4 devices with a knock sensor and a knocking actuator that knocks on the surface where the device is located. The devices communicate to each other wirelessly.
They can be put on or under a table, staircase, ground (preferred wood) in separated rooms in the mill.
When someone knocks a short rhythm on the surface near one device, it will record this sequence and transmit it to the others. If somebody in the other room hear the sequence and repeats this sequence knocking, a 1 to 1 connection will be established and the two communication members will be able to communicate knocking without seeing who is the partner.
Living and working in Kassel, Germany
Making energy-art: with high voltage, incredible loud sound, flamethrowers or smoke