Type of cooling material (ferromagnetic liquid) used primarily with tweeters to keep the driver from overheating by dissipating heat away from the voice coil. Since tweeters in particular move so quickly (up to 20,000 times per second at 20 kHz), they generate fairly large amounts of heat. The cooling fluid prevents the tweeter from overheating and breaking down or suffering from a shortened life span. The cooling effects also lend to improved damping and power handling.