Do you get similar graph when you repeat the measurement?
Yes. Well, some additional information below.
24 measurements at different SPL:
Impulse
Step:
Gets a little confused with the different levels. The four outliers are the lowest SPL, too close to the noise floor. Once well aove the noise floor, the results are rather congruent.
SPL
Ambient noise intrudes at low levels at low frequencies
Distortion is level vs noise dependent, too. Only at the highest SPL do the distortion measurements begin to rise out of the noise (Low SPL at the top (high distortion measured is actually just noise), high SPL at the bottom, where the THD measures begin to bunch up, and rise, with higher SPL, and some actual meaningfult THD from the speakers)