Note that some contaminants are still visible as it is difficult to contain all the particles at the bottom of the electrode. Metal specks that were coated over can barely be seen in the optical image, but the acoustic image provides an immediate indication that contaminants are present as the acoustic density range increased to 0.15-0.19 g/cm 3 from Figure . Although the film has metal particles embedded, it may be treated as having an air-electrode interface with a similar acoustic density as an uncontaminated film. The metal specks would then likely act as points of concentrated mass which increase the acoustic density, as speculated for Figure . However, the incremental acoustic density is not as much as having the contaminants on top of the electrode possibly due to having a rough solid-solid interface (electrode-current collector) instead of a rough solid-air interface which could scatter better.