Many times sensitivity is a derived figure (calculated), but sometimes it is measured over a specified frequency range under specific conditions. Like Sanz said, no sub has 97 dB sensitivity @ 20Hz because that is alignment specific. The low end output is strictly governed by the alignment. That's the whole point of T/S parameters- to predict this output based on user-defined enclosures and tuning.
If the sensitivity is calculated, one can use the equation to verify:
If the DCR of the driver is much lower than 6.8Ω, then the 2.83V sensitivity ends up a bit higher, that I've seen anyways.
BUT, this spec is not for any
frequeny you are dreaming of. This specifies the Mid-band sensitivity, defined by EBP (which is also sometimes used to see how suitable a speaker is for closed ↔ vented type alignments.
EBP = Fs / Qes, where EBP is in units of Hz.
The midband region is: EBP to 2*EBP. ← This range is where the sensitivity manifests itself strongly. Above that, response has to be measured, below that it is completely governed by the alignment, and thus predictable by T/S analysis (aka: subwoofer enclosure software modeling).
This is a picture I had posted on another forum describing where the midband frequency range would be for a woofer with an EBP of about about ~100hz. It will generally extend for about 1 octave without much fluctuation, unless it is wayy into the bass region.
That particular woofer had a midband that extended about 1.5 octaves above the EBP frequency.