Understanding how to map the feedback by fitness apps into concrete actions during the exercise performance is crucial for their effectiveness, for both inexperienced and advanced users. In this paper we focus on audio feedback for running, describing a beat-rhythm representation of the target cadence for helping the user in keeping it. We designed the feedback system in order to balance two conflicting objectives: its effectiveness in helping the user in reaching the training goal and its intrusiveness with respect to concurrent activities (e.g., listening to the music). We detail how we track the user’s cadence through standard smartphone sensors, how and when we generate the audio messages. Finally, we discuss the results of a user-study, showing effectiveness with respect to the adherence to the exercise goal and the overall usability.