The THD measurements at a distance 315 mm and voltage levels 2 - 11.2 Volt were carried out with High Pass Filter "on" (2nd order Buttreworth type, 50 Hz cutoff, HPF2-50).
Before testing the speaker have been good broken-in by applying to it the sinusoidal voltage of 12 Volt/20 Hz during an one hour and resting after that for about 24 hours for the parameters be stabilized.