Dayton Audio DAEX13CT-4 Coin Type 13mm Exciter 3W 4 Ohm Super small size; just over an inch in diameter, about 3/8&quot; high Pre-applied 3M&#8482; VHB&#8482; adhesive for a quick, secure installation Rare-earth neodymium motor 4-ohm impedance for use with small, Class D amplifiers