There is a listening mirror on the north (far) bank of the canal, and a speaking point on the south (near) side. The idea is that you can use the mirror to talk to someone 23 metres away on the opposite bank, but as I was on my own I couldn’t test it!
Update from Paul Devall:
It does work. We have taken many visitors across to have a go!
The information boards on the mirror and on a mini mirror-shaped stand give some brief details of the history of the acoustic mirrors in English and réflecteur acoustique in French. They suggests that you can’t visit the surviving examples, which is a bit misleading, as the Abbot’s Cliff and Redcar ones are publicly accessible, and the close by Hythe ones are on MoD land with permissive access.
The modern mirror was made by Richter Spielgeräte GmbH of Frasdorf in Germany.
The sound mirror is adjacent to the Palmarsh Footbridge over the Royal Military Canal at Pennypot, just west of Hythe in Kent. This Millennium Bridges website has some photos of the bridge, but they pre-date construction of the mirror.
Follow the canal west from Hythe, and you can’t miss the sound mirror. Alternatively there is a bus stop on the road to Dymchurch. An approximate grid reference is TR140340, or N51.065802 E1.053077