accousticmirror

Scheduled Entry text
Monument: World War 1 early warning acoustic mirror on Namey Hill, 570m north of Carley Hill Cricket Ground
Parish: Sunderland
District: Sunderland
County: Tyne and Wear
National monument number: 34835
National grid reference(s): NZ38945960

Description of monument
The monument includes an early 20th century military early warning device known as a sound mirror.  It is located on a gently sloping hillside 2km inland from the coast on the block of land between the Tyne and Wear estuaries.  The mirror was part of a chain of similar acoustic devices located on the north east coast extending from the Tyne to the Humber.  They were erected to provide early warning of potential attacks on the important industrial complexes in the north east from ships and Zeppelins during World War 1.  Little is currently known of the history and development of this particular system and it remains something of an enigma.  Successful experiments in acoustic detection date to 1915 and it is thought that the Tees/Tyne early warning system dates to the last two years of the war.  This mirror faces east and was positioned to cover the approaches to the Tyne and Wear estuaries.  There were probably other mirrors as part of the Tyne and Wear defensive chain but the location of these is currently unknown. 

The mirror is a 'U' - shaped, concrete built structure comprising a thick wall with an inclined face and a shallow concave bowl shaped into its centre.  On either side of the wall are projecting flanking walls, which helped to protect the reflection from noise interference and also supported the structure.  The reflector is a smooth bowl 4.5m in diameter, inclined approximately 11 degrees to the vertical.  The rear wall is 5.8m in length and is 4m high.  The two flanking walls are 3.9m long.  The reflected sound was detected by a microphone placed in front of the dish and then transmitted to the headphones of the operator who sat in a trench to the front.  The location of the operators trench is currently unknown.  It has been suggested that at this mirror the microphone was secured in front of the dish by wires attached to the side walls,  so allowing it to be variably positioned.  This differs from other mirrors in the north east where the microphone was fixed on a metal post in front of the dish.  The monument also includes a margin of 5m beyond the mirror on the eastern side in which remains of the operators trench may survive and a margin of 3m on the remaining sides for the support and preservation of the monument. 

On the northern face of the mirror there is an interpretation plaque which is included in the monument.  The mirror is Listed Grade II.