Ultrasonic waves of frequency $3\times {10}^5 \mathrm{Hz}$ are passed through a medium where the speed of sound is $10$ times that in air (speed of sound in air is $300 \mathrm{m} {\mathrm{s}}^{-1}$). The wavelength of this wave in that medium will be of the order of,

$S_1, S_2$ are two coherent sources (having initial phase difference zero) of sound located along $x-\text{axis}$ separated by $4\mathrm{\lambda }$, where $\mathrm{\lambda }$ is wavelength of sound emitted by them. Number of maxima located on the elliptical boundary around it will be