The Greek historian Herodotus tells of a Persian siege of a Greek colony called Barcus, in North Africa, in 512 BC. During the siege the Persians would tunnel under the city walls, hoping to fill the tunnel with firewood, and when ready the fire in the tunnel would burn the wooden support bracing holding up the tunnel, and the tunnel would collapse, and hopefully the wall above it would also collapse.
This graphic shows typical bronze shields used by the Greeks.
A bronze worker in the city devised a method of detecting the location of hostile tunneling, using a bronze shield "covered over with bronze." Maybe that means they stretched a thin layer of bronze over the back of the shield, to amplify any vibrations as sound. If so, this would be an ancient world speaker, or amplifier.
The defenders of the city would carry the speaker around the wall, and "applied it to the ground". When the shield was near active tunneling activity, the shield made a sound. The defenders of Barca then dug a tunnel from their side, and entered the tunnel and slayed the Persian tunnelers.