ECHO is made up of a microphone inputting sound from a human agent and another one selecting environmental sound above a certain threshold. Another source of input comes from GPS devices installed inside the blimps.
Which of these signals is processed depends on which state ECHO finds itself in. The system scans its surroundings continuously. The moment it detects a sound being picked up by the "human" microphone, or a sound that is above the threshold set for the "environment" microphone, then it processes the signal detected. If no signal is received from either microphone during 30 seconds, ECHO processes the signal coming from the GPS devices.
The sound is used as a signal by a sound processor. The amplitude (energy) of the sound is extracted and used as a parameter. The processor triggers a signal to a wi-fi access point, which redirects it to an array of floating blimps.
Each blimp is fixed to the floor by a long cable and floats in the air. Inside each blimp is an LED lamp, capable of lighting up with varying intensity. When the blimp receives the signal from the sound processor it lights up, with an intensity proportional to the amplitude of the originating sound.
The sound processor calculates the time it takes for the sound wave to reach each of the blimps, and sends out the corresponding control signals with the appropriate delay. The signal reaching each blimp also determines the brightness with which it shines, depending on the natural dampening of the sound wave with time. Thus the shining of the blimps represents the way in which the sound wave propagates.
Finally, the sound is played back through a speaker, after it has reached the end of the array of blimps and returned refracted by an imaginary reflecting surface, the last of the blimps. The sound is played back with the appropriate delay and dampening. Thus, the light is turned into sound again.
In a different mode, the processor receives a stream of data from the GPS device inside each blimp. A processor determines which one of them is moving the fastest and calculates the parameters necessary for the signal propagation.
The blimps will shine with an intensity proportional to the speed of the originating blimp. Thus the wind is turned into light.
When the ECHO project has run its course, the blimps are cut free, or attached to floating buoys. They continue to transmit their location and illuminate as they float through the air or the across sea. Thus the skies and sea are turned into light.