Dayton Audio DAEX19CT-4 Coin Type 19mm Vented Exciter 5W 4 Ohm Small size; just under 1-1/2&quot; diameter, about 1/2&quot; high Pre-applied 3M&#8482; VHB&#8482; adhesive for quick, durable installation Rare-earth neodymium motor with cooling vents for higher power handling 4-ohm impedance for use with small, Class D amplifiers Strategically-placed vent holes cool this exciter&apos;s voice coil for higher power handling