The models used in the regression analysis constitute an extensive search for a data generating distribution that is characteristic of the whole cell population (i.e. characteristic of all cells sold by the manufacturer under a specific catalogue name). Experimentation with a large number of models, having a broad range of model capacities, and using a variety of feature configurations, is what makes this search extensive. The results suggest that such a data generating distribution could not be discovered, and the reasons can be two-fold. Either the dataset itself was insufficient to characterise it; or such a data generating distribution does not exist (meaning that there are no patterns between acoustic features and the SoC that are common across many cells). The latter possibility, which would be an inherent limitation of the acoustic method, should not be readily dismissed. Cell design, manufacturing, and quality control, generally aim to produce cells with consistent electrochemical characteristics. Although this is achieved by carefully controlling the geometry and physical properties of all cell components, the effect of any manufacturing tolerance on the cell's electrochemical characteristics is not necessarily the same as the effect of that same tolerance on the acoustic characteristics. A discrepancy between these two sensitivities might exist, for example, when considering the thickness of electrode layers, the smoothness of the current collectors and their parallelism, or the amount of excess electrolyte in a cell. Investigating this possible discrepancy between acoustic and electrochemical sensitivities to manufacturing would be useful ground for future research.