The WHARFEDALE's DELTA F18B single 18-inch folding horn subwoofer is passive and delivers a whopping 141dB (max SPL). This passive speaker comes with castors, making it a surprisingly portable and manoeuvrable enclosure. When a sound system's low frequency output needs to be extended over medium-long distances, one or more Delta F18B subwoofers will provide high output low frequency improvement to the overall sound system. The Delta F18B's folded horn design allows it to produce ground-shaking low frequencies across extended distances.