Some acoustic cameras use two-dimensional acoustic mapping, which uses a unidirectional microphone array (e.g. a rectangle of microphones, all facing the same direction). Two-dimensional acoustic mapping works best when the surface to be examined is planar and the acoustic camera can be set up facing the surface perpendicularly. However, the surfaces of real-world objects are not often flat, and it is not always possible to optimally position the acoustic camera.[9]