A source (S) of sound has frequency 240 ~Hz . When the observer (O)...

A source (S) of sound has frequency $240 \mathrm{~Hz}$. When the observer $(\mathrm{O})$ and the source move towards each other at a speed $v$ with respect to the ground (as shown in Case 1 in the figure), the observer measures the frequency of the sound to be $288 \mathrm{~Hz}$. However, when the observer and the source move away from each other at the same speed $v$ with respect to the ground (as shown in Case 2 in the figure), the observer measures the frequency of sound to be $n \mathrm{~Hz}$. The value of $n$ is ______