Filter theory in electrical engineering shows that higher-order systems are more efficient than lower-order systems (steeper slopes in electrical filters). Since a loudspeaker can also be described as an electrical filter, this principle applies here as well. A closed box is a 2nd-order filter (one moving mass with suspension), while a bass reflex box is a 4th-order filter (two moving masses: one being the diaphragm and the other the air in the bass reflex tube). This could give rise to the idea of building a 6th-order filter with an additional mass-spring system purely electrically, because electrical resonant circuits have the same differential equation as mechanical mass-spring systems. This is exactly what we do with all our loudspeakers. The increased performance allows us to reduce the internal volume (smaller cabinet) and achieve a lower resonance frequency, enabling the speakers to extend lower than they would otherwise be capable of.

This electrical high-pass filtering does not make the bass louder; instead, the cut-off frequency is lowered by almost an octave (more deep bass!), and the cabinet can be made smaller. We call this enhancement of the lowest octave (30 Hz - 60 Hz) Bass Extension, and by inserting the jumper, you can achieve a boost of 4 dB at 40 Hz. If you plug one of our BE modules into these sockets, you can vary the boost in four steps. You can find out more about this under Bass Extension.