Two coherent sound waves of equal amplitude A = 2.0 mm meet at a point with a phase difference of 120 degrees (2 pi/3 radians). What is the amplitude of the resultant wave? Is the interference closer to constructive or destructive? Solution: Resultant amplitude = 2A cos(phi/2) = 2(2.0 mm) cos(60 deg) = 4.0 x 0.5 = 2.0 mm. The resultant amplitude (2.0 mm) is larger than each individual amplitude but smaller than the maximum 2A = 4.0 mm, and much larger than zero, so the interference is partly constructive. If the phase difference were 180 degrees, the result would be zero (complete cancellation).