An ultrasound head consists of many individual, densly packed ultrasound sources. From these slightly different directions, the ultrasound picture is made. This picture is only sharp if all the ultrasound sources are aimed at the same focus point at the same time. That's why the time of flight of an ultrasound wave from its emission until its desired destination, the focus point, is important. If this time of flight is known, the individual sources can send out signals with just the right delays so that all waves reach the focus point at the same time. We try to calculate this time of flight from specific medium parameters so that it can be used for making sharp images. And if the needed medium parameters are not known, they can be reconstructed by several measurements.