Whispering Arch, Grand Central Station

A whispering gallery found in a surprising place, Grand Central Terminal in New York.

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There’s an area under 4 archways, on the way down to the lower concourse, where you can experience an amazing sound effect. If you and a friend stand at opposite ends of the underpass and one of you speaks towards the wall at a normal volume, the other person can hear you perfectly even though you are a good 10 metres away and facing in the opposite direction. The stone walls and ceiling do a great job of reflecting the sound on a path across to the opposite side of the underpass. For more on the cause of this effect, see Whispering Gallery in St Paul’s Cathedral.

Logistics and location

In the lower concourse outside the Oyster bar. Whispering galleries need to be visited when it isn’t too busy otherwise the effect can’t be heard above other noise.

Sources

  • Site suggested by Charlie Mydlarz
  • Photo Nick Gray

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.