Thermal Distortion your loudspeaker most likely suffers from it. But do you care?

OMG, I from suffer TD and had no idea!

@johnk --

Thanks for starting this thread. 

"But do you care?"

Most certainly, but not for the reasons outlined by quite a few here about avoiding one's speakers going up in smoke. What's interesting to me is how thermal compression/-distortion can impact the sound at a much earlier juncture than speaker failure or overheating per se: with "thermal modulation," eloquently put into words by poster @audiokinesis ..

"During the recent "great recession", I spent most of my time over on the prosound side of things. Imo, there's a lot of validity to Mtrot's observation: "I'm beginning to think the ability of speakers to achieve that sense of dynamic "liveness" may be as or more important to a sense of realism than frequency response accuracy."

On the acoustics side, I take the word to mean unrestrained dynamic transients. Compression can come from amplifier clipping or loudspeaker thermal or mechanical limitations. I believe that the most common culprit in loudspeakers is "thermal modulation", a quick-onset compression that results from the near-instantaneous heating of the voice coil from a high-power transient.

On the psychoacoustics side, "slam" registers when a limbic system response ("fight or flight" startle) is triggered. It is a function of transient dynamics and raw SPL. If there's not much dynamic contrast, it doesn't come across as "slam". If there's good dynamic contrast but the sound pressure level is still soft, it doesn't come across as "slam".

From a loudspeaker design perspective, the solutions include high efficiency and/or large diameter (or multiple) voice coils. If a loudspeaker system is being pushed close to its RMS thermal rating on peaks, your peaks are softened and so is the emotion conveyed. If a loudspeaker system is just loafing along at fairly high SPL, it will deliver plenty of slam. That's why 5 watts into a 98 dB efficient speaker almost always sounds so much more lively than 200 watts into an 82 dB efficient speaker, even though "on paper" both are 105 dB capable.

Duke
dealer/manufacturer"

https://forum.audiogon.com/discussions/quot-slam-quot-what-is-it-is-it-really-accurate

The proof is in the eating of the pudding, as they say. 

@ditusa --

Thanks for provided article on the subject (and to @ieales for the link). 

"It is clear, however, that even the best transducers will power compress and yield less than desirable performance at the limits of their power capacity. It therefore makes the most sense to operate drivers well below maximum power to achieve desirable performance. The ideal operating level will track with power handling, and drivers with high power ratings and good shortterm capability (high T and low Rg) will be the best choice."

Indeed, "well below maximum power" says it clearly; it's about maintaining ample headroom, even at the highest SPL's to achieve the best sonic outcome, and so this is really not about the risk of frying one's speakers (though it may pertain more readily to pro installations) as much as it is sonic implications. 

Heat also affects passive cross-overs and in effect the sound from such passive speakers, which has been pointed to above, consequences that are happily avoided with active configurations. 

I am a big fan of efficient speakers.  I don't understand why manufactures make 4 ohm speakers for example.  I believe you need more power to drive them.  If that is the case, I would imagine this in turn creates more distortion.  Not sure if I understand this correctly.  Just an observation I have had.  I would think higher efficient speakers can create higher amounts of head room as well.

4Ω or 8Ω speakers are a myth.

Just look at the impedance curves on Stereophile. Some go from 2 to 20 and beyond.

SPL efficiency @ 1w or 2.83v is also highly suspect as few speakers are 8Ω @ 1kHz.

I think this heat issue may effect high frequencies most.

And it may be that the AG sold, “Deulund-Mundorf Ultra Speaker Purifiers” could be the solution to the heat issue. After 100++ hours of breakin they have had a profound effect on the soundstage and imaging. I now hear layers in a wide and deep soundstage and imaging has come into focus like never before.

These can be self made but the seller adjusts the values of the cap and resistor to match your speaker’s needs. 
This idea is not new. They are also known as Zobel Networks.

They only effect frequencies 12K and above. So above the crossover point of most.

Radically improved my listening experience.