Listen carefully

[ppppenguin]

A guy has photographed all the remaining "sound mirror" aircraft detectors that were built in the approach to WW2. Though the article claims they were built in WW1, well before any practical radar. I'm not sure on that point.
https://www.bbc.co.uk/news/in-pictures-46348917

As we now know, radar was how it was really done in WW2 but both sound and infra red offered possible alternatives. RV Jones discusses both in his excellent book "Most Secret War". He worked on IR methods which weren't developed as practical systems until rather later. "LIDAR". It's hard to be sure if the sound mirrors and other acoustic detectors were a panic measure or a kind of hoax to give cover for radar. Possibly a bit of both. They had a very limited range, perhaps 3 miles maximum, nothng like enough time to dispatch RAF fighters to meet an incoming threat. They also gave very little information about range. Since sound travels slowly, about 1 mile per second, the information they give was rather historical -  they told you where the aircraft was, not where it is.

[JohnBS]

I'm suprised the larger mirrors would only provice a 3 mile range.

BTW, I think the velocity of sound in air is c 340m/s, which is much slower than a mile /sec (more like 4.5 seconds/mile). In fact it's so slow you can see a wavefront advance: if you watch the video of Queen at Live Aid during "We will rock you" , the audience is clapping in time to the beat, and you can see the "wave" advance from the stage!

John

[Murphyv310]

There was a sound mirror built at the gunnery at Loch Doon circa 1919 I've a photo taken by my grandfather in the loft. I'm not sure that they were ever used to detect enemy air craft but to confuse them. Sadly my grandfather is long gone as is all the relatives that can help on this. When Anne is better I'll dig out the photo and have a better look.
http://www.secretscotland.org.uk/index.p...nerySchool

[ppppenguin]

John, you're right about the speed of sound. Wasn't thinking straight when I made my original post.

This article has some truly wonderful photos. Caution, you may find yourself laughing out loud: http://www.douglas-self.com/MUSEUM/COMMS/ear/ear.htm

In the UK, the true visionary was Henry Tizard. He was convinced that radar was practicable and formulated the concept of early detection with fighters taking off in time to intercept. His name came into common useage in WW2 due to the "Tizzy Angle", a simple but brilliant idea for vectoring the defending fighters on to the attacking aircraft: http://beyourfinest.com/the-tizzy-angle/

Dowding was the other great figure in this work, who devised what was probably the world's first combined command and control system. This was able to make use of all the info from radar and dispatch our fighters to do their job. The plotting table at the heart of this is a staple for WW2 movies.

The alternative method of defending against enemy aircraft was the old concept of the standing patrol. This would have been hugely expensive in planes, fuel and pilots.

PS: This article suggest a usable range of 5 miles for acoustic locators: https://en.wikipedia.org/wiki/Dowding_system

[Murphyv310]

I'm not sure that they were ever used to detect enemy air craft but to confuse them.
http://www.secretscotland.org.uk/index.p...nerySchool

Please ignore that sentence, I'd been looking at something else and added this by mistake.
Sorry.

[Crackle]

This a copy of a post I put on UKVRRR, where they have also discussed the subject. https://www.vintage-radio.net/forum/show...p?t=152927

They have a couple of demonstration sound mirrors in Wat Tyler park Pitsea, Essex.
I went there today for a stroll and we filmed the mirrors.
Here are a couple of photos,

The mirrors are about 100m apart and some thick scrub bushes have grown up between them, masking them so they are difficult to see from each other.
       
But they do work, if you stand in front of each mirror and talk in a normal voice you can clearly hear one another over the distance of about 100m.
It is quite critical where you listen from, if you move your head forward or backward by more than 6 inches then the sound fades away.
Here is a short demonstration, the video is confusing because Gill could not make out where I was to point the camera. But the audio comes out clearly.
https://www.youtube.com/watch?v=D2tTDYGkiJ8

Mike

[Terry]

Here's a more permanent st-up at Joddrel Bank. They work very well indeed!