Tuning forks are used to tune musical instruments and ensure they are producing the correct notes. A tuner will strike the fork and play the note at the same time. If the two frequencies are not the same, a periodic "beating" sound will be heard.
Suppose a 261.6 Hz tuning fork is struck while middle C (which is supposed to be 261.6 Hz) is played on a piano. However, a beat frequency of 30 Hz is observed.
Assuming the fork makes the correct sound, which of the following must be true?