Abstract: This paper introduces the concept of bubble factors for assessing the communication bubble created by a near-field magnetic induction (NFMI) communication system. First, the coupling coefficient as a function of distance between two magnetic transmitters is derived and used to show that the induced magnetic field reduces in proportion to the inverse sixth power of distance. This idea is used to define and analyse the communication bubble around the source. Three bubble factors are defined and shown to provide the best approach for quantifying the cone of silence around the transmitter and receiver. The decaying power based on the distance-bubble factor and receiver-load- bubble-factor shows that the transmitted power reduces by 60.25 dB/m. This provides the basis for receiver design and the distance at which interception of the NFMI communication is most secure.