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 negligible. Two common solutions exist: 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 to 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.

Power compression is usually considered a long-term problem, arising over time with extended strong signal sent to the loudspeaker. However, if the change in resistance is short term, observed as heating up and cooling down with each cycle of low frequency waves, then then the loudspeaker will increase in total harmonic distortion.