Answer
There is no minimum.
Work Step by Step
First, we can use the given frequency, $f=1.0*10^3Hz$, and the speed of sound in water, $v=1450m/s$, to find the wavelength.
$\lambda = v / f$
$\lambda = (1450m/s) / (25*10^3Hz)$
$\lambda =1.45m$
Now we can use the given diameter, $d=0.60m$, in equation (36-12), to find the required value of $\sin \theta$
$\sin \theta= 1.22 \frac{\lambda}{d}$
$\sin \theta= 1.22 \frac{1.45m}{0.60m}$
$\sin \theta= 2.9$
We require the value of $\sin \theta $ to be 2.9, but since the value of $\sin \theta$ can't be greater than 1, there is no minimum.
