context: Based on eqn (12), the effective stiffness can be determined if the wave velocity and the material density are known. The wave velocity can be determined from the first arrival of the wave and cell thickness. To confirm that the measured wave velocity is accurate, calibration metals of known thicknesses and wave velocities were tested. Table 4 indicates metal foil thicknesses above 500 μm in thickness are accurately measured, whereas foil thicknesses less than 250 μm are underestimated. We attribute this error to the greater impact of the acoustic gel couplant at these lower thicknesses. The liquid gel couplant, which is necessary to induce low acoustic attenuation at the interface, is of a finite thickness and should be accounted for. Liquid gel couplant typically has a relatively lower wave velocity of around 1500 m s−1 (similar to water) and would therefore result in a significant underestimation of the wave velocity for thin metal foils where the couplant contributes to a greater proportion of the total propagation path. Fortunately, pouch cells are 500 μm thick at the minimum and can be accurately measured. To confirm the consistency of results regardless of battery thickness, pouch cells were constructed with n = 1 to n = 30 layers, with one layer (n = 1) being defined as: anode + separator + cathode. The subsequent layer is then: cathode (other side) + separator + anode. n = 30 is the full 210 mA h LiCoO2/graphite pouch cell, with 15 double-sided cathodes and 16 double-sided anodes. A schematic of the configuration is demonstrated in Fig. 1. In Fig. 3b, the last data point corresponds to n = 34, which was obtained from a slightly thicker commercial pouch cell of the same chemistry and configuration. The resulting thickness vs. first break (Fig. 3b) shows a linear relationship, indicating a constant wave velocity of approximately 1700 m s−1 and resulting in a calculated effective stiffness of 4.76 GPa (Fig. 3a). Therefore, the measured wave velocity and the resulting effective stiffness is confirmed to be the same regardless of how many repeating cell layers there are, and thicker cell stacks do not slow down the wave velocity. The measurement of 4.76 GPa is comparable to a prior ex situ study by Knehr and Hodson, where a digital caliper was used to measure the pouch cell thickness. The careful calibration and confirmatory studies here demonstrate the reliability of the acoustic measurement not only for relative shifts but also in calculating an intrinsic material stiffness.