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

@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. 

@pcrhkr —

“Thermal distortion from my speakers. My ears would blow first! :)”

One of those instances where it may feel inappropriate stating that this is actually a desirable outset :) Question is if you would know whether thermal distortion/modulation had krept in in the first place, because that’s the real bug for the ones in particular who don’t have high efficiency speakers (and speakers that are passively configured to boot), and at what juncture - much earlier than one would imagine - it may pose an issue. In your case it may be a lesser issue than what many other audiophiles (unknowingly) are facing, at least.

@johnk --

Thanks for clarifying. Had no intention to put down Alnico magnets in any way, in fact I'd only cherish their inclusion in a speaker setup.  

@atmasphere --

"Back when tubes were king, high efficiency speakers were very common because tube power was (and continues to be) expensive.

When solid state amps were commonplace, speaker efficiency started to go down.

The problem here is that it never helps to have an amplifier drive a speaker that is a difficult load and in particular low efficiency. The result will be higher distortion from the amplifier if nothing else, and that distortion usually manifests as higher ordered harmonics, to which the ear assigns the tonality of 'harsh and bright' and is keenly sensitive to their presence since it uses them to sense sound pressure.

So we've been hearing 'harsh and bright' for 50 years now. Some is the fault of amplifier design of course, but difficult to drive speakers don't help. 

So inefficient and low impedance speakers should be avoided if you want to get the most out of your amplifier dollar investment."

But what makes a speaker a difficult load also and not least involves the effects of passive cross-overs (unless this is implicit to what you're saying here, but it's unclear to me) in terms of their complexity and the (steep) phase angles presented, not to mention smearing of the signal here.

This is interesting due to observations I've made going from passive to active speaker designs (with the same speakers), where I've found going active with solid state amps makes for a sound more akin to the sonic imprinting of SET's; passive sounds more sluggish, heavier even and less resolved (but to some more pleasing this way), whereas active makes for a more transiently clean (less smeared), less grainy and more liquid and open presentation. Easier on the ears, and more fleshed out.

I'd have cherished the opportunity of using 16 ohm versions of the drivers used in my speakers (not least the compression driver, which is available in a 16 ohm version), having then both high efficiency, high impedance and passive filter-less speakers. I would assume though passive filters and their negation to be a similarly or even more important means of making the amp seeing into an easier load, certainly bypassing more complex passive filters. A "purer" low impedance load seems preferable to a higher ditto marred by steep phase angles, but I guess it also depends on the specific amp. 

@ditusa --

"Yes That is one of the many reason for using ALNICO in permanent magnet speakers, it’s impervious to heat so it’s flux is not affected by the change in temperature."

Alnico magnet’s were/are sometimes prone to demagnetize:

... there was one major disadvantage in Alnico loudspeakers that would have to be addressed � the susceptibility to permanent demagnetization due to overpowering.

This phenomenon is a result of variability in strength of the permanent magnetic field caused by interference from the voice coil�s electromagnetic field. This is referred to as flux modulation and is a leading cause of distortion in any dynamic loudspeaker. The electromagnetic field generated in the coil pushes against the global magnetic field set up by the permanent magnet and return circuit, causing it to �bend�. Under normal operating conditions, Alnico magnets actually resist this bending better than most other magnet materials. However, should this shift become large enough, it will exceed the coercivity of Alnico and cause it to partially demagnetize.

http://www.audioheritage.org/html/projectmay/technology/1500al.htm

Though in most cases, not least with high efficiency drivers used domestically I gather there are no real drawbacks with Alnico magnets to speak of.

@fiesta75 --

"You’ve got more to worry about in passive crossovers than heat"

Not least that they’re there in the first place, and their more complex iterations usually make matters worse. Heat at some point only adds to those issues, and used in low efficiency speakers this will only be more prevalent.

High eff. horn speakers ideally, or at least often call for steep filter slopes to avoid out-of-band irregularities, and as such active filters offer themselves much better here compared to passive XO’s.

@ieales --

"If high efficiency speakers are so darned good, why did the industry move away from them? So electronics makers could sell more expensive Power? Methinks not."

Actually more power got less expensive with the advent of the transistor, at which point Edgar Villchur also made his entry with his "acoustic-suspension" AR-1’s as a much smaller and much less efficient speaker package - a package that needed the extra power, of course.

It was and largely still is about (size-)convenience and the introduction of a mass domestic market, albeit at the time (and reiterated today by the likes of John Atkinson) it was sold off with the marketing bling as offering the same extension from a fraction of a size with less distortion. What’s not to like?

Well, Mr. Atkinson was (and likely still is) an avid supporter of MQA, so let that seep for a while like a good Earl Grey.

In the context of this thread it’s about thermal compression/distortion/modulation, and there’s no escaping physics here with regard to overall size requirement of a speaker system that naturally accommodates high efficiency, and thus is much more impervious to thermal issues.

The question though also seems to be: does it matter, or how much does it matter in a domestic environment with typically moderately sized listening rooms? Here’s a quote from yet another fine article supplied by poster @ditusa on the subject of efficiency:

"In all fairness, this limitation in dynamic range [with a small, inefficient speaker system] is of little interest to many listeners. At "average" loudness, neither type of system is apt to be momentarily overloaded. But the difference can be easily demonstrated under the right conditions. The man who wants to hear the smash of cymbals, the "bite" of a Steinway grand, a full concert intensity, will not be able to duplicate these sounds readily with a bookshelf-type loudspeaker system.
"This is all very interesting, no doubt," says the prospective costumer, "but you still haven’t told me which type of system is better."
The answer is that if all other considerations can be ignored, a good big system is almost always better than a good small system."

http://www.lansingheritage.org/images/jbl/reference/technical/efficiency/page03.jpg