We should reject hard-to-drive speakers more often

"Some compression occurs in planar drivers, including ESLs, because the membranes are stretched across a frame. The mechanical impedance is not linear."

True for all drivers, except perhaps the massive fan subwoofers. I think the difference in measurement/thinking about thermal compression vs. mechanical is that thermal compression changes the behavior of the speaker in time, sometimes within milliseconds, while mechanical compression is always there, until you blow the driver. :)

As a mechanical compression artefact this is hardly true for all drivers. Dynamic drivers have suspensions, and the "stretch" of a membrane here wouldn't occur in a way comparable unless the suspension iself is (getting close to being) mechanically edged out. 

I do think it’s odd audiophiles have fixated on thermal compression, specifically, as being the only one that matters, though I do agree that higher efficiency drivers seem to be at an advantage here.

Question is when, and perhaps not least how thermal compression starts becoming prevalent and an actual audible effect. Thermal 'modulation' may be a better term to explain or correlate what happens sonically; thermal compression impacts SPL envelope and ultimately driver failure, but it also appears to dull transient cleanliness and snap at a much earlier juncture as a very dynamic phenomena. 

There’s a reason JBL professional drivers are so expensive, and one of the main reasons that is that they are built specifically to avoid thermal compression even at constant power levels that would make most audiophile systems weep.

Other pro brands would do equally well, and at a cheaper price, but yes pro drivers are simply on another level here. 

As a mechanical compression artefact this is hardly true for all drivers

@phusis

Then I’d really like to know what you think happens when your reach the maximum excursion of a driver. Either they have limited excursion, and therefore compression, or they have infinite excursion and no compression.

I wish I could find them but I remember seeing tone burst tests showing that thermal compression could happen in a tweeter in less than half a second. You could see the first tone burst perform perfectly, and then half way through the second compression sets in.

@erik_squires wrote:

Then I’d really like to know what you think happens when your reach the maximum excursion of a driver. Either they have limited excursion, and therefore compression, or they have infinite excursion and no compression.

That’s indeed what I’m talking about in regards to mechanical compression, but how often do we reach, let alone exceed X-mech? My point is, before that happens the driver’s suspension has a range of motion within which compression per se as a mechanically induced phenomenon isn’t relevant. What’s arguably more relevant mechanically is hysteresis (or magnetically, hysteresis distortion) as that which happens through the range of cone movement, but that’s hardly a compression issue, no?

Although: smaller (<10"), low eff. woofers certainly makes their effort in reproducing the range <40Hz at elevated, but hardly prodigious levels be known, and when you’re used to a pair of corner loaded, high eff. tapped horns fitted with 15" pro woofers in 20cf. enclosures where the cones barely more than a few mm’s at bonkers SPL’s, it’s all the more obvious. With smaller hifi woofer cones working hard it’s likely a combo of mechanical noise and -compression, as well as the onset of thermally induced compression.

I wish I could find them but I remember seeing tone burst tests showing that thermal compression could happen in a tweeter in less than half a second. You could see the first tone burst perform perfectly, and then half way through the second compression sets in.

Interesting; I take it what could also be referred to as thermal modulation as a more dynamic phenomenon. It’s a shame this area isn’t more well documented, leaving us with the more general ’thermal compression’ description and correlation as heat build-up in the voice coils over longer time.

which compression per se as a mechanically induced phenomenon isn’t relevant.

@phusis I don’t claim to be an expert in all measurements but this seems not to go along with what I’ve seen or measured.

At some point significantly below Xmax I believe there begins to appear evidence of compression, both within the FR and distortion.  I'm not sure how we could attribute any/all of it to thermal without tone burst testing. Take a look at the SoundStage speaker measurements, just about any of them, since they are the only mag I know of that takes compression measurements regularly.

The alternative is to own amps that will drive anything.  I was lucky enough to buy several Class  A amps that were stable into 1 ohm loads early in my audio hobby - a pair were driving the heck out of Apogee Scintillas at 1 ohm setting.