Bandpass Subwoofer: Driver Between Two Chambers

The driver does not radiate directly to the outside. It sits on a partition between two chambers; the sound only reaches the outside through one or more ports. The enclosure thus acts like an acoustic bandpass filter: it lets only a limited frequency range through.

• 4th order: The rear of the cone works into a sealed chamber, the front into a vented chamber.
• 6th order: Both chambers have a port. Such enclosures are considerably more difficult to design and are sensitive to the characteristics of the driver.

At Speakerplans.com such designs are called “Coupled Cavity”, at 18 Sound “Double Reflex Band-Pass”.

• 18 Sound 121BP – 21″ bandpass with three driver options and presets
• Speakerplans 184 Sub – compact 18″ bandpass with tube lengths per driver
• Speakerplans C Sub – 18″ bandpass with crossover at 80 Hz

Because the bandpass is closely tuned to the driver, the rule is: only build it with a driver named by the designer. With the 184 Sub, the tube lengths even differ depending on the driver; according to the designer, the design does not work at all with the Eminence Kilomax Pro 18.

The chamber volumes are also precisely calculated. Driver and ports take up space in them. Rog Mogale shows in his FAQ how to take this into account: an 18-inch driver displaces approx. 7 litres on average, the slot port in his example around 25 litres – the chamber must hold both in addition.

The boundary is fluid. According to Rog Mogale, a folded horn with a path length of less than 1.67 m works both as a horn and as a 4th-order bandpass: rear chamber, front chamber and horn – the horn then acts like a flaring port.