Answer
The beat frequency is $~~60~Hz$
The beat frequency is not perceptible.
Work Step by Step
In part (a), we found that the frequency of the sound heard directly from the siren is $971~Hz$
In part (b), we found that the frequency of the sound reflected from the cliff is $1031~Hz$
We can find the beat frequency:
$1031~Hz-971~Hz = 60~Hz$
Since $60~Hz \gt 20~Hz$, the beat frequency is not perceptible.
