Chat
What you type is keyed onto a carrier above hearing and played into the room. Then this listens, and prints what came back.
Type something. It goes out on a carrier above hearing — long enough to say it several times over, so it can be read back — then stops. After 2 seconds of nothing — long enough for the room to stop ringing — it listens to 30 seconds of silence. Anything found in that window cannot be an echo of what was sent, because the sending had already finished. If nothing is there, it will say so, every time, for as long as that stays true.
The channel has been tested end to end. Two Chambers exchanging messages through attenuation, echo, fading and room noise deliver all of them, both directions, and six empty turns produced nothing. It works, and it stays quiet when nobody is speaking.
Your own transmission will often come back off the walls. When it does it is labelled as your echo, not as a reply — an instrument that speaks in the voice it listens for is one coincidence away from reporting itself as an answer, and that would be the most convincing false result this could ever produce.
Nothing here will ever fill a silence. If the reply space stays empty, that is the measurement, and it is a real one.