+Normal box speakers consist of two or three separate drivers. A box with three speakers is called a three -way, and has a tweeter for high sounds, a midrange speaker for voice range reproduction, and a bass speaker for the low notes. The bass speaker is usually between six inches to 12 inches in diameter, called a woofer. . . Sub-woofers are separate, large single drivers in a separate box that are tuned to handle very low sounds , or reinforce the low end output of smaller speakers., thus sub (below) woofer. All normal box speakers need amplifiers to drive them. normally this is achieved by using a receiver or a set composed of a sound processing unit called a pre amplifier and a separately powered amplifier. A reciever contains both of these parts, and a radio. Box speakers connect to the amplifier with two individual wires that are usually fused together in one casing something called lamp chord, or zip-wire (chord). Like all electrical appliances, or battery poles, one wire is marked positive and one is marked negative. both are connected to the speaker box. Inside the box, there is an electonic package called a crossover, that sends the proper tones to the proper driver inside it. some owners buy speaker boxes that are Bi-ampable, that is, use two amplifiers to separately power the woofer from one, and the remaining two drivers from the other, hoping for better performance. In such a system, the Crossover is partially bypassed, or a separate filter is used to isolate the woofer from the unwanted high sounds woofers cannot make well. In such cases Four wires (or two pairs) lead into the box. Self-Powered speakers would, in addition have a power chord exiting the box, in order to power the built in amplifier. All amplifiers are electrical. All speakers , like lightbulbs, need to be driven from an electrical source. For normal speakers that source is the amplifier.