We didn't always have radar to detect incoming aircraft. During the First and Second World Wars, there were multiple attempts, and multiple apparatuses, used to detect incoming planes based on the sounds they made. Check out how people spotted enemy planes, using tubas — and modern art.
Radar seems like a natural way to locate airplanes nowadays, when it's in full swing getting commuter flights lined up on the runway and spotting nuclear submarines. But the idea of finding an aircraft using a radio wasn't always obvious. And this was especially true during the First and Second World Wars, when resources were strained and airplanes had a fairly obvious way of announcing themselves anyway. Back when pilots and engineers were too busy trying to keep planes in the air to get them to go high, or to quiet their engines, pretty much anyone could hear them coming.
That all-encompassing sound was the plane's best camouflage. The sound of a plane's engine is so far away, and reverberates off the ground so well, that it sounds like it's coming from everywhere. There's plenty of volume but no specific locality. That's where the innovation comes in.
The most famous radar-less plane-spotter of the Second World War was the war tuba — which has to win the prize for the least-intimidating instrument (heh) of war ever designed. Used primarily by the Japanese, the tuba also showed up on the southern shores of England. The war tuba looks like the bell of a tuba, but tapers down enough to fit against the human ear, or into a contraption that feeds into the human ear. It's worn in pairs — and it's actually an amazing tool for spotting planes, because it uses what we have.
In life, we judge the direction of sound by the amount of time it takes to reach both of our ears. The judgement may not be conscious. We think we hear sound pretty much simultaneously in both ears, but the timing is everything. Sound moves at a relatively sedate 1000 feet per second. Our ears are about half a foot apart. That gives us a half-a-millisecond pause before the sound hits one ear and the other. The bells of war tubas are obviously a way of harnessing all the sound possible to be gotten in a small stretch of air. It's the distance between the bells that irons out the details. If the tubas are spaced two or four feet apart, the difference between when each ear gets the sound becomes roughly that of an eighth note on a musical score. Some of the larger tuba sets have bells ten or more feet apart, allowing a second to go by between when one ear hears and the other one does. It's easy to orient yourself to that.
But not everyone likes to rely on a human ear and a tin can. One of the lasting legacies of World War I along the English coast are the sound mirrors. Giant concrete structures, they look like modern art, with one side a hunched mound, and one a rounded bowl.
The sound mirrors are placed in pairs, with a microphone at the focus of each curve. The huge rounds of the curves catch the sounds and the time it takes for each microphone to pick up the signal lets people monitoring it know the direction of incoming aircraft. Each pair of mirrors were experiments, and so they come in different configurations and sizes, but the inner curve of each is a perfect sphere.
This was also a matter of physics. A parabola allows a more specific focus than a section of sphere would, but the engineers who made these five meter high concrete mirrors couldn't manage specificity. The structures were way too big to move around, and so the only way to change their direction was to move the microphones inside the curve. A sphere works equally well for all directions. A parabola will lose efficiency unless it's pointed directly at the incoming plane, which would negate the point.
The war tubas have gone, but the sound mirrors, or listening ears, still stand to this day at a Royal Air Force site in Dungeness and in a coastal town in Kent. If microphones were hooked up to them, we could probably still hear incoming planes.
Image: Wiki Commons
Second Image: Wiki Commons