A method for determining a velocity of an object (O), such as a ball, over a span length (SL) extending from a span start (SS) to a span end (SE). An audio signal (AS) is received by a mobile terminal's (MT) microphone (MP). An approximate travel time (TT) for a traversal of the span length (SL) by the object (O) is determined, which act time comprises processing the audio signal (AS) and detecting a first shape (210) and a second shape (212) which respectively correspond to an approximate start time (T1) and approximate end time (T2) of the traversal, and determining the approximate travel time (TT) based on the detected first shape (210) and second shape (212). The object's velocity is determined based on the span length (SL) and approximate travel time (TT). The velocity may be determined by the mobile terminal or by an external server.