Understanding the stress dependence of elastic rock properties is an essential tool for various applications in the earth sciences and the resource industries. The stress sensitivity approach presented here derives the nonlinear stress-strain relationship for a porous medium, by considering the stress induced changes of the pore geometry. We compare the stress dependence of ultrasonic velocity measurements for three rock samples that were specifically chosen for validation of the stress sensitivity approach. We used a low porosity granite,as a rock that satisfies the assumptions of the stress sensitivity approach; a clean sandstone, which strains the assumption of low porosity; and a clay-rich sandstone, that no longer can be considered as a homogenous rock. We incorporated microstructural analysis into the interpretation of the ultrasonic velocity measurements. Interpretation of the velocity measurements is straight forward for the granite. Surprisingly even porosities above 20% do not limit the interpretive power of the stress sensitivity approach. However, the influence of a high clay content does complicate the interpretation of our experimental results and further investigation maybe needed.