The strength of a reflected pulse depends mainly on the differences in density of the two materials on either side of the interface. It also depends on the sound velocity in each material, but this normally has little effect since in most tissues the velocity is within a few percent of the average 1540 m/s. Exceptions are bone (4000 m/s) and air (340 m/s). At interfaces involving bone or the lungs, most of the sound pulse is reflected.