 3/4 MDF or ply to build the enclosure.  The driver's T/S parameters are as follows: Vas:1.6 cu.ft., Fs: 35 Hz, Qts: 0.42.  You want the enclosure to have one vented section and two sealed sections.  You have measured the driver's displacement volume to be 0.10 cu.ft.

Design Process
A simple 4th order bandpass alignment for one of these drivers with a cutoff frequency of  42 Hz has the following design parameters: V(sealed) = 0.64 cu.ft., V(vented) = 0.55 cu.ft., and Fb = 65.5 Hz.  The total net volume is 1.19 cu.ft., which means that the total net volume of a trichamber bandpass box using this alignment will be 2.38 cu.ft.

The minimum depth and width for any section is set by the size of the driver, in this case 11 in. (the driver's diameter, plus 0.5" clearance). The required length of each sealed section therefore works out to (0.64+0.10)*1728/(11*11) =   10.5  in., and the required length of the vented section works out as follows (0.55*2*1728)/(11*11) =  15.75 in.  We're going to have to leave a little extra space for the port(s), so let's use 16.25 in for the length of the vented section. The length of the entire enclosure therefore works out to be 0.75 + 10.5 + 0.75 + 16.25 +0.75 +10.5 + 0.75 = 40.25 in..  To use these dimensions, we will need to use some extra spacers to ensure that the enclosure doesn't bounce around between the wheel wells, as you will end up with 1.75" of clearance.

The ported section of the enclosure needs to be tuned to 65.5 Hz.   As we're using 10 in. drivers, we will want to use nothing less than double 3" ports, or one 4" port.  We want to use the largest port possible.  Also, for a nice-looking box, we want the port to actually fit inside the box.  To tune a (0.55*2)=1.10 cu.ft. enclosure to 65.5 Hz, we can use a port 5 in. in diameter that's 7.56 in. long.  As the enclosure is 11 in. deep, this will leave enough space between the entrance to the port and the back wall of the enclosure (minimum clearance = approximately one port length).   Using two 4 in. ports is out of the question, as each port will have to be almost 11.5 in. long.  Using two 3 in. ports might work, but the surface area is less than on 5 in. port, and plus we now have TWO ports to cut instead of one.  So one 5 in. port remains as our best choice, followed by two 3 in. ports.

We now have enough information to design and build our tri-chamber bandpass enclosure.