The beat frequency is determined by the absolute difference between two frequencies. To find the combination that produces the lowest beat frequency, we calculate the differences:
- 500 Hz and 501 Hz: |500 - 501| = 1 Hz
- 500 Hz and 600 Hz: |500 - 600| = 100 Hz
- 10 Hz and 20 Hz: |10 - 20| = 10 Hz
- 10 Hz and 15 Hz: |10 - 15| = 5 Hz
Among these, the combination of 500 Hz and 501 Hz produces the lowest beat frequency, which is 1 Hz.