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

But prior to all that, the industry moved away from field coil to permanent magnets, not because permanent magnets were better, but because they were cheaper as well!

That is correct.

@larryi,

The interesting thing about Alnico magnets and compression is that some theorize that the particular sound of Alnico magnet speakers is that they are actually subject to more compression than other magnet types because flux density is lower under dynamic conditions when the voice coil is excited.

I think they are confusing dynamic instabilities within the voice coil with compression, I could be wrong. Also, JBL uses Alnico 5dg magnets. The 4dg and 3dg magnets are lower in strength as some design require less strength. Just when you thought you had perfect sound. LOL 😎See link below: Suspension Bounce as a Distortion.

Mike

This "thermal compression" issue really impacts those that listen for long periods at higher levels.     In the studio where I go often, this is a MAJOR problem affecting lots of speakers. Active ones and especially passive ones. Cheap drivers or entry level speakers with little cooling can be heated up (long hours of loud music) and become "power compressed".  The effect is distortion but also more than that: its reduced dynamics.  For many tracking engineers, the reduced dynamics of the speaker is THE major issue affecting their work as tracking live sources requires intense dynamic range.  

I don't think there are many consumer customers listening at high levels for hours on end.

Brad