24. The mobile face-to-face interaction monitoring method according to claim 23 , wherein the creating the volume topography based on the sound signals is performed by using a feature vector P(t), wherein the feature vector P(t) is defined as P(t)=(p(t,1), p(t,2), . . . , p(t,np)), where p(t, i) is an average of a square of the sound signals in each mobile device i of the mobile devices at a given time t, and where np is a quantity of the mobile devices in the conversation group.
24. The mobile face-to-face interaction monitoring method according to claim 23 , wherein the creating the volume topography based on the sound signals is performed by using a feature vector P(t), wherein the feature vector P(t) is defined as P(t)=(p(t,1), p(t,2), . . . , p(t,np)), where p(t, i) is an average of a square of the sound signals in each mobile device i of the mobile devices at a given time t, and where np is a quantity of the mobile devices in the conversation group. 25. The mobile face-to-face interaction monitoring method according to claim 24 , wherein the quantity np of the mobile devices is equal to or less than a number of the interactants in the conversation group.