Author: Trevor Cox

I am a Professor of Acoustic Engineering at the University of Salford where I carry out research and teaching focussing on architectural acoustics, signal processing and audio perception. I am also an author and radio broadcaster having presented many documentaries on BBC radio and written books for academics and the general public.

Sound Mirrors

Acoustic mirrors were an attempt to detect enemy aircraft flying towards England in the early twentieth century.

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These amazing structures are sound mirrors. Concrete concave dishes designed to capture the sound of incoming enemy aircraft as they flew over the north sea and the channel towards England.  Acting on sound like a concave shaving mirror focusses light, the sound from the aircraft is concentrated to a point where a microphone picks up the sound.  The largest structures allowed aircraft to be detected 6.5 miles away, as well as determing the direction of attack to an accuracy of 1.5 degrees [4]. Overall they were not terribly effective, however, and became obsolete and abandoned when RADAR was invented.

Sound mirror
Sound mirror, Kilsnea, Yorkshire {1}. The focussing point where the sound is concentrated is at the top of the metal pole.

At Kilnsea, Yorkshire, UK the mirror is about 4.5m high. It was used in World War I to try and pick up the engine noise from Zeplin aircraft. This gave 3-4 minutes of extra warning before attack [2]. Zeppelins raided the North East of England fifteen times between 1915 and 1917.

Sound mirrors denge
3 Sound mirrors, Denge {3}

At Denge, Kent, UK there are three mirrors. The various acoustic mirrors were constructed in the 1920s & 1930s. The different designs are evidence of experimentation to discover which shape and size worked best. There are a 6m concrete concave mirror, a 9m hemispherical bowl and a curved 60m long mirror.

Logistics

Easy to visit

Abbot's cliff sound mirror {6}
Abbot’s cliff sound mirror {6}

1. Abbot’s Cliff, Kent, UK: grid reference TR27083867. A ten minute walk from the Folkestone – Dover road along a tarmac path [5].

Sound mirror at Hythe [6]
Sound mirror at Hythe {6}

2. The Roughs near Hythe, Kent, UK are on MoD property but several websites say the dishes can be visited. The picture is of a 9m mirror, a smaller 6m mirror lies nearby on its face [5].

3. The Redcar sound mirror is in a modern housing estate at the junction of Holyhead Drive and Greenstones Road.

Sound mirror Sunderland
Sound mirror Sunderland {8}

4. In Sunderland alongside a bridleway about 200 metres west off the Newcastle Road, Fulwell.

On private land and probably only viewable from a distance

1. Boulby, Yorkshire, UK: West of Boulby Barns Cottage on Boulby Bank. On private land, although the rear of the mirror is visible from a nearby minor road [5].

2. Kilnsea, Yorkshire, UK: Not far from a road but on private land.

3. Il Widna (“The Ear”), Malta: On private land but can be seen from a distance through a fence [5]. Large curved strip mirror similar to the large device at Denge.

Accessible only on a guided tour

Denge, near Dungeness, Kent, UK. These are not accessible to the public except via guided walks by. [5]

Sources & Links

Website dedicated to sound mirrors

[1] Photo Paul Glazzard, Creative Commons Attribution-ShareAlike 2.0 license.

[2] http://www.bbc.co.uk/humber/content/articles/2008/10/15/kilnseaabandoned_feature.shtml

[3] Photo Paul Russon, Creative Commons Attribution-ShareAlike 2.0 license.

[4] http://www.southdown-amateur-radio-society.org.uk/HTML/Soundmirrors.html

[5] http://www.andrewgrantham.co.uk/soundmirrors/

[6] Abbot’s Cliff and  Hythe photos Between a Rock, Creative Commons Attribution License.

[7] Redcarr photo © Copyright Mick Garratt and licensed for reuse under this Creative Commons Licence.

[8] Sunderland sound mirror photo, Phil Thirkell, Creative Commons License Attribution-Non-Commercial 2.0 Generic.

Great Court British Museum

An amazing cacophony in the largest covered public square in Europe 

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This place has a remarkable acoustic which complements the beautiful architecture. Opened in 2000, the court wraps around the circular central reading room, providing a huge circulation space with cafes, information points and shops. The sound of people talking and walking echoes throughout the space. The soundscape is a cacophony of indistinct speech and other blurred sounds.

Large reverberant spaces are familiar to us all, but the sheer size of the Great Court along with the number of people creating the sound makes this space ear-conic. The only downside to the great sound effect is that it makes conversations difficult to hold in the cafes – as I found out when I foolishly tried to hold a business meeting there.

Incidentally, audio nerds can also wander around and look for the Intellivox loudspeakers; an interesting technology which attempts to make public address announcements intelligible in cavernous spaces.

Logistics

The British Museum, Great Russell Street, London, WC1B 3DG. Can only be visited when the museum is open. The Great Court is free to enter. Most impressive when busiest (weekends, school holidays etc.)

Credits

  1. http://www.britishmuseum.org/the_museum/history_and_the_building/great_court.aspx
  2. Photo: M.chohan
  3. Sound ERH (c) some rights reserved