The J-INFRA from d&b audiotechnik is the cardioid infra subwoofer for the J-Series. It is tuned to fit below the J-SUB subwoofer in terms of phase, sound, and impact and extends the frequency response of the system down to 27Hz. It is an actively driven two-way bass-reflex design housing three long excursion neodymium 21" drivers of which two face front while the other radiates to the rear. The two forward drivers are housed in a bass-reflex chamber and driven from a single channel of the D12 amplifier. The backward driver is in a second chamber and driven by the other D12 channel. The cabinets can only be used in ground-stacked configurations in conventional left/right setups or in an Infra subwoofer array. One J-INFRA subwoofer provides low frequency energy for two J subwoofers.