I saw a TV programme about this a while ago.

One of the guys who was developing it enjoyed pointing it down towards people on the street and speaking to them. He said they got really creeped out and started looking around to see if anyone else heard it.

I think it'd be more fun to involve two people and two speakers. Have a sort of devil and angel think going on.

"Kill that guy over there!"
"No, don't do it"
"Go on, don't listen to that pussy!"