Japanese early warning systems---pre radar.

[emgcr]

Here is an amusing sight. Might they make wonderful gramophone horns if used in reverse ?! Not exactly exponential...............

[ImperialGuardsman]

How did they work? Did they measure sound or pressure waves in order to determine if an aircraft was in the sky?

[De Soto Frank]

The carriages look like they were repurposed from Model T Ford running-gear...



Even Old Deaf Tom should have been able to hear with an ear-trumpet like these !

[banjo_picker]

How did they work? Did they measure sound or pressure waves in order to determine if an aircraft was in the sky?

Apparently it collected low frequency sound, and piped it to an earpiece that somebody would listen to. They would pick up the sound from a bomber's engines.

[emgcr]

How did they work? Did they measure sound or pressure waves in order to determine if an aircraft was in the sky?

Yes, just like a giant magnifying ear trumpet. In the UK, we had (and still have) equivalent concrete structures along the coast facing Holland, Germany, Belgium and France for similar purposes.

[estott]

Armies tried a remarkable array of acoustic listening devices: https://www.google.com/search?q=army+li ... 66&bih=643

[emgcr]

Here are some of the English "sound mirrors".

http://www.andrewgrantham.co.uk/soundmi ... ons/denge/

[Curt A]

Then in 1964, Dr. Amar G. Bose, professor of electrical engineering at the Massachusetts Institute of Technology looked at this picture and decided to turn this into a home audio system... the birth of the acoustic wave technology.