The Subwoofer DIY Page
4th Order Bandpass Systems: Design Notes
last updated: 09 August 2015
The Subwoofer DIY Page

Choosing an alignment
Be careful when choosing a 4th order bandpass alignment for your driver. An ultra-large box with an ultra-low cutoff frequency may not produce the best results, because of the reduced power handling and lower efficiency. Examine carefully the cutoff frequency and box size and tradeoff in efficiency to determine whether or not this alignment lives up to your expectations.

Driver orientation
For bandpass systems, you may get better results if you orient the driver so that its magnet is in the vented section. Also, locate the port so that the speaker is not visible through the port. Both of these techniques will help to reduce the out of band noise that can be a problem with bandpass systems.

The total volume of the vented section will now be given by

Vf' = Vf+Vs+Vp+Vm,

where

Vf = net vented volume given by calculations,
Vs = volume displaced by driver,
Vp = volume displaced by the port
Vm = volume displaced by any bracing and any other miscellaneous items used within this section.

High power applications
If you're designing your system for high-power applications, use a slightly lower tuning frequency. This should produce better results at higher volumes.

Car Audio applications
If you're designing a system for car-audio use, remember that the interior of the car is going to boost the bass by about 12dB/octave below 60~80 Hz. A 4th order bandpass box that sounds flat in open air may sound boomy and flabby in a vehicle. Try aiming for cutoff frequencies in the 40-50 Hz region..

Where should I put the ports?
Ports should be placed at least one diameter away from any adjacent walls. If this is not possible to do this, the tuning frequency for a given port length will be lower than that predicted by the equations, and this may adversely affect the results.

Stuffing the sealed section
If you stuff the sealed section of the enclosure, you can reduce the volume requirement. If using 100% fiberglass stuffing, you can reduce the volume requirement by about 25%, e.g. if the equations predict a 1 cu.ft. sealed volume is required, you can use a 0.75 cu.ft. sealed volume that's 100% stuffed.

Lining the vented section
One layer of lining on every wall for the vented section will generally give better results. Ensure that no lining obstructs the ports. Fiberglass will work here, but make sure that none's located near the port entrance, as air turbulence can rip chuinks of it off the walls and eject it through the port. Any change in the tuning frequency should be measured and adjusting by shortening the port(s) slightly. The lining will help to reduce the out of band noise.

Port size
Use the largest port possible for your design. This will reduce power compression effects and port noise caused by turbulence. Flaring the ends of the port will also produce better results.