Please help me understand what is wrong with my thinking. I have two high frequency sources into an oscilloscope with one source a few hertz difference. I have a spectrum analyzer and expected a low frequency signal due to the beat frequency of the sources. What is going wrong? I'm confused. This is from EM voltages. But I have a related question. Why can the human ear hear audio ranges from two ultrasonic heterodyned sources? Isn't that just a beat frequency in the audio range. Why do I not see the same principle on my spectrum analyzer? Do the ear create some kind of diode effect?