Thank you for that information and for digging up those examples, Erik!
Interesting that the less expensive speaker beats the more expensive one not only in deviation from linearity, but also in radiation pattern smoothness - look at the 45-60-75 degree off-axis curves. I tip my virtual hat to Paradigm.
Considering how SoundStage makes their Deviation from Linearity measurement, I think it includes not only mechanical effects but thermal ones also, because at 90 dB the speaker is seeing 10 times as much excursion but 100 times as much wattage as at 70 dB. At any rate, given that peaks 20 dB above the average are quite common in recordings that aren’t overly compressed, the deviation from linearity going from 70 to 90 dB may just be the "tip of the iceberg" for real-world effects, if the speaker is driven to average levels higher than 70 dB/1 meter.
My impression is that excursion-related non-linearities decrease gradually with level until they reach a certain point and then they shoot up rapidly.
The same thing is more or less true for thermal effects: It’s not uncommon for a driver to exhibit less than 1 dB of thermal compression at 10% of its AES rated power, often rising to about 2 dB at 50% of its rated power and then maybe 3.5 dB at 100% of its rated power. In other words, in that last doubling of input power, we only get about half as much increase in SPL as we "should have" (1.5 dB instead of 3 dB), Now these ballpark figures come from eyeballing the spec sheets of those few prosound manufacturers who publish compression specs. These numbers are for long-term thermal compression rather than short-term thermal modulation (which has a very rapid onset and then a slower release, unless another peak comes along before the voice coil has had a chance to cool down). My assumption is that there is a correlation between the short-term thermal modulation behavior and the long-term thermal compression behavior.
(Note that the AES rated power is a fairly conservative yardstick; typically the "music program" power rating is double that, and then the "peak" power rating may be double the music program rating, and we probably don't know which of these the manufacturer is using... and real-world, the excursion-limited power handling may be significantly lower than any of these at low frequencies.)
Duke