The Subwoofer DIY Page
26 October, 2018
Bandpass subwoofer systems are basically systems where the driver's output is subjected to one or more "acoustic filters" in the form of an additional vented section added in front of the driver. The two most common type of bandpass subwoofer systems are the 4th order bandpass system and the 6th order bandpass system (which can be be "parallel-tuned" or "series-tuned"). There are other higher-order bandpass systems of course, but they are more difficult to design and construct, and it might be more practical to choose a design and build a simpler system using a better driver.
As all of the output of a bandpass system is via its vent(s), careful attention needs to be placed on the design of the vent or vents in order to get the best results. For example, the largest port diameter possible for the enclosure should be used in order to minimize vent compression effects, and the ports should be flared whenever possible, for the same reasons.
Bandpass systems rarely exhibit a perfect bandpass response - there is usually some out-of-band noise present in its output. A low-pass filter can be used to reduce this out of band noise if it is audible, but the impact of this low-pass filter on the overall output of the bandpass system needs to be taken into consideration when designing it. Careful choice of enclosure dimensions, the driver's location within the enclosure and the use of lining or stuffing within the sections can also be used to minimize out of band noise.
Designing a Bandpass System
Pictured below are (1) the input screen for a 4th order bandpass system in Hornresp. The driver in question is the Dayton Audio PA310-8, and the semi-inductance option is enabled. The model includes a simulated sealed section that is 32 litres in volume and 51 cm deep, and the vented section is 17.2 litres in volume and vented with a vent that is 27.5 cm long with 200 sq.cm. of cross sectional area.
..and this is what the simulated response of this 4th order bandpass system looks like:
As you can see from the simulation, the out of band noise starts from around 350 Hz, and the predicted level peaks at around 3dB below the passband level. This out of band noise will be very audible, unless it is dealt with using one of the techniques previously described. Pictured below is what the impact on the response would be if a 200 Hz 12dB/octave line-level filter was used with this particular system. The vent has been trimmed slightly to minimize the effect of the filter on the response in the passband.
Pictured below is the Hornresp input screen for a series-tuned 6th order bandpass system using the same driver. In this case the rear section is 30.5 litres in volume and tuned with a vent 49.1 cm long with a cross-sectional area of 150 sq.cm. that connects to the front section, and the front section is 40 litres in volume and tuned with a vent 12.5 cm long with a cross-sectional area of 200 sq.cm. The resulting passband is wider than that of the 4th order bandpass system that was previously simulated with the same driver. However the resulting box size is also quite a bit bigger.
...and this is what the simulated response of this 6th order bandpass looks like:
Apart from a very narrow spike at 300 Hz which will very likely not appear in a built version of this design (Hornresp does not include the impact of box losses in its simulations), the out of band noise appears much further up in the frequency range, above 600 Hz, making it a lot easier to deal with with a simple low-pass filter that does not impact the response in the passband.
Bandpass Subwoofer projects on the Internet