ScanSpeak Ellipticor 18WE/4542T00

The THD measurements at distance of 315 mm and voltage levels from 2 Volt to 8 Volt were carried out with High Pass Filter "on" (2nd order Butterworth type, 50 Hz cutoff, HPF2-50)

The THD measurements at distance of 315 mm and voltage level 11.2 Volt were carried out with High Pass Filter "on" (2nd order Butterworth type, 80 Hz cutoff, HPF2-80)

On the harmonic distortion measurements at distance of 20 mm around the frequency of 220 Hz  you can notice distortion spikes. I don't think the reason is the midwoofer itself but most likely the resonance of a removable test bafle for speaker driver due its insufficient rigid fixation. Those HD spikes doesn't manifest itself on the HD spectrums at voltage levels 2.83, 4 and 5.6 Volt, but appear again at 8 Volt.