So far every source I have read define "beat" frequency as the difference between two frequencies or f2-f1. The common example is in the audio world where we have two tones very close in value, say 100 MHz and 101 MHz. In this case the beat frequency would simply be 1MHz. These same sources pull out one of the trig terms in the result for adding two sine waves and refer to the beat frequency as the "envelope" of the sum of the two waves. This envelop or beat frequency is easy to see with two sinusoid waves close in frequency, but when I have two waves far apart in frequency.....say 100 MHz and 1 MHz the "envelope" visually looks like the lower frequency of 1 MHz although the math says it should be at 99 MHz since it's the difference frequency. So what is the issue here? Am I misunderstanding the definition of beat frequency or am I simply tracing out the envelope incorrectly? Would the "beat" frequency or "envelops" be 99 MHz if I had two waves adding together at 100 MHz and 1 MHz?