Here's a page with interesting “accoustic mirrors” in the UK that have long been abandoned. They must have been built after recognizing the thread of incoming aircraft, but according to the page they became irrelevant once radar could do the same job.http://www.artificialowl.net/2009/03/abandoned-acoustic-mirrors-denge-uk.html
Glad to be of help. 😉 Actually, I'm fascinated by the fact that these structures – which were once the subject of considerable planning and resources – are now forgotten by time. It's amazing how what was once so “new” is now so “old”.