From our previous study, the average phase velocities calculated from SAW signals were , , and which yields a Young’s modulus of , , and for the 3%, 2%, and 1% phantoms, respectively.29 In Fig. 3(a), the estimated slopes of FFT amplitude curves for 1% agar layer and 2% agar layer are and , respectively. The slope ratio of 2% and 1% is 2.64, which approximates the inverse ratio of Young’s modulus of the two concentrations (). Similarly in Fig. 3(b), the estimated slope of FFT amplitude curves for 2% agar layer and 3% agar layer is and . The ratio is 2.58, which approximates the inverse ratio of Young’s modulus of 3% and 2% agar (). We then try the phantom of agar over agar, which shows a more significant difference in Young’s modulus (results shown in Fig. 4). As expected, the slope ratio of 1% agar and 3% agar vibration amplitude is 6.64 (), which is similar to the inverse ration of Young’s modulus of 3% and 1% agar ().