Acoustic Mirrors: How to Find Planes without RADAR

A lot of science museums and parks feature something called an acoustic mirror. The one at Houston’s Discovery Green park is called the listening vessels. [Doug Hollis] created two acoustic mirrors 70 feet apart, pointing at each other. If you stand or sit near one of the vessels, you can hear a whisper from someone near the other vessel. The limestone installations (see right) are concave and focus sound like a parabolic mirror will focus light.

mirrorJust a science curiosity, right? Maybe today, but not always. The story of these devices runs through World War II and is an object lesson in how new technology requires new ways of thinking about things.

Adapting to the New

I’ll get back to the acoustic mirrors, but first, consider how changes in technology can sometimes change how we do things in a fundamental way. That’s easy to see in retrospect. There used to be a huge industry revolving around harvesting and shipping ice from cold climates to other locations. Refrigeration killed that off. Even though there are still people who make horseshoes, it’s obvious today’s need is a shadow of the demand found prior to the automobile.

It is harder to see that while it is happening. It looks like widespread acceptance of bank cards has all but killed off the paper check, for example. It used to be common for people to use travel agents. Big companies that used to provide dial up Internet access are all but gone or–at least–transformed beyond recognition.

Recognizing the changes in real time can be the difference between success and failure. One example of this is the advent of RADAR during the second world war, and the recognition of how it was different from previous airplane detection methods like acoustic mirrors.

The World War II Connection

World War II was a time of a lot of technical innovations that changed combat. One well-known story is that the Battle of Britain was won by the timely development of RADAR. Without RADAR, the British couldn’t defend their entire airspace with the number of planes available. The whole story is a bit more complicated and–believe it or not–starts with acoustic mirrors.

chainBoth the British and the Germans had RADAR. The British, however, realized that just detecting aircraft wasn’t enough. You needed a sophisticated command and control system that could rapidly develop a picture from multiple inputs, allocate resources to threats, and dispatch them.

If you have a picture of a modern RADAR installation, you have to scale back your imagination. The British Chain Home RADAR stations sent out signals between 20 MHz and 30 MHz at 350 kW. The antennas were more like ham radio antennas than what we think of as RADAR dishes today (see left). In fact, the towers supported several dipoles and reflector wires. The transmitters were in wooden buildings.

On the other hand, the Germans did knock out at least one Chain Home station, but the towers were too hard to permanently disable and the station would return to the air quickly. The Luftwaffe decided it wasn’t worth the effort and focused on more conventional targets.

This would turn out to be a mistake. Spending time destroying the RADAR would allow future bombing raids to be far more successful and could have turned the tide of the war.

Remember Acoustic Mirrors?

amirrorWhile today the acoustic mirror is a museum curiosity, before World War II, it was a method of detecting aircraft. The mirror could focus the sound from an aircraft engine allowing early detection. There are several of these stations still on the coast of Britain (see right) and one in Malta. A microphone picked up the sound and the construction wasn’t actually parabolic, they were spherical mirrors. The reason is that a parabolic mirror has to move to determine direction, while a spherical mirror could detect direction by moving the microphone.(As an aside, the observatory at Arecibo is also a spherical reflector, but for radio waves.)

Acoustic mirrors were never very successful and as planes became faster, the detection range of the mirrors become less useful. With the advent of RADAR they quickly fell by the wayside. However, the program of using the intelligence from the mirrors (led by [William Tucker]) became the core of the British command and control system.

Many historians agree the German RADAR systems of the time were more sophisticated than the British. They featured a magnetron at 600 MHz for an emitter. They also employed modern features like lobe switching and duplexers. But the British made more effective use of RADAR thanks to [Tucker’s] methods.

Lesson Learned

We’ve all seen a guy trying to fish with an expensive set of new gear catch nothing while an old timer with a stick and a worm catches the limit. A smart engineer with a volt meter can do more than a dummy with an oscilloscope. It might be simplistic, but from a technology point of view, the British won the Battle of Britain for two reasons: they understood the ramifications of RADAR (thanks, at least in part, to previous experience with acoustic mirrors) and–perhaps more importantly–the Germans didn’t.

The point is that understanding technology is important. We all like to do that part. But sometimes, understanding how the technology fits in with people and processes and how the tech might fundamentally change things is also important. Sometimes we cling to old ways too long. Other times we develop something new and fail to see how it could be used in the best way.

As new tools are available to us, we should constantly be thinking how to best use them. Sometimes that answer will be technical and sometimes it won’t be. Are we using, say, 3D printing in the best possible way? If you want key chains and vases, maybe. But what are the fundamental ways we can use them to do something new and different? As embedded processors get more and more powerful, are we finding new and better ways to develop for them? Or are we clinging to our favorite platform and language instead of using something new and different?

World War II was a time of incredible technological development. Besides RADAR, the war saw better tanks and planes, miniature spy radios, big radios, and high-tech bomb sights.

Photo Credit: Acoustic Mirror, [Paul Glazzard] CC-BY-SA 2.0

41 thoughts on “Acoustic Mirrors: How to Find Planes without RADAR

  1. It’s not actually true that the Germans invented the cavity magnetron. They derived it from one that they recovered from a British plane.

    The British were arguably the first to invent radar in WW2, though several other countries did so independently (like Germany and japan, in Japan’s case possibly twice and at the same time), and the British were also the ones to develop the magnetron, sharing the discovery with the Americans in hopes of information on the Norden bombsight. They were also the first to develop airborne radar using the device.

    The reason for the confusion is that while the British went on to develop massively sophisticated radar systems for their airplanes, the chain home radar system remained almost exactly the same since it’s inception on through the war. So, while a British magnetron was eventually captured by the Germans (it turns out that they’re really hard to destroy with scuttling charges), the British continued to use antennas and stations which were effectively the same as the BBC broadcasting station which was used for the first trials of radar.

    1. The chain home radar system operated on HF (around 10 Meters, I think) and was used through the war because it was a OTH radar system. The magnetron sets, although having far better resolution were line-of-sight, as they operated at a far shorter wavelength.

    2. There is a big difference between a simple magnetron and the British developed cavity magnetron. The magnetron had been known for some time, the cavity magnetron was a game changer.

      1. A great book on the development of the cavity magnetron, microwave radar, and it’s use in and past the war, is “The Invention That Changed the World”. It’s very well written and I enjoyed the heck out of it when I found it by accident in the library in high school.

  2. Does someone have a link to an explanation about the statement of not being able to detect direction using a parabolic mirror? (Yes, I know there are a zillion explanations about how parabolic mirrors work, that’s not what I’m asking for.)

    I always thought that a parallel signal coming in off-axis to a parabolic mirror would focus to a point off-axis of the focal point of the mirror, so that you *could* move a microphone around to detect direction.

    Does someone have a good reference that explains this?

    1. A parabolic mirror must be moved to point the center axis directly at the target:
      https://en.wikipedia.org/wiki/Parabolic_reflector#/media/File:Parabola_with_focus_and_arbitrary_line.svg

      A movable focal axis can be used with a spherical mirror:
      https://en.wikipedia.org/wiki/Curved_mirror#/media/File:Convexmirror_raydiagram.svg

      So simplistically, you move the equipment the to determine location with a parabolic reflector, or you move the listener to determine location with a spherical reflector.

    2. Allowing some generalization:

      Basically, a parabola always focuses to a point and your detector is either at that point or it’s not. This is good for solar heating and radio/radar dishes where you intend to aim the dish and you want maximum energy in a small spot.

      A spherical mirror focuses to an image which includes elements that are on the axis as well as off the axis (similar to the way a lens focusses an image). By moving the detector, you can choose which part of the entire field of view your wish to examine. The trade off is the energy coming from the field of view may be “spread around” in the image area instead of concentrated in a small point so total detected energy may be less for a given setup.

    3. For off-axis signals, both the parabolic and spherical reflectors will focus the signal to the side of the central focus point. However, it’s my understanding that the spherical reflector will do this better across a wider range. The parabolic shape is optimized for the on-axis signal, and the more off-axis you get, the worse the signal gets. The spherical shape is not well optimized for any particular direction, but works okay across a wide range of directions.

      This is my general understanding. I may be wrong.

      1. Your absolutely correct, because on a satellite dish, which are usually parabolic, you can have multiple off axis LNBFs to receive signals from multiple satellite orbital locations. The thing is that if you go too far off center performance will drop too much with a parabolic dish.

  3. “Echos of War, the story of H2S radar”. A good interesting book about brittish radar development in WW2 era, recommend it. The main intel fault the Germans did before WW2 was missing that the brittish masts along the coast where a radar system, so it took quite some time until they realised it, and never really focused on knocking it out due to other things that took priority.

    There are a few acoustic mirrors still standing in different parts of the european coastline. Many due to not worth the effort to destory.

      1. It is not exactly new technology. There is the whispering wall in Bologna, which allegedly allowed lepers to confess their sins during the middle ages.

        You can whisper into the wall, with packed crowds surging through the Palazzo Podestá Archway, and only a person in the opposite corner, also pointing in towards the wall, can hear your whisper as clear as day.

          1. … and in the US Capitol building, and… An oval has two focal points, so you almost always get this effect. You also get it off of some gothicy barrel-vault ceilings. And the awesome 360-degree echo from standing in the middle of a perfect circle is not to be missed either.

            I always wish architects would pay more attention to acoustics. There’s just so much that happens randomly, and so much more that could be happening on purpose.

  4. Not much use once aircraft began flying faster than the speed of sound.

    Hmm then again, if sound waves change the characteristics of air electromagnetic-radiation interactions, because that EMR parameter shift, as the shockwave passes through a given volume of the atmosphere, would propagate at the speed of light even if the wave that triggered it can’t.

  5. Besides Tojo’s tubas there is more. There is a German one (ugly), a French one (flowery fractal design), an English one (practical) and they are all in stereo. While sweeping, the sound of in phase centered mono is quite sensitive in degrees of bearing. A pair of mics or sound tubes with the binaural distance between them would scan in the case of the big fixed installations. I read where 2 of these WW2 sites are being restored and open to visits where one can talk across the English Channel England to France with no radio of course. The delay must be like talking to Mars by radio!
    Now we are aiming them up to hear daytime meteors.

    1. Surly the german design would be described as efficient. :D
      I like the concept of being able to literally have a conversation over the channel. Although having someone eavesdrop on a conversation you’re having on a south coast beach, says the person who has a phone next to them… never mind.
      I thought maybe using an air vortex cannon. I’m wondering how accurate it could be if tubes were used to provide phasing information, with the angle measured mechanically, accounting for air pressures and wind speed, with a little trig and using three or more of them you’d have a fairly accurate sonar.

    2. You wouldn’t happen to know the name, or anything I can Google, about those cross-channel acoustic tuba things that are still there today? Searching “cross channel acoustic”, as in trans-manche, tends to get lots of results about audio channels, rather than 21-mile fish-bearing ones. I ask because I’d like to go visit one.

      1. The English Channel is 22 miles wide at its narrowest point; I don’t think human voices, even with big mirrors, would get that far. But it would be cool to hear seagulls, or sailors, half a mile away.

    1. Naaaah, someone in the group will pipe up and go “Hey, I know what this is! See that concave spherical surface? This is caused when naked killer robots arrive from the future, I’ve seen it in a movie once…!”

  6. My father told me of his grandfathers cold war acoustic reflector, part of the civil defense line in northern Wisconsin USA in the early to mid 50s. Apparently it piped into their living room and he had a special hotline phone to the military on which he would call if he ID’d an unidentified aircraft heading south.

  7. I think the Viet Cong made acoustic receivers by digging pits with the right geometry. They used them to detect the “chopper” sound of approaching helicopters used by South Viet/US forces.

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