1) Warning system
Found on
https://www.crosswordclues.com/clue/acoustic-mirror
An acoustic mirror is a passive device used to reflect and perhaps to focus (concentrate) sound waves. Parabolic acoustic mirrors are widely used in parabolic microphones to pick up sound from great distances, employed in surveillance and reporting of outdoor sporting events. Pairs of large parabolic acoustic mirrors which function as `whisper ga....
Found on
http://en.wikipedia.org/wiki/Acoustic_mirror
No exact match found.
