Beats With Two Tone Generators
The image below is the beat pattern produced by the two two small loudspeakers side-by-side that are driven by two tone generators.
When the two tones are close together in frequency, the ear hears what seems to be a single pitch, and the measurement above gives a frequency of 231 Hz. But the tone pulses in loudness at the beat frequency, the difference between the two frequencies. Measuring the period of the pulsing gives a beat frequency of about 8Hz. We don't get an accurate frequency of either tone, but we know that the two tones center about 231 Hz and that they are 8 Hz apart in frequency.
Traveling wave concepts