Two identical loudspeakers separated by distance $\mathrm{\u25b3}x$ each emit sound waves of wavelength $\lambda $ and amplitude a along the and amplitude a along the x-axis.

What is the minimum value of the ratio $\mathrm{\u25b3}x/\lambda $ for which the amplitude of their superposition is also a?

What is the minimum value of the ratio $\mathrm{\u25b3}x/\lambda $ for which the amplitude of their superposition is also a?