The standard deviation of acoustic density further supports the indication of contaminants as a source of material inhomogeneity. In the electrode with metal specks on top, the area for statistics calculation is shown as a black dashed box in Figure and had a standard deviation of 0.0122 g/cm 3 (n = 8750). This variance is statistically larger (F test = 2.2686 > F crit = 1.0343) than the variance of the electrode without contaminants (Figure : 0.0081 g/cm 3 ). For the electrode with embedded contaminants, the area used for statistics is again showed as a black dashed box in Figure with a standard deviation of 0.0123 g/cm 3 (n = 7475). Similarly, the variance of the electrode with embedded contaminants is statistically larger (F test = 2.3059 > F crit = 1.0358) than the uncontaminated electrode. Both statistical tests strongly suggest inhomogeneity due to metal particles when compared to a pristine electrode. This further highlights the capability of using the standard deviation of acoustic density as a quality control metric.