Abstract: A method of detecting occurrences of a sound event in an audio signal comprising a sequence of frames of audio data, each frame corresponding to a respective time in the audio signal, the method comprising: for each frame in the sequence: determining, using the audio data of the frame, a first conditional probability value that a transition occurred from a sound event not having started to the sound event being ongoing, and a second conditional probability value that a transition occurred from a sound event being ongoing to the sound event having ended; and determining a marginal probability value that a sound event was ongoing at the time corresponding to the frame, the marginal probability value being determined using the first and second conditional probability values for the frame and a previously determined marginal probability value that a sound event was ongoing at a time corresponding to a frame preceding the frame in the sequence.