It is also important to remember that ToF as a metric is sensitive to changes in both the distance traveled and the speed of sound within a sample. Therefore a change in ToF through a battery could point to changes in a component's thickness, density, or stiffness. These changes are often coupled -a material may both swell and decrease in modulus during cycling, causing an increase in ToF. An example of the competing effects of material stiffening and swelling on ToF are shown in Figure .