A strange kind of stereo

Although we think of stereo as a modern invention, defence forces were using acoustic stereo detectors to listen out for enemy warplanes, before radar was invented...

Suppose for a moment that you're way back in history and you're in the middle of a war. Enemy warplanes often fly over to try and bomb you in your home, but radar has not yet been invented. How do you know when to run and hide in the air-raid shelter.

The answer is in acoustic location. Instead of bouncing radio waves off aircraft, which you don't know how to do yet, you listen out carefully for the characteristic sound of an air-raid on the way.

Obviously, the earlier you hear the planes, the more time you have to make preparations. So how do you make your hearing more sensitive? You could use a microphone, but a conventional microphone is sensitive to background noise, and if you turn up the gain all you will get is more background noise, obscuring the sound of engines.

The two-prong answer is to make your hearing more sensitive, and at the same time more directional. And since the human ear is the most sensitive microphone yet invented, you need a device that collects sound over a wide area, like a dish antenna collecting the weak signal from a satellite 22,000 miles away in space.

You can either use a funnel, or a reflector that focuses sound onto your detector. Either will work effectively, although it is probably easier to make a bigger reflector than a funnel.

But you go with the funnel and connect it via a simple tube to an acoustic headset. No electronics involved nor necessary. Now you can hear the planes loud and clear. The only trouble is that you have difficulty telling exactly where they are coming from.

You can swing the funnel from side to side to see where the signal is loudest. But what about having two funnels - one connected to each ear? Now you can listen in 'super stereo' and tell exactly where the enemy is.

But you need to get your guns pointing in the right direction, including height, in readiness for the earliest moment when you can open fire and stand a chance of a hit.

Simple, set up another pair of super-stereo funnels and orientate them vertically. Now you can hear very clearly the height of the planes. With two 'super stereo' funnel operators, you have a system almost as good as early radar.

Makes me wonder whether we have scratched the surface of what stereo is all about, even today.

Of course, you can always try out an experiment for yourself. Get two funnels, a couple of lengths of hose and some gaffer tape to hold everything together. Stick the hoses in your ears and hold the funnels at arms' length.

Don't forget to send in a photo (and a recording!).

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