Dayton Audio DAEX25VT-4 Vented 25mm Exciter 20W 4 Ohm Small size; just over 2&quot; diameter, about 7/8&quot; high Pre-applied 3M&#8482; VHB&#8482; adhesive for quick, secure installation Rare-earth neodymium motor and a proprietary voice coil 4-ohm impedance for use with small, Class D amplifiers 20 watt RMS power handling for high output