Ultrasonic attenuation in acoustic touch panels

Acoustic touch panels are increasingly used in commercial display monitors as user-friendly touch sensitive input devices. Water droplets can produce false touch signals on conventional touch panels which use Rayleigh waves on glass. To avoid this defect, touch panels are now being developed that use Love waves rather than Rayleigh waves. This paper presents a quantitative investigation of the sensitivity of Rayleigh waves and Love waves to water and finger touches as well as oil. Both theoretical and experimental results are presented. In the theoretical analysis, the influence of water on Rayleigh waves is analyzed using the leaky wave model. Damping of Love waves by water is modeled by a perturbation calculation of power transfer to a viscous fluid. Experimental measurements of attenuation for both Rayleigh and Love waves are based on the observation of signal amplitude changes when material is placed on the acoustic path between a pair of wedge transducers. Results confirm that Love-wave touch panels are indeed much more immune to water contamination than Rayleigh-wave touch panels. The non-zero viscosity of water leads to a small amount of Love wave damping by water. Data suggests that with appropriate thresholds, Love-wave touch panels may respond to finger touches while rejecting signals due to water and even light oils. A direction for future research is the theoretical calculation of a finger touch sensitivity of Love-waves using a visco-elastic model of the human finger.