Walker, Halliday & Resnick — Waves-II: Problem 28

28 Figure 17-28 shows four isotropic point sources of sound that are uniformly spaced on an x axis. The sources emit sound at the same wavelength \(\lambda\) and same amplitude \(s_m\), 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) \(\lambda/4\), (b) \(\lambda/2\), and (c) \(\lambda\)?