Ultrasonic transducer arrays suffer from the effects of acoustic cross coupling. The beam pattern and temporal response are degraded. The mechanism for acoustic cross coupling has been postulated to be surface wave propagation along the array. Our theory has shown that surface waves are indeed responsible, and that they may propagate in either the backing or load medium. The new theory is based on an extension to periodic systems of the theory for excitation of a uniform semi‐infinite substrate by a single line source. It provides a quantitative model for cross coupling, as well as physical insight into its mechanism.