Sonifications are auditory displays that can help operators monitor safety critical data. However, there are very few guidelines that describe the design of effective sonifications. Two parameters that may impact the effectiveness of a sonification are discriminability and perceived urgency. We designed and evaluated four sonifications which varied timbre and the presence of a redundant pulse rate mapping using measures based on discriminability and perceived urgency. Our results show that participants were faster in detecting changes in a simple pure-tone timbre when compared to a more complex but natural sounding propeller engine timbre. Perceived urgency was heavily influenced by the presence of a redundant pulse rate map-ping that changed the tempo of the sonification. When the redundant mapping was present, participants rated that the sonification sounded more urgent than when the mapping was absent. Further examination of how context influences discriminability and perceived urgency can extend the findings of this study