On a piano tuned to the American equal-tempered scale, the frequency of the third harmonic of C4 string is 784.87 Hz. The fundamental frequency of G5 string is 783.99 Hz. Beats between these frequencies can be heard in the following way:
Hold the key for C4 down, so that the string can vibrate. Strike the G5 key loudly, so that the third harmonic of the C4 string will be excited. Before that note dies down, let the G5 key go (this quiets the G string). Then strike the G string again more softly, so that the volume of the two strings are matched.
What is the frequency of the beat between the notes?