There is a mention of sound mirrors in the article Mirrors and smoke: A. V. Hill, his Brigands, and the science of anti-aircraft gunnery in World War I, William Van der Kloot. DOI: 10.1098/rsnr.2010.0090 Published 20 July 2011.
… One of the largest projects funded by the Aggregate Levy Sustainability Fund and managed by English Heritage was concerned with stabilizing these structures and undertaking research into their repair.
This paper aims to outline the conservation approach to the project and to detail the concrete repair techniques trialled. It also highlights some pointers for the repair of twentieth-century concrete based on the advice of a master mason and a concrete repair contractor. Finally, the long-term monitoring that is in place for the carbonation inhibitors and cathodic protection systems that are installed on these structures are detailed.
The Listening Mirrors – A Conservation Approach to Concrete Repair Techniques by Alan Wright and Peter Kendall. Journal of Architectural Conservation, Volume 14, Issue 1, March 2008
Douglas Butler illustrates how to create an Autograph file to test the parabolicality of a sound mirror, at the Association of Teachers of Mathematics website.
Alec Muffett at Dropsafe on sound mirrors.
Britain is a hothouse of brains and creativity, doubly-so for having to make-do-and-mend from underinvesment and underappreciation, and this leads to startling solutions that fuel incredible innovation – even if most of those subsequently flop for lack of business nous.
Occasionally, these innovations leave footprints in the sand. Bletchley Park is onesuch. Another of which I have long known a little, but never known a lot, are the sound mirrors.
The Whipple Museum of the History of Science in Cambridge has a webpage about some parabolic sound mirrors to reflect and focus sound.
This apparatus consists of a pair of brass parabolic reflectors mounted on wooden stands. One of the reflectors has a bracket with a hook for hanging a pocket watch. The demonstrator would use this apparatus to show how the sound of a ticking watch may be heard at a considerable distance having first been made into a beam, projected over a distance and refocused. Very little is known about this pair of mirrors in the Whipple’s collection. It is thought that they were made in France or Germany during the second quarter of the nineteenth century.
Source: Whipple Museum of the History of Science