Headphones are placed on a rock and, along with a mic, located outside. The system is linked by cable to a mixer and speakers inside.
Observers in the room respond by listening for sound. The system captures sound,
communicates it through a mixer to speakers, and feeds the sound back by
headphones to the originators of sound, the rocks. The presence of speakers
creates ambiguity about the presence or absence of communication within an ecosystem
that includes all participants.