Loudspeaker specifications and technical explanations
Thermal compression is a term used to describe the effect on a loudspeaker of the voice coil heating up during use.
Consider a hypothetical 8 ohm 100w speaker that has not been used and is completely cold or at least at room temperature, if you were to measure the impedance of the voice coil it would read about 8 ohms, if the speaker is then used with 100w amplifier (or 28.3 RMS) at full power the speaker will be dissipating 100w.
However if you were to run this speaker at full power for an extended period of time the voice coil will get very, hot possibly more than 200 Degrees and its resistance will increase, this will increase the total impedance of the speaker, now if the hot voice coil were to cause the impedance to increase by say - double, not an unreasonable estimate ie to16 ohms, then the power dissipation will only be 50w, so the speaker has 'compressed' the output by 50%
Obviously speaker manufacturers take this (or should do) effect into consideration when rating particular drivers, generally a speaker with a larger size voice coil and good thermal management will thermally compress the output to a lesser extent than a driver with a smaller voice coil and less than optimum thermal management (a cheaper speaker).
Modern driver voice coils are able to withstand much higher temperatures than speakers from the 60s and 70s, due to better high temperature adhesives and coil formers but the ability to handle more power thermally does not necessarily translate into a louder sound, although it will guarantee a warmer room.
The reality is that there is a practical limit to how much acoustic output (loudness) a 12"- 15" or 18" speaker can achieve because loudness is actually air pressure acting on the ear drum, and for a certain size cone there are limits to the amount of air pressure it can generate, this will of course vary by the size of the venue - a 12" 100w speaker will be able to achieve an unbelievably high acoustic output in a toilet cubical at Wembley stadium, but move it to the middle of the football pitch and the volume produced will be far less. (actually the volume right in front of the speaker will be exactly the same, but the pressure that it can generate in a huge stadium on your ears is tiny).
The modern trend of having say a 1000w per channel amplifier and 2 x expensive1000w speakers is really complete nonsense, all you are doing is wasting electricity, a far better set up would be to use 8 inexpensive 12" 250w speakers per channel, This would guarantee a far louder sound, although it would also be somewhat impractical to move 16 speaker cabs around, to sum up, a very high power speaker is more robust and will probably prove more reliable than a lower powered speaker but will not necessarily be able produce any more volume than a lower power model, as most of the additional power handling capacity is wasted as heat and not used to produce sound - that is thermal compression.
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