The self inductance of the voice coil is increased by the presence of the pole piece due to its relatively high permeability. As the coil penetrates further into the gap, the effect increases, increasing the self inductance of the coil. The currents induced in a low impedance shorted turn (or "Faraday Ring") around the magnet pole pieces tend to oppose the currents which induce them (This is Lenz's law the electrical equivalent of Newton's second law of motion). The shorted turn consists of a length of extruded copper fixed to the rear of the pole piece
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