The 21iD is an 21 inch neodymium high performance subwoofer. The loudspeaker has been optimized for vented and bandpass subwoofer cabinet designs and is recommended to use a Class D or iPal (tm*) amplifier able to deliver 3600 Watt program power without clipping. Eighteen Sound engineers have obtained the best possible results with today's available materials in terms of clean and undistorted LF reproduction at a ultra high SPL, with the lowest possible power compression figure. The transducer design features include a large displacement suspension system specifically designed for matching the composite fiber reinforced, straight ribbed cone. Thanks to the Triple Silicon Spider (TSS) technology, the 21iD is able to control the moving mass with high linearity, showing an exceptional stability of mechanical parameter values in the long term. Bl force factor, as well as all other electro-dynamic parameters, are linear within the working range. This, together with the exceptional high excursion behavior - 70mm before damage, ±14mm linear Xmax - makes the 21iD an extremely low distortion, highly dynamic transducer. The 21iD features a state-of-the-art 5,3" inside outside copper ISV (Interleaved Sandwich Voice-Coil) enabling the 21iD to deliver extraordinary transient results. The 21iD has been developed after intense FEA and fluidodynamics simulation and testing,focusing on dissipating the heat generated by the powerful voice coil. Special attention was given to the optimization of air flow into the gap without introducing audible noise. A low-density material air diffractor placed into the heatsink acts as a cooling system, increasing the power handling capability and lowering the power compression figure.