Power compression

In a loudspeaker, power compression or thermal compression is a loss of efficiency observed as the voice coil heats up under operation, increasing the DC resistance of the voice coil and decreasing the effective available power of the audio amplifier. A loudspeaker that becomes hot from use may not produce as much sound pressure level as when it is cold. The problem is much greater for hard-driven professional concert systems than it is for loudspeakers in the home, where it is rarely seen. Two main pathways exist to mitigate the problem: to design a way for the voice coil to dissipate more heat during operation, and to design a more efficient transducer that generates less heat for a given sound output level.

High-power audio transducers have a low efficiency, with less than 5% of the amplifier signal turned into sound waves. The other 95% or more of the electrical energy is turned into unwanted heat, which causes the voice coil to increase in temperature. Too much heat – more than 200 °C (390 °F) – can destroy the voice coil, but long before that happens, the loudspeaker will experience power compression. A voice coil made of copper wire will have its DC resistance increase by about 72% when heating up from 20 °C (room temperature) to 200 °C, and its sensitivity will decrease by 4.7 decibels. Silver wire has a slightly worse problem with power compression, while aluminum wire is slightly better.