This is an idea that some bus travellers appear to believe in, that there is some magical bubble that surrounds only the people they are talking to, holding the conversation within its walls . It is a myth, there is no magic sound inhibiting force field which prevents other passengers from hearing your conversations. Although I often wish there were!
Also a myth the idea that we can choose not to hear someone else's conversation no matter what the volume of the participants. I have heard brave souls challenging the blue content of another passengers language admonished with that myth, "well if you don't want to hear stop listening to a private conversation", only those with hearing aids they can turn off can truly manage that. Those using in ear music devices may also be able to tune out literally but to what damage to their ears as they up the volume.
A conversation not held in private, i.e only the people you want involved present, is not a private conversation! This is beyond the exchanges of bus friends and ventures some distance into exhibitionism and for the other passengers forced voyeurism. Discussions of such a personal nature they astound me. Sexual exploits including affairs are regularly aired, criminal activities referred to, prison expectations on the way to court, results of meetings with social services the list goes on. Of course it is not just the topics that can be uncomfortable to be privy to, the language used can be considerably less than edifying.
Those using mobile phones appear to believe in a variation of the bubble, that as they are speaking into a phone the sound is only going as directed and not that their half of the conversation is easily audible to all on board. Mind some are so loud I often wonder if they need a phone at all and can probably be heard by the recipient anyway.
My tolerance of others mythical bubbles tends to relate directly to the other recipients on the bus of this assault to the ears and sensibilities. It is considerably diminished by the presence of children who hear and remember anything adults would prefer they did not.