Think of a bat. It clicks and the reverb is what its ears interpret to understand surroundings. Output can be used to trigger/power passive listening devices. I

Think of a bat. It clicks and the reverb is what its ears interpret to understand surroundings. Output can be used to trigger/power passive listening devices. I always think of the Russian gift to the US embassy that was a wooden seal with a passive bug in it. Scans didn't catch it for years.