Answer

It is well understood that low frequency energy from subwoofers is effectively omnidirectional in nature (equally loud in all directions). This is because low frequency wavelengths are much longer than the dimensions of the subwoofer enclosure, and therefore “wrap” around it. 

A cardioid Subwoofer arrangement employs both front- and rear-firing woofers, and through some clever use of spacing, delay and polarity, a “null” or cancellation of rear-firing energy is created behind the subwoofer, while also enhancing the forward-firing energy.