Look at Neumann’s webpage. They show Distortion for 80dB, 85, 90 and 95dB. You can compare to Amir’s dataI did some measurements to confirm the resonance at low levels. Just a quick setup, speaker at listening position, mic at 50cm, level calibrated at 1m distance. No gating - so there is all the room influence included.
My measurements look different. Not the steep resonance at 330Hz but still, there is a leftover at 400-500Hz at low levels. When going even lower in level (70dBSpl and less) THD sinks in the noise floor (of the measurement mic).
View attachment 298823
There is also a little hump in the THD % measurement, but by far not what amirm measured.
View attachment 298824
THD at 80dBSpl from H2 and H3:
View attachment 298825
I first thought about noise artefacts but it probably isn't. Not sure what's going on.
@amirm - can you repeat your 80dBSpl measurement, probably with a different speaker? What does Neumann say to your data? And can you show a logarithmic THD scale - with these low distortion speakers the linear scale lacks information. And humans don't work linear - we are logarithmic people
So I can not confirm the spike at 330Hz from these measurements but there is probably some small effect going on here. But at 0,2% THD I would not overestimate this effect.