Given the number of audio recordings involving members of the University of Hong Kong council leaked recently, one fears Hong Kong will become a place where everybody either watches what they say or chooses not to say anything for fear of being tried and fried by public opinion. The obvious question – who’s been doing the recording? – on everyone’s minds notwithstanding, it can’t be that hard to screen for listening devices.

Clandestine listening devices in ancient China were simple cylindrical tubes pressed against the wall, which gave rise to the saying geqiang you’er (“the walls have ears”).

“Listening urns”, which were detailed in a military treatise some 2,500 years ago, were used on battlefields to provide advance warning of enemy approach. A wide-bodied urn would be buried with its small opening above ground, over which a thin piece of leather was stretched. By pressing one’s ear to the leather, one could detect the direction from which an enemy was approaching. For precision, huge urns were used, with someone sitting inside, at times. The visually impaired were preferred, for their supposedly acute sense of hearing.