World War I acoustic mirror 335m north east of Kilnsea Grange

The World War I acoustic mirror near Kilnsea Grange is a concrete relic of early air defence, standing in a field on the Yorkshire coast close to Kilnsea, where the land narrows towards Spurn Head. It is protected as a scheduled monument.

What is an acoustic mirror?

Before radar existed, the only way to detect an aircraft or airship at a distance was by ear. Engineers therefore built large concave structures, often called sound mirrors, that gathered distant engine noise and focused it on one point. A microphone placed at that focus was connected to headphones, and a trained listener could pick up the faint drone of an approaching machine well before it came into view.

The Kilnsea example is one of several concrete mirrors built along the north-east coast of England during the First World War. It is generally thought to date from 1918. A shallow dish, roughly 15 feet (about 4.5 metres) across, is formed in the concrete on the side facing the sea.

Why was it built here?

The mirror faces due east, out over the North Sea. Its position was chosen to watch the northern approaches to the Humber estuary and, beyond it, the port of Hull. Hull was an important industrial and shipping centre and had been hit by Zeppelin raids between 1915 and 1917, which killed 52 people. Early warning of raiders was a pressing concern for the local population and for the military.

Why does it matter today?

Sound mirrors were overtaken quickly by faster aircraft. By the time a plane had been located by ear, it was often already in sight, which limited the usefulness of the technique. Radar replaced it during the following decades.

Historic England records this site as one of only four known surviving mirrors of this kind in the north-east of England. It is also described as the only known example in the country that still has its sounding column, the part where the listener was positioned. That makes it a rare and informative piece of military and technological heritage.

Visiting

The mirror lies in a field north of Kilnsea, so it is best seen from the surrounding paths and lanes. Take care not to climb on the structure, as it is a protected monument. The area suits a walk combined with birdwatching, since the Spurn peninsula and the Humber estuary are well known for migrating birds. Wear sturdy footwear, bring layers, and expect an exposed coastal landscape where the wind can be strong.

Frequently asked questions

When was the Kilnsea acoustic mirror built?

It is thought to have been built in 1918, during the final year of the First World War.

What was it used for?

It was an early-warning device, used to detect the sound of approaching enemy aircraft and airships over the sea.

Is it protected?

Yes. It is a scheduled monument listed by Historic England.

What makes it unusual?

It is one of only four known survivors in the north-east of England, and the only one in the country known to retain its sounding column.

Where is it?

In a field north of Kilnsea, in the Easington area of the East Riding of Yorkshire, near the landward end of Spurn Head.