The occurrence of microcracks and pore space reduces the P and S wave velocities in rocks from those expected for the intrinsic mineral matrix. In the low-pressure regime, before microcracks close, the elastic properties and wave velocities are characteristically nonlinear. This nonlinear behavior reflects the effects of the crack closure spectra on the P and S wave velocities. Several efforts have been made in the past to describe or model this behavior using phenomenological relations. However, success has been limited to a rather restricted pressure range. In the present study, two simple relations are considered that have a clear physical basis and are convenient to use and interpret. Both are tied to the functional form V2=A+BP+Ce-P/&tgr;. The fitting parameters A and B can be identified directly as second- and third-order elastic properties of the intrinsic mineral matrix of the rock specimen, and C and &tgr; describe the nature of the microcrack distribution. The results from fitting a large number of published igneous rock data sets indicate that the relations studied yield rms errors that are (1) always within the stated accuracy of the data, (2) generally within the stated precision of the data, (3) comparable in ''goodness of fit'' to previously published but more complex relations over limited pressure range data sets, and (4) clearly superior over extended pressure ranges. Because of their comparative simplicity, demonstrated accuracy, and clearly defined fitting parameters, they are well suited for systematic studies of large numbers of igneous rock data sets. ¿ American Geophysical Union 1996