How did this ugly concrete lump on Kent’s coast help to defend the capital?

This concrete dish was once part of Britain’s national defence strategy. Known as ‘sound mirrors’ or ‘listening ears’, objects like this were used during the First World War and beyond to detect enemy aircraft.

Notice the bowl-shaped hole scooped out of the centre. This concave shape was deliberately designed to ‘catch’ sound waves coming from approaching planes. The sound waves were then relayed back through microphones to an operator, who could raise the alarm. This gave the military a fifteen-minute warning and time to organise their anti-aircraft defences.

Sound mirror at Dungeness © Explore Kent

Several sound mirrors were built along England’s east coast - in Kent, Yorkshire and Durham - to detect airborne threats from across the Channel and North Sea. The one here at Abbot’s Cliff, built in 1928, is one of seven in Kent alone, strategically positioned to detect aircraft aiming to attack London.

The system was never really successful however. Weather conditions affected how the sound waves were detected. Developments in aircraft design meant faster planes would already be too close for the ‘ears’ to hear them. In 1939, the invention of radar effectively drowned out the listening ears.   

Now abandoned, they still stand sentinel along our coast. These days they attract the attention of psychogeographers and military historians. Fashion photographers use them as the backdrop for their shoots and bands including Turin Brakes, have featured them on their album covers.

The Folkestone acoustic mirror also features in wartime artist Eric Ravilious’ 1941 watercolour Bombing the Channel Ports.

Written by Caroline Millar, Discovering Britain Project Manager, RGS-IBG

Whispering wall

Sound mirror at Abbot's Cliff, Folkestone © Caroline Millar

Folkestone Abbot's Cliff viewpoint

This large, square piece of concrete with a circular hole scooped out of it looks like a modern sculpture. Yet there’s nothing nearby to tell us the artist or any information panel to read. Without a clue to what it’s for, it towers over us mysteriously, rather like the monolith in Stanley Kubrick’s film 2001.

Here on the North Downs Way between Folkestone and Dover we are just 20 miles from the French coast. This might offer a clue as to its function.

This viewpoint is one of 12 created in partnership with Kent Downs AONB to celebrate their 50 year anniversary in 2018. 


How did this ugly concrete lump on Kent’s coast help to defend the capital? Click to reveal the answer


Abbott's Cliff, near West Hougham, Kent, CT18 7HZ

Grid reference:

TR 27083867

Getting there:

The sound mirror is on the on North Downs Way coast path between Folkestone and Dover

Keep an eye out for:

Samphire Hoe Country Park - an entirely new piece of land, created from the earth dug out for Eurotunnel.

Kent Downs AONB Anniversary Year :

The Folkestone sound mirror is in the Kent Downs Area of Outstanding Natural Beauty

This year the AONB celebrates its 50th anniversary. Find out more and Head for the Hills.

Dungeness sound mirrors:

If this has piqued your interest in sound mirrors, the site at Dungeness runs monthly photography and study trips. 

Find out more:

Andrew Grantham's website has lots of useful information on the sound mirrors 

Folkestone Abbot's Cliff viewpoint credits

This viewpoint is one of 12 new views created in partnership with Kent Downs AONB to celebrate their 50 year anniversary in 2018. The events and activities throughout the Anniversary Year of the Kent Downs Area of Outstanding Natural Beauty are supported by Heritage Lottery Fund.  

Thanks to National Lottery players, The Heritage Lottery Fund invest money to help people across the UK explore, enjoy and protect the heritage they care about - from the archaeology under our feet to the historic parks and buildings we love, from precious memories and collections to rare wildlife.   



"...spectacular remnants of a dead-end technology on the British coast" - historian Andrew Grantham