The effect of refraction caused by a sound‐speed gradient at the surface of a diffracting obstacle is investigated by examining both exact and asymptotic solutions for the sound field behind a rigid cylinder. Depending on the sign of the gradient, the curvature due to refraction over the surface of the obstacle either adds or subtracts from the geometric surface curvature and thereby alters the shadow zone of the obstacle. Results of exact and asymptotic solutions are found to display similar trends. Implications regarding improved acoustic performance of barriers are discussed.