The signi?cance of free will

A surface wave device having a periodic frequency response characteristic is disclosed. At least two reflective grating structures are located at the surface of the surface wave device such that acoustic surface wave standing wave resonance can occur in the region between the two reflector structures. The spacing between the reflector structures is sufficiently large to insure that standing wave resonance can occur at any of a plurality of discrete resonance frequencies, all falling within the bandwidth of the grating reflectors themselves.