Ultra high frequency (similar to 1GHz) ultrasonic bulk acoustic waves (BAW) characterization has been already integrated in a lab-on-a-chip silicon platform. The acoustic wave guided in three dimensions (3D) was achieved via 45 degrees mirrors in a silicon wafer and applied for the characterization of chemical solution and fluids actuation in micro-channel. The main problem is that strong conversion mode occurs at the silicon-air interface with the incidence angle of 45 degrees. To avoid conversion mode we need to optimize the reflection on the 45 degrees mirrors in order to maximize the acoustic energy of the longitudinal wave and reduce signal to noise ratio. We demonstrated that longitudinal wave transmission is efficiently strengthened using gold coating (average gain factor of 8 compared to reference case without mirror coating). The experimental validation of the losses improvement is achieved using S-21 scattering parameter in a transmission acoustic integrated system. (C) 2015 The Authors. Published by Elsevier B.V.