After the fall to lower frequencies, a sub-frequency curve (or post-frequency curve, see red arrow ) reappears. Anyone know why this is so? It started suddenly, before it was not there. But I have seen the same sub-frequency in measurements I made more than 10 years ago. I have also seen that other measurements online sometimes have the same sub-frequency. I have measured 4 inch and 12 inch elements without a box in a 1 cm near field. If I move the microphone to 2 cm, the level of the frequency curve decreases but the sub-frequency curve does not change. Anyone can explain the reason for this and why some measurements do not turn out that way?