in the reflection path 26 to detect the reflected surface waves and to provide an output signal representative thereof at output terminals generally indicated at 30 of course if a second reflection of the surface waves is not desired there is no need for the second acoustic discontinuity 12 39 and the output transducer 28 may be disposed in the reflection path 22 in operation an electrical signal supplied from the input signal source 18 to the input transducer 16 generates surface waves which propagate along the propagation path 20 as was previously mentioned undesirable bulk waves may also be produced the surface waves strike the reflecting surface defined by the acoustic discontinuity 12 and are reflected along the reflection path 22 if the device is to be employed for relatively short highly accurate signal delays or for other applications in which bulk wave discrimination is the primary concern the acoustic discontinuity 12 may extend beneath the surface 14 of the substrate 10 by an amount sufficient to reflect a substantial portion of the acoustic surface wave energy propagating along the path 20 without reflecting a substantial amount of bulk wave energy thus most of the bulk wave energy will continue in the direction of the propagating path 20 beyond the acoustic discontinuity 12 and only relatively pure surface mode energy will be reflected along the reflection path 22 if a.