Dayton Audio DAEX25FHE-4 Framed High Efficiency 25mm Exciter 24W 4 Ohm Small size; just under 2&quot; diameter, about 7/8&quot; high Pre-applied 3M&#8482; VHB&#8482; adhesive for quick, durable installation Rare-earth neodymium motor and a proprietary voice coil 4-ohm impedance for use with small, Class D amplifiers 24 watt RMS power handling for high output