The enemy always keeps coming. That’s part of what being an enemy is about; they come, and we really don’t want them to. Or it’s us doing the coming and them doing the not-wanting-us-to-come-ing. Either way enemies are always unwanted, and so we’ve devised ways of alerting the people upon hostile invasion.
To begin with, we shouted. Then, we shouted louder, as this proved more effective. Then, cupping our hands, banging on drums, smoke singals, lighting fires on mountaintops, all that. With the radio invented, we soon heard the booming radiograph alarm signal telling us a distress message is about to be transmitted and would you please turn your radio on right away and pay attention because the government is about to tell you something bloody important. This is still the best solution we’ve got, but as with all forms of alarm, its effect depends on it being well ahead of the danger it’s announcing. So we had to come up with ways for being ahead of time, and this is where it gets interesting. Before the radar, would you know, we used fancy hearing aids!
I’ll let the pictures speak for themselves, please visit WebUrbanist.com or DamnInteresting.com for more insight.

A.M.Mayer's patent from 1879. Image via Michael Leong.

Radar O'Reilly. Image via DarkRoastedBlend

Acoustic Locator - Czechoslovakia circa 1920

Two-horn aircraft locator, Washington D.C., 1921

Japanese emperor Hirohito inspecting his hearing aid. Impressive.

Concrete sound mirror at Dungeness, England. Photo: David Barrington