Today, however, the sound has changed, at least for now. It's now three audio frequencies instead of two, so the "wheel" really needs oil. This spectrogram was made from the U. of Twente WebSDR tuned to 3828 kHz USB, with the passband opened to 3 kHz to get all of the tones. Note how the 2800-Hz tone appears to be somewhat distorted.
Now we can get headaches even faster.
