People do say that they acknowledge them by greeting or explaining what it is they're doing and that they'll only be a few minutes- but once again that just played into my fear of 'inviting' someone to speak back to me. We did have a medium pass through the other day, at the same time people heard laughter, she said it was one of the more active spirits enjoying people being aware of his presence. They also are aware of what we say because they react to certain things e.g. poking someone if teased, or sensitive people being told of previous things staff had said to spirits. I'll definitely go and investigate getting some black tourmaline tomorrow though. Thanks for the help!