Figure shows four isotropic point sources of sound that are uniformly spaced on an x axis. The sources emit sound at the same wavelength `lamda` and same amplitudes `s_m` and they emit in phase. A point P is shown on the x axis. Assume that as the sound waves travel to P, the decrease in their amplitude is negligible. What multiple of `s_m` is the amplitude of the net wave at P if distance d in the figure is `(a) lamda//4 (b) lamda//2 and (c ) lamda ` ?

Explanation

`(a) 0 , (b) 0 , (c ) 4S_m`