into widespread use in European live concert sound in 2006. The end-fire array trades a few decibels of output power for directionality, so it requires more enclosures for the same output power as a tight-packed, flat-fronted array of enclosures. Sixteen enclosures in four rows were used in 2007 at one of the stages of the Ultra Music Festival, to reduce low-frequency interference to neighboring stages. Because of the physical size of the end-fire array, few concert venues are able to implement it. The output pattern suffers from comb-filtering off-axis, but can be further shaped by adjusting the frequency response of each row of subwoofers. Delay-shaded array A long line of subwoofers placed horizontally along the front edge of the stage can be delayed such that the center subwoofers fire several milliseconds prior to the ones flanking them, which fire several milliseconds prior to their neighbors, continuing in this fashion until the last subwoofers are reached at the outside ends of the subwoofer row (beamforming). This method helps to counteract the extreme narrowing of the horizontal dispersion pattern seen with a horizontal subwoofer array. Such delay shading can be used to virtually reshape a loudspeaker array. Directional enclosure Some subwoofer enclosure designs rely on drivers facing to the sides or to the rear in order to achieve a degree of directionality. End-fire drivers can be positioned within a single enclosure that houses more than one driver. Variants Some less commonly-used bass enclosures are variants of the subwoofer enclosure's normal range, such as the mid-bass cabinet (60–160 Hz) and the infrasonic (extra low) subwoofer (below 30 Hz). Enclosure designs Front-loaded subwoofers have one or more subwoofer speakers in a cabinet, typically with a grille to protect the speakers. In practice, many front-loaded subwoofer cabinets have a vent or port in the speaker cabinet, thus creating a bass reflex enclosure. Even though a bass reflex port or vent creates some additional phase delay, it adds SPL, which is often a key factor in PA and sound reinforcement system applications. As such, non-vented front-firing subwoofer cabinets are rare in pro audio applications. Horn-loaded subwoofers have a subwoofer speaker that has a pathway following the loudspeaker. To save space, the pathway is often folded, so that the folded pathway will fit into a box-style cabinet. Cerwin-Vega states that its folded horn subwoofer cabinets, "...on average, produce 6dB more output at 1 watt than a dual