a hlcd is a horn loaded compression driver. like what is used in many PA setups. their advantage is that they play quite low and have retardedly good sensitivity. their downside is, in a car, they require a horn (the flared "port" that the compression driver attaches to) to be fairly wide(usually 12-24" depending on design), and the compression driver itself is usually the size of a baseball or softball.
02 Mustang GT... Tuned by Nelsons. Low 12's, anyone?

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