I have made a recording inside a quite office (office noise) with the Zoom H6 mobile recorder and XY microphones and got the following spectrum from the signal:
There is a peak at about 19.8 kHz. Even though I am not able to hear such high tones, I am pretty sure the tone is not present in the acoustic environment. I have seen the same tone from a USB audio interface connected to a smart phone.
I can use a filter to get rid of the tone, but I was curious why it is there.
Does anybody know where the tone comes from? And what is the purpose of this tone?
Thanks in advance