Speaker voice coils are made of copper or aluminum. As these voice coils increase in temperature during normal operation, their resistance increases. Greater voice coil resistance means less power transfer from the amplifier. As a result, the speaker will not play as loud when it’s “warmed up” as it did when it was “cold”. Some speakers may exhibit 3 to 6 dB of power compression. A mere 3 dB of power compression is equivalent to cutting the available wattage of your power amps in half. Speaker manufacturers who develop systems for use in demanding applications such as concerts or nightclubs spend a great deal of their research and development energy working on ways to keep speaker voice coils cool while in operation.