In a

loudspeaker A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or " ...

, power compression or thermal compression is a loss of efficiency observed as the

voice coil A voice coil (consisting of a former, collar, and winding) is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it. The term ...

heats up under operation, increasing the DC resistance of the voice coil and decreasing the effective available power of the

audio amplifier An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspea ...

. A loudspeaker that becomes hot from use may not produce as much

sound pressure level Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone ...

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 – 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. Link t
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In multi-way systems, power compression is often observed to occur first in one of the low frequency bandpasses. This causes the total system to have an imbalance in frequency response, a reduction of level in one bandpass compared to the others. In passive loudspeakers with internal crossover components, power compression will change the electrical characteristics of the crossover filters, and the crossover point can shift, introducing distortions related to an incorrect crossover filter. To counteract power compression, one solution is to increase heat dissipation. Typical methods include cooling fins on the magnet housing, a larger diameter voice coil,

ferrofluid Ferrofluid is a liquid that is attracted to the poles of a magnet. It is a colloidal liquid made of nanoscale ferromagnetic or ferrimagnetic particles suspended in a carrier fluid (usually an organic solvent or water). Each magnetic particle ...

in the gap between voice coil and magnet, venting of the

pole piece A pole piece is a structure composed of material of high magnetic permeability that serves to direct the magnetic field produced by a magnet. A pole piece attaches to and in a sense extends a pole of the magnet, hence the name. Pole pieces are us ...

, metal parts that conduct heat to the outside, increasing the enclosure's internal chamber volume behind the magnet, and electric cooling fans. Another solution is to design a system that increases efficiency, such as by using a

horn loudspeaker A horn loudspeaker is a loudspeaker or loudspeaker element which uses an horn (acoustic), acoustic horn to increase the overall efficiency of the driving element(s). A common form ''(right)'' consists of a compression driver which produces sound w ...

rather than a direct-radiating design. Or by choosing a transducer other than the voice coil, such as Bruce Thigpen's

rotary woofer A rotary woofer is a subwoofer-style loudspeaker which reproduces very low frequency content by using a conventional speaker voice coil's motion to change the pitch of an impeller rotating at a constant speed. The pitch of the fan blades is controll ...

(1974) or

Tom Danley Tom or TOM may refer to: * Tom (given name), a diminutive of Thomas or Tomás or an independent Aramaic given name (and a list of people with the name) Characters * Tom Anderson, a character in ''Beavis and Butt-Head'' * Tom Beck, a character ...

's servo-motor subwoofer (1983). Danley, Thomas J. (1986) "The Elimination of Power Compression in Servo Drive Loudspeakers," Presented at the 81st Convention of the AES. 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 the loudspeaker will increase in

total harmonic distortion The total harmonic distortion (THD or THDi) is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Distortion facto ...

References

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