Sensitivity in sound propagation modeling is a measure of the ability of environmental parameters to affect the sound field. The most sensitive parameters are often considered most important because errors or uncertainties in their values are among the most likely sources of error in the model predictions. There are many ways to define the measure of sensitivity. It generally depends on the sound field itself, on the energy flow through regions where the geoacoustic parameters under consideration lie, and on the way the field is monitored by receivers. But it is useful to define sensitivity without reference to a particular field or source--receiver configuration, as if it were a parameter of the environment alone. Then conclusions drawn from a sensitivity study apply as much as possible to all source--receiver configurations, and for both inverse and forward modeling. Such a measure has been derived using an adiabatic mode model for range-dependent media, exploiting the fact that the normal modes of vibration embody the essential properties of the field without reference to a particular source--receiver configuration. The new measure is demonstrated for range-dependent media using computer simulations.