Much of the existing literature is focused on reducing waveforms to specific features, investigating their individual dependencies on the SoC, and possibly on additional parameters such as the temperature and C-rate. Gold et al. performed tests at relatively low ultrasonic frequencies, near 200 kHz, where they observed the separation of travelling waveforms into a fast and a slow wave according to Biot theory . They then argued in favour of using features of the slow wave and demonstrated state estimation by linear regression. Wei et al. also searched for a feature that would be linear in the SoC, and their recommendation was termed the 'initial rise time' -the time between the front and the centroid of the transmitted signal.