The social nature of laughter invites people to laugh together. This joint vocal action often results in overlapping laughter. In this paper, we show that the acoustics of overlapping laughter are different from non-overlapping laughter. We found that overlapping laughs are stronger prosodically marked than non-overlapping ones, in terms of higher values for duration, mean F0, mean and maximum intensity, and the amount of voicing. This effect is intensified by the number of people joining in the laughter event, which suggests that entrainment is at work. We also found that group size affects the amount of overlapping laughs which illustrates the contagious nature of laughter. Finally, people appear to join laughter simultaneously at a delay of approximately 500 ms: this means that spoken dialogue systems have some time to decide how to respond to a user's laugh.

Index Terms: laughter, conversation, overlap, entrainment