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Monument details

HER Number:TR 13 SW 19
Type of record:Monument
Name:Hythe Sound Mirror

Summary

This ‘bowl’ mirror dates from 1929. It is a survivor of three sound mirrors constructed just below the crest of ‘The Roughs’, an area of rising ground to landward of Hythe Ranges. It was intended to give early-warning of approaching enemy aircraft by detecting the sound of their engines at long distance and formed part of a chain of partly experimental sound mirrors at points along the Kentish coast from Greatstone to Dover.

Sound location was achieved by collecting and focussing the sounds of an aircraft engine striking the concrete ‘mirror’ in a metal trumpet connected to a stethoscope worn by a listener who sat in a small chamber below the front of the bowl. By moving the trumpet and recording the angle of best reception of sound the direction of the target could be established. Then by matching this to an angle of sound taken from another of the coastal mirrors, it was possible to establish the position and height of the target aircraft and track it in flight. Fighter interceptors could then be ordered airborne. This system was tested with varying degrees of success during a number of air defence exercises in the early-mid 1930s.

Situated near the bowl mirror were an earlier 20 ft. diameter slab mirror (1922) and an experimental horizontal disc mirror (1931). There is a residue of the slab mirror, which collapsed over on its face in recent years, but no trace is evident of the disc mirror.

A now vanished timber hut erected as a control room was an early feature of the site.A concrete building a short distance SSE of the bowl mirror has been described elsewhere as a replacement control room.

Sound location by fixed detectors ceased with the establishment of RADAR stations in the later 1930s.


Grid Reference:TR 1398 3451
Map Sheet:TR13SW
Parish:HYTHE, SHEPWAY, KENT

Monument Types

Protected Status:Selected Heritage Inventory for Natural England: Hythe Sound Mirror, The Roughs, Saltwood

Full description

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TR 138345. An anti-aircraft acoustic-detection device similar to the northern device at the installation at Greatstone, Lydd (TR 02 SE 12) which is a scheduled ancient monument, survives at The Roughs, Burmarsh, Hythe (TR 13823443). The Lydd device was built for the RAF 1930-4 but went out of use with the introduction of radar by 1935. (1)

This ‘bowl’ mirror dates from 1929. It is a survivor of three sound mirrors constructed just below the crest of ‘The Roughs’, an area of rising ground to landward of Hythe Ranges. It was intended to give early-warning of approaching enemy aircraft by detecting the sound of their engines at long distance and formed part of a chain of partly experimental sound mirrors at points along the Kentish coast from Greatstone to Dover.

Sound location was achieved by collecting and focussing the sounds of an aircraft engine striking the concrete ‘mirror’ in a metal trumpet connected to a stethoscope worn by a listener who sat in a small chamber below the front of the bowl. By moving the trumpet and recording the angle of best reception of sound the direction of the target could be established. Then by matching this to an angle of sound taken from another of the coastal mirrors, it was possible to establish the position and height of the target aircraft and track it in flight. Fighter interceptors could then be ordered airborne. This system was tested with varying degrees of success during a number of air defence exercises in the early-mid 1930s.

Situated near the bowl mirror were an earlier 20 ft. diameter slab mirror (1922) and an experimental horizontal disc mirror (1931). There is a residue of the slab mirror, which collapsed over on its face in recent years, but no trace is evident of the disc mirror.

A now vanished timber hut erected as a control room was an early feature of the site.A concrete building a short distance SSE of the bowl mirror has been described elsewhere as a replacement control room.

Sound location by fixed detectors ceased with the establishment of RADAR stations in the later 1930s.

The mirror consists of a thirty-foot diameter concrete bowl angled skywards. It is supported at the back and at either side by buttresses of steel girders fixed into concrete ground pedestals and by two intervening webs of concrete, with metal supports and concrete bearers at the front of the bowl. Under the bowl and reached down steps from its western side is a small chamber for the listener.

Spalling shows the bowl to have a formwork of steel mesh into which a mortar of sand and cement was plastered. In the lower part of the front of the bowl is the cut-off pillar for the sound trumpet.

There is a detached concrete building 45 m. SSE of the mirror. It is a single room structure 10m x 4m, with a flat roof. There is a door at either end and two windows on its landward long side. The lower part of its north long wall is pierced with several rectangular apertures at floor level.

Importance
15

The mirror expresses a distinct stage of technical innovation in aircraft detection in the pre-RADAR era.

Historic importance - B

Structural survival – B


Condition
16

The mirror is in a deteriorating and spalled condition.

Current condition – D

Ownership (or tenancy)
17
Ministry of Defence

Use(s) of the site
18
Abandoned

Statutory protection
19
For consideration. See field 15. This is one of a small number of surviving sound mirrors nationally. It merits protection on historical grounds as a symbol of pre-RADAR methods of aircraft detection and, aesthetically, as a distinctive and specific architectural form.

Nature conservation interests
20

AONB: Kent Downs
SSSI: Lympne Escarpment

Public access to the site
21
Although within an army training area it is not far (500m) from a public footpath.

Relationship to other sites
22

The bowl mirror was one of a small number of sound mirrors of which there are other surviving examples at Greatstone (3) and at Abbotscliff (1).

Potential as part of an economic regeneration package 2 3None.Interpretational potential as an educational resource 24 Its accessibility and interest make it a key site for interpretation of the pre-radar acoustic detection system. Tourism potential 25. Its special technical interest and striking architectural form merits a visit possibly as part of a heritage trail.(4)

SOUND MIRROR. SRF, with 2 photographs. SRF. The Roughs. Bowl Type. Recorder- C. Urquhart, and Goodson. (5)

An Acoustical Research Station and associated sound mirrors located at Hythe (TR 134 338). The first mirror built in early 1923 was a vertically-mounted, reinforced concrete slab with a central shallow dish 20ft (6.1m) in diameter and a frontal plinth mounting for a microphone stand. The second built in 1929 was a mirror with a larger, deeper curved bowl of 30ft (9.1m) diameter. Sound mirrors were superceded by radar in the 1930s. (6)


Wessex Archaeology, 2005, Architectural Survey of the Concrete Sound Mirror, Cinque Ports Training Area, Hythe, Kent. (Unpublished document). SKE54575.

<1> DOE (IAM) Rec Form (OS Card Reference). SKE40714.

<2> Chris Blandford Associates, 1994, A259 Dymchurch to M20 (J11) Draft Brief for Archaeological Field Evaluation (Unpublished document). SKE6884.

<3> Chris Blandford Associates, 1992, A259 Dymchurch to M20 (Junction 11) Stage 1 Heritage (Unpublished document). SKE6769.

<4> Victor Smith and Andrew Saunders, 2001, Kent's Defence Heritage (Unpublished document). SKE6956.

<5> Council for British Archaeology, 2002, Defence of Britain Project (Digital archive). SKE32029.

<6> CBA, 01/01/1996, Twentieth Century Fortifications in England Vol. I.2 Anti-aircraft artillery 1914-46 - Site gazetteer WW1 (Bibliographic reference). SKE24120.

Sources and further reading

Cross-ref. Source description
---Unpublished document: Wessex Archaeology. 2005. Architectural Survey of the Concrete Sound Mirror, Cinque Ports Training Area, Hythe, Kent..
<1>OS Card Reference: DOE (IAM) Rec Form.
<2>Unpublished document: Chris Blandford Associates. 1994. A259 Dymchurch to M20 (J11) Draft Brief for Archaeological Field Evaluation.
<3>Unpublished document: Chris Blandford Associates. 1992. A259 Dymchurch to M20 (Junction 11) Stage 1 Heritage.
<4>Unpublished document: Victor Smith and Andrew Saunders. 2001. Kent's Defence Heritage.
<5>Digital archive: Council for British Archaeology. 2002. Defence of Britain Project.
<6>XYBibliographic reference: CBA. 01/01/1996. Twentieth Century Fortifications in England Vol. I.2 Anti-aircraft artillery 1914-46 - Site gazetteer WW1. [Mapped feature: #42123 sound mirror, ]