Here is a picture of that range (FFT 8192 bands and blackmann-harris window). Interestingly there seem to be two signals oscilating synchronously in frequency at about 0.8Hz.
Do you have any info on the other tone? It is centered at 18440Hz.
Maybe it could also be a computer monitor set to whatever resolution and vertical refresh rate.
QuoteMaybe it could also be a computer monitor set to whatever resolution and vertical refresh rate.That's unlikely, if you ask me. Look at this:...The 18.44 kHz tone is too low for computer monitors. They usually operate at much higher horizontal frequencies, i.e. 31 to over 100 kHz - depending on resolution and refresh rate.