Acoustic-surface-wave bandpass filters

Acoustic surface waves in solids are a form of mechanical transport of energy in which the physical disturbance in the material is very closely confined to the surface. Their properties were first investigated theoretically towards the end of last century by Lord Rayleigh and hence they are also known as Rayleigh waves. Until the last 10 or 15 years, interest in them was largely confined to seismologists as they are generated in the earth´s surface by earthquakes. Their attraction for signal-processing applications lies in the facts that they are accessible, since they are on the surface rather than in the bulk of a material, and that, as their wavelengths are typically 105 less than those of electromagnetic waves, devices employing them are correspondingly small. Typical wavelengths for acoustic surface waves are in the range 15¿30 ¿m at 100 MHz, and this gives the order of the dimensions of surface-wave devices