You don't want to meet Michael Palin there

27 May 2012 — dan

A reader writes:

Why do nuclear power stations (and other power stations, for that matter) have cooling towers in that weird half-hourglass shape?

I presume the guys who built them knew what they were doing, but what did they know that I don't?

Ian

Cooling tower

I pledge to eventually answer your question, Ian, but first I'm going to rabbit on interminably about power stations.

The cooling tower has become emblematic of nuclear power stations, and the white "smoke" drifting from the top of them is a source of vague nervousness for a lot of people.

But, as you say, other kinds of power stations have cooling towers too. I live less than an hour's drive from Lithgow and the Mount Piper and Wallerawang Power Stations, able to produce 3.4 gigawatts of coal-fired electricity between them; Mount Piper has two cooling towers, Wallerawang has one. The "smoke" that comes out of these towers is actually just clouds of tiny water droplets.

(Once again, if you can see it, it's not "water vapour". Clouds, and the visible "steam" squirting out of a kettle or a steam locomotive, are liquid water droplets with a ceiling temperature of 100°C at sea-level air pressure. It's possible for actual invisible-vapour steam to be swirled in with condensed droplets as it mixes more or less chaotically with the outside air, but "pure" steam is invisible, and has no ceiling temperature. Put your hand in the visible portion of the steam coming out of the side of a locomotive and you may get scalded, but putting your hand in the invisible jet close to where it's exiting may flense the flesh from your bones.)

Power stations need cooling towers, or some other heat-sink like water from a convenient river, because they are heat engines. Heat engines, as I've written before, become more and more effective as the temperature difference between their "hot end" and their "cold end" increases.

A heat-engine that makes this fact obvious is the now-quite-standardised sort of "coffee cup" Stirling engine...

...which stands on a wide circular displacer-piston cylinder and can run on the heat from a cup of coffee or tea, or backwards on a cup of ice-water. I've got one that runs like this, but really low-friction versions of the design can run on the heat from a human hand, if the ambient temperature is cool.

(You can pay quite a lot of money for a jewel-like Stirling engine {or, more interestingly, a kit to build one}, but this eBay dealer, in addition to being called "Stirlingeezer" which ought to be a reason to buy from him all by itself, sells quite beautiful engines and kits that are guaranteed to run from hand-heat. If enough people buy stuff via the above affiliate link to Stirlingeezer, I shall soon be able to afford one of his engines!)

(Oh, and if you're short of money, you can get a Stirling kit for $US30 delivered, or conceivably less if you get lucky with your bids, from this guy in China.)

Conventional power stations, whether fired by coal, combustible gas of one kind or another, or a nuclear reactor, make their electricity by turning a turbine connected to a generator. Gas-fired stations can do this directly with a gas turbine, which is essentially a jet engine tuned for shaft-turning power, rather than thrust. Coal and nuclear stations make electricity less directly, by using the heat of combustion or nuclear fission to boil water and run a steam turbine.

(I think there are also gas power stations that use steam turbines. There are definitely gas power stations that burn the gas in one turbine, and then run another, different turbine from the hot exhaust of the first one.)

Anyway, that's the hot end. A well-designed heat engine will try to get its cold end as distant in temperature from the hot end as is practically possible. The ratio between the two temperatures, expressed in Kelvin (or any other temperature scale, as long as it starts at absolute zero), determines the maximum possible efficiency of a heat engine.

Sometimes "the cold end" is synonymous with "the exhaust temperature"; that's how it works for internal-combustion piston vehicle engines, and steam engines too. A classic example of the latter is the triple-expansion compound steam engine. This has one small piston for the fresh, hot, high-pressure steam right out of the boiler. The medium-heat, medium-pressure exhaust from this first piston powers a medium-sized piston, and the low-heat, low-pressure exhaust from that piston in turn runs one or more even bigger pistons. (This can theoretically be extended to even more stages, but in practice quadruple-expansion was about as far as anyone could get before the gain in efficiency wasn't worth the extra complexity and friction.)

Steam-turbine power stations, on the other hand, may emit exhaust gases from the burning of fossil fuels, but the system that makes the actual electricity is a closed, Rankine-cycle steam/water circuit. The burning fuel or fissioning atoms heat cool water to steam, the steam turns a turbine or three, and the turbine exhaust then goes to some sort of cooling device, generally a heat exchanger, that dumps the final unusable portion of the water's heat somewhere.

This "somewhere" can be a separate water supply, either a river, large lake or sea, or it can be evaporating water in a cooling tower. Once the heat exchanger has cooled the closed system's water in whichever way, that water is pumped into the boiler again, and the cycle continues.

You might wonder why you need to dump heat from the turbine exhaust, when you're only going to heat the water up again in the boiler. There are two practical reasons for this.

The first reason is that the exhaust from a power turbine is almost all still water vapour, because, in brief, turbines made to run on a flow of hot gas do not like it if the gas condenses to liquid inside them.

The second reason is that the pump that returns the water to the boiler has the opposite preference; it only works with liquid water. It would be possible to use a gas pump instead and make a system in which the working fluid is always vapour, but the energy needed to run a gas pump against pressure from the boiler is high, while the energy needed to run a water pump is trivial (by power-station standards), on account of the incompressibility of the water.

The upshot of all this is that standard 20th-century power stations are pretty miserably inefficient. Today, there's much more effort being made to reduce the heat wasted, by for instance transferring some of the heat of the turbine exhaust to the water feed between the pump and the boiler, or by using some of the waste heat to keep nearby buildings warm ("cogeneration"). These sorts of measures can only go so far, though, so cooling towers of one shape or another will continue to be built.

Which, finally, brings us back to the classic cooling-tower shape.

Cooling towers actually come in all shapes and sizes; large air conditioners, for instance, often have evaporative coolers for their chillers, but those coolers don't look anything like a power-station cooling tower.

Power-station coolers have to have very large capacity, so they inescapably have to be very large. Power-station coolers also have to provide a decent convective "stack effect", also known as "draught" (or "draft", in the less-demented American spelling). But, importantly, power-station coolers don't really need to be able to hold up much more than their own weight, plus any remotely plausible wind loads or shifts of their foundations.

The classic curvy cooling-tower shape fits all of these requirements. In engineering terms, because cooling towers don't need to hold up an interior full of offices, they can be built as a "thin-shell structure". You could build a cooling tower out of giant Great-Pyramid stone blocks if you wanted to, but a surprisingly thin reinforced-concrete shell, built in layers from bottom to top (not unlike the way 3D printers work), is the usual solution. And the builders almost never balls it up.

Objects of this shape are called "hyperboloid structures"; they're strong for their weight and so have been used for all sorts of masts and towers and, sometimes, ordinary buildings too, and they're particularly suited for use as cooling towers. The large area at the bottom of the hyperboloid gives lots of room for evaporation, the "waist" accelerates the gas mixture (I think because of the venturi effect), and then the widening opening at the top encourages turbulent mixing with the ambient air. (Air gets into the tower in the first place via an open latticework section around the base.)

(Oh, and I just have to take a moment, here: Segmentally Constructed Prestressed Concrete Hyperboloid Cooling Tower! Segmentally Constructed Prestressed Concrete Hyperboloid Cooling Tower! Segmentally Constructed Prestressed Concrete Hyperboloid Cooling Tower! Thank you.)

The final question that occurs to me in this area is why cooling towers are hyperboloids, but factory chimneys are cylindrical (or close to it - they often taper a bit toward the top).

This is because the cooling tower wants to move a vast amount of low-pressure air. The evaporating warm water at the bottom of the tower produces a steam/air/water mixture that isn't much warmer, and thus less dense, than the ambient air, so it has little buoyancy compared with the ambient air, won't move terribly fast, and so has to pass through a really wide pipe. Factory chimneys, on the other hand, are moving a much smaller volume of much warmer gas, usually combustion-product "flue gas". This is usually quite a lot hotter than ambient, so it rather wants to go up a chimney and doesn't need a wide one; you just need a nice long chimney, both to get a strong stack effect and to discharge the gas as high up as possible, to spread the pollution by dilution, as it were.

(Incidentally, The Secret Life of Machines addresses the stack effect in episode five, on central heating. And while I'm on the subject, the extraordinary documentary Fred Dibnah, Steeplejack features the titular working-class hero climbing hundreds of feet up a brick chimney and then perching on scaffolding that looks as if it were assembled by blind drunkards and knocking the chimney down by bashing bricks, one by one, into the flue. It has to be seen to be believed.)

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

Posted in Electricity, Psycho Science, Science. 18 Comments »

Further shiny things

23 May 2012 — dan

Crystalline silicon carbide

This extraordinary object looks as if it came from outer space.

Silicon carbide crystals

It's around 11 centimetres long (4.3 inches). It weighs a bit more than 170 grams (six ounces). Its overall colour is a sort of greenish black. But it's entirely composed of darkly reflective crystalline facets, ranging in size from microscopic to about 8mm (5/16ths of an inch) in length.

Close-up of silicon carbide crystals

On close inspection, the mass of crystals comes in a dark rainbow of different colours; yellows, blues, purples and reds.

Extreme close-up of silicon carbide crystals

Just like solid chromium, this stuff doesn't look real. Like the chromium, it looks more like some sort of movie prop. But my chromium lumps look like rocks spray-painted silver for an Original Series Star Trek episode in which those silver rocks were the most valuable object to appear. This stuff isn't nearly as shiny, but is much flashier, if you get my meaning. It's detailed. It's high-definition. It looks like a prop from a sci-fi movie we won't have the technology to make for another ten years.

You know what it looks like? It looks like black kryptonite, that's what it looks like.

(If you ask me, it looks better than the actual black kryptonite prop from Smallville. I presume you all share my incredulity that it took them until 2004, the 66th year of Superman's existence, to add the seemingly obvious black to the host of other kryptonite colours. Oh, and this Flickr user had the same thought, about what looks to me like the same material.)

Or possibly this stuff is what was left over after Gus Gorman boiled some home-made kryptonite in ammonia and ether and then smoked it.

This mass of black crystals is actually a chunk of crystalline silicon carbide (SiC), which I purchased quite cheaply on eBay (see below. There's video, too!).

This means it must be man-made, because natural, "native", silicon carbide does exist, but it's fantastically rare. It takes a lot of energy to persuade silicon and carbon to form a molecule.

Native silicon carbide is known as "moissanite", and so are simulated diamonds made from high-purity SiC. Far, far more SiC is made for use as an industrial abrasive or super-hard coating for tools, though, and some such process probably made this lump of the stuff.

Silicon carbide is so widely used as an abrasive because its Mohs hardness is as high as 9.5, between corundum (sapphire, ruby, and a component of emery) at 9, and diamond at 10.

Those numbers are misleading, because the Mohs scale is ordinal; it tells you what's harder than what, but not by how much. There are different ways of measuring the hardness of a material - compare and contrast the Knoop, Vickers and Rockwell tests, for instance. Whatever method you use, if you do a relative-hardness test, pretty much everything looks sick compared with diamond.

Assign a relative-hardness score of 10 to diamond and, depending on what test you use, corundum may score as high as 2.63 or as low as 2.2, and silicon carbide may score as high as 4.63 or as low as 2.5. For further comparison, quartz, generally regarded as a pretty hard material, scores down around one, if diamond is 10.

(In view of this, the fact that humans are now finally more-or-less managing to make exotic materials that are actually harder than diamond is quite amazing. The most widely used new super-hard material is diamond-like carbon, which as the name suggests isn't actually "better" than diamond, and two of the other candidates are actually just novel forms of diamond. Only minuscule amounts of the best non-diamond candidate have been made to date - with some debate over whether any of it has actually been made at all. But one way or another, we're doing it, and the achievement is a lot more impressive than a mere Mohs hardness of "11" would suggest.)

I don't know exactly how my lump of SiC was made, but I suspect it was an unwanted byproduct of some industrial process, perhaps one or another kind of vapour deposition. The carbide is meant to coat drills or saws or something, but it deposits elsewhere on the equipment too. When some lucky duck gets to clean out the machinery, stuff like this crystal mass ends up in their bucket.

Most industrial waste is not particularly decorative, but every now and then, something extraordinary comes along.

I also don't know how pure this carbide is. High-purity silicon carbide can be black, just like this material, but there may be various impurities in there too.

The surface is definitely not pure SiC; the rainbow reflections are created by a very thin layer of silicon dioxide on the surface. This interacts with light in the same way as various other super-thin coatings, like the surface of anodised titanium, the "rainbow of temper" on steel, or a soap bubble, for that matter.

Silicon carbide is very hard, but rather brittle. If you buy a chunk mail order like I did, you're going to get a few broken-off crumbs in the box along with the main piece, unless the seller packed the carbide in thick cotton wool. And if they did pack it in cotton wool, you're going to spend forever picking cotton shreds off the pointy crystals. Just hitting the thing with a blowtorch might be a faster solution. Or it might heat-shatter.

You don't really have to treat SiC like the egg of a tiny bird, though. When I deliberately broke off a little crystal stuck to the main mass by a couple of millimetres of hair-thin filament, I was surprised to see the filament bend a good five degrees before it snapped. And tiny crumbs coming off even a small SiC lump won't make any obvious difference to its appearance.

One thing you probably don't need to worry about your silicon carbide doing is melting. The melting point of pure silicon carbide is 2730°C, 3003K, or 4946°F in the old money. So you may be able to melt it with an oxy-acetylene torch; the theoretical perfect-combustion temperature for that is around 3500°C. Oxy-hydrogen might manage it, too. MAPP-gas and oxygen probably won't cut it, though, and no cheap butane torch will come within a hundred miles.

Silicon carbide was the material used for the very first light-emitting diode, way back in 1907, though this discovery was largely ignored at the time. That could be why nobody managed to make an LED bright enough to be useful for anything until the Sixties. Henry Round's original discovery was still scientifically important, though, and I swear I managed to get a tiny spot of my chunk of SiC to light up under a pin connected to minus 12 volts. But once I set my camera up, it refused to do it again, no matter what I poked with the pin or where I attached the positive cable's alligator clip.

I think having an alligator clip as the positive terminal, rather than for instance sitting your SiC chunk on aluminium foil that's connected to positive, is important - you need pressure on the SiC to get a decent contact, and the positive connector needs to be close to the point you're poking with the negative pin, or the semiconducting SiC won't let any current flow. With the clip close to the pin (less than a centimetre), something above 20 volts always persuades my SiC to allow current to flow, but that doesn't give the LED effect, just little blue sparks. You're looking for something greenish-yellow, as in this Wikipedia picture:

Silicon carbide LED effect

(The picture is from this page, which contains further instructions on how to try this experiment yourself. And then there's this dude, whose carbide lump seems happy to light up all over, damn his eyes.)

Trying, and failing, to make my own carbide-LED picture was quite frustrating. I can see why people in the early days of radio were so happy when they could buy machine-made vacuum-tube diodes so they didn't have to fool around with super-fine wires and lumps of galena any more, poking around all over the crystal like a tiny pirate seeking one buried treasure chest on the whole island of Barbados.

Aaaaanyway, you may be pleased to know that I am now finally going to tell you where I got this stuff, and what it costs.

Getting some

I bought a little chunk of crystalline silicon carbide on eBay a few years ago, from this seller, but they don't have any SiC for sale at the moment. This new bigger chunk was another eBay purchase, for $US28.17 including delivery to me here in Australia, from this seller (who's here on eBay Australia, here on eBay Canada, and here on eBay UK).

As I write this, they've got one more lump of the stuff, closer to spherical than mine and weighing 210 grams.

The inimitable Theodore Gray has a chunk of this stuff too; he bought it on eBay as well, but from a seller who called it "native bismuth". Dark SiC crystals resemble bismuth hopper crystals (see here) in colour, but that's as far as the resemblance goes.

(Theo also has this different-looking SiC sample, which was also sold as bismuth. And then there are these high-purity crystals, transparent green with no oxide layer. Oh, and on the subject, if you get a solid block of carbide but your plutonium hasn't arrived yet, you can pass the time with some microwave metal melting!)

Nobody on eBay seems to be selling silicon-carbide "bismuth" at the moment; there's plenty of "native bismuth" crystals that're obviously actually purified bismuth crystallised by the standard stovetop method, but at least those actually are bismuth, so by eBay fake-minerals standards no great crime is being committed.

And now: Twinkling!

Behold, the silicon carbide lump, and the chromium, and a couple of large oval-cut cubic zirconias ("CZs") into the bargain. They all look impressive in sunlight.

(These videos don't have sound. Feel free to add your own vocal "ting" sound effects to synchronise with the reflections and refractions.)

The smaller CZ is, at about 36 by 29 by 22 millimetres (1.4 by 1.1 by 0.9 inches), comfortably in the Crown Jewels size range. If it were a diamond, it'd be around 155 carats (as a CZ, it's 255 carats - CZ is about 1.6 to 1.7 times as dense as diamond). This is a bit less than the original cut of the Koh-i-Noor, but about 1.5 times the Koh-i-Noor's current size.

The larger CZ is about 52 by 38 by 28 millimetres (2 by 1.5 by 1.1 inches), and weighs 132.5 grams; a diamond the same size would be about 400 carats, far larger than any of the world's famous colourless diamonds, and a little less than the total weight of all of the multicoloured diamonds in the two "Aurora" displays.

I bought both CZs in 2009 from this eBay seller; the smaller one cost me $US19.95 delivered, and the bigger one was $US37.95.

That seller doesn't seem to have a lot of huge CZs on offer today, but if you use the always-entertaining "highest price first" sorting option but set a price ceiling at, say, $100, then in among the eBay listings for bags containing many small CZs, there are plenty of monster white and coloured stones.

(Here's that search on eBay Australia; it's here on eBay UK, and here on eBay Canada. I strongly recommend you buy at least a pocketful of small CZs; they make novel presents, and you can also wrap them in black felt, go to a cafe with a friend, wait for people to look, and then make everyone think some serious state secrets are being sold.)

Posted in Nerdery, Science, Toys. 5 Comments »

Fnord!

23 May 2012 — dan

From: "freemason illuminati" <noreply@freemason.org>
To: yourorder@fi.org
Reply-To: order@illuminati.umail.net
Subject: fi
Date: Tue, 20 Mar 2012 07:10:41 +1200

You are receiving this mail in regards of the freemason confraternity of the
whole wide world (FCWWW).

You are moving well in what you are doing but in order to make it easier for
you, we have concluded for you to be a part of us as a member to sign your
life to us and have any thing you need.

Be it any thing in the whole wide world.

You can't refuse us now for it's too late.

Get back to us now for your Illuminati membership Order and also for you to
know more about the ancient ILLUMINATI FORUM and also the Orientation and
goals that we pursue.

Get back to acquire your goal now.

I would appear to not be the only person who is moving well, et cetera. The Freemason Confraternity of the Whole Wide World also seem to be offering a better deal than the LaRouche people.

(Yes, the local LaRouchies continue to e-mail me periodically, unconcerned by their continuing terrible prophetic record, and not inclined to admit any errors.)

Fi.org is registered to someone in Denmark, and rather appropriately www.fi.org currently redirects to shady.dk. But that's just a parked domain now; it looked something like this in 2007. The Internet Archive have numerous copies of fi.org going back as far as 1998, but they all seem to be parked-domain redirectors too.

(I suppose archive.org just didn't know the secret handshake.)

Posted in Spam, Strange Tales. 1 Comment »

"I'm not an ageist, but..."

21 May 2012 — dan

A reader writes:

Something has been bugging me for a while, but I didn't want to ask anyone in case it sounded racist, which it isn't, because some of my best friends are members of the inferior races which Asians like me will soon enslave.

Internet anonymity lets me ask YOU, though:

Why do old black people so often not look as old as identically old white people?

I'm asking now because I've seen a few excellent examples in just the last few days.

I was watching Joe Morton in Eureka, and he looks EXACTLY THE SAME as he did in Terminator 2, 20 years earlier.

And look at this guy! He just died at age 75, but in the picture of him performing a year ago he could be 50, or 40 even.

And then I watched a recent Daily Show where George Clinton did a walk-on, he's 70 but looked 50, tops.

And the flipside: Just now Reddit brings me a young basketballer who only needed some fake white hair and beard and bang, plausible old black man!

(OK, there was some latex work there too. But you can't see that clearly even in HD, and he still looks old.)

I know black people don't actually LIVE any longer, quite the opposite here in the States, but looking young your whole life has to be some consolation. How/why does it happen?!

Z

I have heard this phenomenon described as "black don't crack", but I, like you, don't know whether it's safe for non-black people to call it that in company.

(This whole situation, especially in the USA, seems to have taken a terribly wrong turn at some point after Blazing Saddles.)

In the case of people in TV and movies this phenomenon is, of course, at least partially the result of makeup, lighting and plastic surgery. But you're right when you say that it happens in "real life" too.

The reason is actually quite simple.

When you get older, your skin loses elasticity and you get more wrinkly. The principal factor in the visibility of wrinkles is light, or more precisely shadow. Wrinkle-hills cast shadows in wrinkle-valleys, and those shadows play a big part in making a face look old.

If you've got pale skin, wrinkle-shadows show up very clearly. But the darker your skin is, the closer to the shadow shade it all is naturally, and the less obvious are the wrinkle-shadows, and the less old you look. That's really all there is to it.

Rub your face with lampblack and, no matter what colour your skin was before, it'll now be so dark that wrinkle-shadows will be almost invisible. Do the same thing with titanium dioxide powder and every tiny line will stand out clearly, unless you're only illuminated by a light right next to the viewer.

(This is why the built-in flash of a compact camera tends to make everybody's face look flat and weird - but not wrinkly! A photographer may use a "beauty dish" to add a controlled amount of this effect to a portrait.)

This same phenomenon can be seen in some peculiar places. Take the moon, for instance.

A full moon is much more than twice as bright as a half moon, because of what's called the "opposition effect". The effect is partly caused by the retroreflective qualities of lunar regolith - it tends to reflect light back the way it came. There may be some quantum weirdness involved too. But the opposition effect occurs mainly because the lunar surface is very uneven, thanks to meteorite impacts and no erosive forces. So there are lots and lots of shadows when the moon is illuminated from the side from our point of view, making it half-full, but there are almost no shadows at all when it's full, and illuminated by the sun looking over the earth's shoulder, as it were.

(The albedo of the moon is surprisingly low - it's about as dark as an asphalt road. It seems so brilliant in the night sky because it's illuminated by direct sunlight, not because it's actually the pale grey it seems to be when compared with the surrounding dark sky.)

The "black don't crack" phenomenon is one small part of numerous more-or-less-racist theories that explain one or another kind of physical advantage that dark people are supposed to have over pale people.

One of the more popular of these theories is that black slaves were literally bred to be stronger and healthier, since there wasn't much of a market for longsighted asthmatic cotton-pickers. Whether the claim is that this breeding was forced by slave-owners, or was just a result of brutal natural selection that caused weak slaves to often die before reproducing, though, it's pretty clear from genetics and genealogy that it actually didn't happen.

There is evidence for something like this in some situations. It's hardly surprising, for instance, that a number of successful very-long-distance runners have come from cultures where, for centuries or even millennia, being good at cursorial or persistence hunting has been a way to get more wives and offspring.

Even in these situations, though, there are many confounding factors. Running is something almost anybody can do, almost anywhere. It requires no expensive equipment or special facilities. So poor countries, regardless of culture, produce more runners than they do, say, golf or polo players. (And every now and then along comes a little white guy who's accustomed to spending days on end rounding up sheep, on foot.) For the same reason, you don't see many bobsled teams from countries where it doesn't snow.

(While I'm digressing, here's a note even less relevant to the original question: Because I'm in Australia, thedailyshow.com doesn't want to show me that George Clinton video. I just get a "Sorry, this video is unavailable from your location" error. If you have the same problem, you can solve it with the Modify Headers Firefox extension, which lets your browser say it's asking for the page on behalf of a US IP address. Find instructions on how to do this here.)

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

Posted in Psycho Science, Science. 3 Comments »

Like piranhas with a cow

19 May 2012 — dan

It is possible that you do not, as a matter of course, have at least fifty cockatoos turn up in your back yard for a feed every afternoon.

If you do, I advise you to, as I do, purchase your bird seed in 25-kilo sacks, and not attempt to feed the cockatoos with a supermarket seed bell.

Those don't last.

(Available in HD!)

The loud scrapey-thud noises happen every time a 900-gram bird uses the microphone on top of the camera as a perch.

Extension of this behaviour brought the video-shooting to a halt when only about half of the seed bell was gone. But, as I said, you get the idea.

(I know it's not good to give them a small source of food so they fight over it. The regular seed I spread on the table, and on the deck itself, which greatly reduces the Skesis act.)

Posted in Animals, Birds. 2 Comments »

The music goes round and round and comes out backwards

16 May 2012 — dan

A reader writes:

Sometimes when I plug in my headphones it seems as if I'm not receiving any vocals. Its still stereo, but I find that the headphone jack isn't completely in. What's going on here? And why does it work so effectively at removing vocals?

Simon

The quick answer: You're hearing the two stereo channels mixed, with one of them out of phase with the other.

I can't for the life of me figure out how this happened, though. In the interminable rambling below I talk about a couple of other crossed connections that can, and often do, happen, but unless there's a bit of wire stuck in the headphone socket or some other such oddity, I don't know how you could have the exact symptoms you report. Unless, of course, what you think you're hearing isn't what's actually happening, which is eminently possible since the ear is as easy to fool as the eye.

Anyway, mixing one stereo channel with an opposite-phase version of the other channel means that any component of the musical mix which is essentially monophonic - in the middle of the stereo "soundstage", the same on both channels - will be cancelled out.

Singing in popular music is, usually, pretty much in mono in the middle of the stereo mix. So mixing one side with a phase-flipped version of the other side will cancel out said singing; all you'll hear of the singer is any stereo reverb or difference in volume between the two sides (a.k.a. panning).

This technique, called Out Of Phase Stereo or OOPS, is used in this simplest form by old and/or cheap karaoke machines that let you "mute" the vocals on normal music, so you don't need a special vocal-less karaoke version of every song.

Simple OOPS doesn't work very well, because vocals are seldom exactly in mono (they usually have some stereo reverb, for instance), and if any other component of the mix - drums, bass, whatever - is also mono, then that'll be muted too. Smarter OOPS vocal-muting tries to identify and mute only the vocals, based on the pitch and possibly even the timbre of the sound.

This weirdness can come about because headphone plugs use TRS (Tip, Ring, Sleeve) connectors. TRS connectors come in a variety of sizes - the big old-fashioned 6.35mm (quarter-inch), the ubiquitous modern 3.5mm (eighth-inch), and the usually unnecessary and irritating 2.5mm (3/32 or 1/10 inch, depending on who you ask). All three-contact audio TRS connectors are, or should be, wired the same way; the tip of the plug is left channel, the ring is right channel, and the sleeve at the base of the plug is a shared ground.

(This sort of cylindrical plug connector should only be called "TRS" if it has these three contacts. Mono-audio cables with the same sort of plug, like for instance guitar leads, omit the ring contact and should therefore be called just "TS" connectors. Connectors with two ring contacts, as for instance used for stereo headsets with a microphone, should be called "TRRS". Sometimes you'll see any plug of this basic form called a "TRS" plug, though, regardless of how many contacts it actually has.)

Most headphones today have a 3.5mm plug; fancier ones...

Sennheiser plug

...come with a 6.35mm adapter for it, too.

Some headphones mix it up a bit. One of these Sennheisers has a simple 3.5mm TRS plug on both ends of its cable, so you can very easily and cheaply replace the cable if it's damaged...

Sennheiser cable plugs

...but the other has a 3.5mm TRS on one end of its cable, and at the other end the cable splits into a pair of 2.5mm TS plugs, one for each side of the headphones.

If a TRS plug isn't fully inserted, contacts on the plug can touch the wrong contacts in the socket.

(This characteristic makes this shape of plug a bad fit for many applications. I once bodged up a two-voltage power-supply connection for an external SCSI drive box for, of course, my Amiga, using a 6.35mm TRS plug and socket. I avoided the dangers of connecting one voltage to the other's contact and the other voltage to the ground contact by only connecting or disconnecting the power supply when everything was turned off. Or, at least, connecting or disconnecting it really fast.)

A partially-inserted TRS plug could, for instance, leave the tip contact on the plug touching the ring contact of the socket, and the plug's ring contact not touching anything, and the sleeve of the plug still touching the sleeve of the socket, because the sleeve is much longer than the other two contacts. This sort of mis-connection will give you the signal meant for your right ear in your left ear, and no sound from the right side of the headphones.

(The sleeve of the plug is connected to the braided or foil shield of the cable, and any or all metalwork on the plug that is not the tip or ring contact will also be connected to the shield - it's all one big sleeve contact, basically. TRS sockets almost always work the same way; the socket has small tip and ring contacts, but the rest of the socket's metal is all sleeve. This makes it very easy for the sleeve of plug and socket to remain connected when the plug isn't all the way in, and if the plug's a long way out of the socket it also makes it pretty easy for the plug's ring contact to touch the socket sleeve contact.)

I encourage readers, by the way, to try this out yourselves; it's pretty much impossible to hurt your headphones or anything you plug them into by only plugging them in half-way. So do that, especially if you find my interminable blather confusing, or just want to see if you can create Simon's symptoms for yourself.

Another mis-connection could have the plug-tip touching the socket-ring, and plug-ring touching socket-sleeve. Now you'll hear the signal meant for the right side through both ears, but it'll sound weird, because it's out of phase, as mentioned above.

This is because the left and right "drivers" of the headphones (the electromagnetic transducers that actually make the sound) are, with this mis-connection, now wired in series, with the wire going "out" of the left driver that's meant to connect directly to the sleeve contact instead only being able to connect to the sleeve through the right driver and the plug's ring contact, thereby feeding the right driver "backwards".

This causes the right side to be out of phase - when the left driver is moving toward your head, the right one should be too, but now its phase is inverted and it's moving away.

There are two reasons why I'm rabbiting on about this stuff at such outrageous length. The first is that basic audio cabling like this is something that almost everybody has to deal with, and it pays to know what connects to what even if you don't intend to be soldering up any cables of your own - though basic soldering is easy to learn, cheap, and can save you quite a bit of money.

(Just yesterday, I turned a couple of not-that-cheap Apple laptop power supplies, one electrically dead and the other with a fractured and fiendish-to-repair MagSafe plug, into one working power supply with an extra-long cable. Even the dodgiest off-brand MagSafe power supplies are about $US20; a genuine Apple one is $79. With basic soldering skills and equipment, this sort of thing is a literal five-minute job.)

The second reason for this ridiculously large post is that phase problems are actually very common in home audio, and not only among people who fix their own headphones and get one set of Profanity-Allowance-consuming minuscule wires backwards.

People have been creating phase problems ever since the invention of stereo, occasionally by mis-connecting a turntable cartridge, but usually by wiring one of their stereo speakers the right way around (red terminal on the amplifier to red terminal on the speaker, and black to black) and the other the wrong way (red to black, black to red). In this case, each speaker actually is playing the signal that it should be, but one is phase-inverted.

(It doesn't really matter whether you've connected red-to-red or the other way around, as long as both speakers are the same. In theory, having both out of phase with the way they should be, but in phase with each other, could cause an audible difference, but in practice it's only detectable by golden-eared audiophiles who see no need for tiresome things like blinded tests.)

If you've got two working ears, you definitely can hear when stereo has one side out of phase. It's hard to describe, though; it's sort of like having a head cold that's blocked one of your Eustachian tubes but not the other, or how things sound when an air-pressure change has popped only one of your ears. Here's a page that explains this, with nifty audio samples including one (MP3 link) that should sound mono in the first half and out-of-phase pseudo-stereo in the second half.

Out-of-phase audio sounds different on headphones and speakers, because of the basic differences between the two devices. Headphones deliver pretty much pure left-signal to your left ear and right-signal to your right, but each of your ears hears both members of a stereo speaker pair, plus umpteen reflections and resonances from the room you're in.

For this reason, if you're listening to stereo speakers with one side out of phase, there will be strangely little bass, because low-frequency sound has a long enough wavelength that the out-of-phase speakers can mix their sound even if they're separated by a few metres. Higher-pitched centre-mixed components of the music won't cancel as much, though.

Just to make things even more complicated, sometimes it's good to reverse the phase of a subwoofer or surround speakers, to compensate for subwoofer location or the distance of the surround speakers. There's often a hardware switch on a subwoofer or a configuration option on a surround receiver that'll let you do this, or of course you can just switch the wires around or make a crossover RCA cable.

There are also some stereo recordings on which phase reversal is undetectable, because the two channels share nothing at all. Many early stereo rock tracks are like this, and are pretty much intolerable to listen to with headphones because the stereo mix puts each instrument entirely on the left or right channel. Ringo and Paul on the left, John and George on the right. Nobody minded this very much at the time, because almost everyone heard this music in the mono mix, but to make these tracks listenable with headphones you need a fancy headphone amplifier, or music-playing software, that has a "crossfeed" control, to deliberately mix some right into the left and some left into the right.

Out-of-phase mono - the same signal on both sides, but one way round on the left and the other way round on the right - kind of sounds like stereo, because you genuinely are hearing something different on each side. So what I think you, Simon, are hearing from your partially-connected headphones is a mixture of left and right, with one side's waveform inverted, the resultant mono signal being heard one way round on the left side and the other way around on the right. I just can't figure out how you could electro-mechanically get this to happen by only partially plugging in the headphones. I presume there's one contact on plug or socket that's touching two contacts on socket or plug, but I don't know which.

It is, again, entirely possible that you're not perceiving what's going on correctly; psychoacoustic effects can be powerful. But perhaps I'm just insufficiently imaginative. Any ideas (or experiment reports!), commenters?

Posted in Electricity, Electronics, Music. 9 Comments »

Giant watery balls

11 May 2012 — dan

A reader writes:

I recently saw a news article that linked to this government page:
http://ga.water.usgs.gov/edu/earthhowmuch.html
...which says if all Earth's water (liquid, ice, freshwater, saline) was put into a sphere it would be about 860 miles in diameter.

Now I understand an 860-mile sphere is massive, so even though that sounded small I could accept it, until they state the estimated volume of water on earth at 332.5 million cubic miles.

So how do you cram 332,500,000 cubic miles into a 860 mile sphere?

Matthew

Quite easily, actually!

The volume of a sphere is four-thirds pi times the radius squared cubed [Sorry I left that error there for so long, commenters!]. So if the radius is 1 unit, the volume is 4.19 cubic units.

The radius of an 860-mile sphere is 430 miles. 430 cubed is 79,507,000. Four-thirds pi is about 4.1888. Multiply that by 79,507,000 and you get about 333,038,143, a number less than 0.2% larger than 332,500,000. The difference is accounted for by variations in precision in working out the number, since this is really only a ballpark figure and taking it to nine significant digits is silly.

To "sanity check" this if, like me, you always feel mildly nervous about the order of operations for a calculation like 4/3Πr^3, consider the volume of a cube 860 miles on a side.

The volume of a cube is of course just its edge-length cubed, and an edge length of 860 miles gives a volume of 636,056,000, a nice sane-sounding 1.91 times the volume of the sphere that'd neatly fit in that cube.

My own second-favourite way-to-visualise-the-quantity-of-something is that all the gold in the world (not including gold we have yet to dig up or somehow extract from seawater) would make a cube only 20 to 22 metres on a side, depending on who you ask. To help visualise the size of the cube, 21-ish-metres is about the length of two city buses parked nose to tail.

Because gold weighs 19.3 grams per cubic centimetre, though (11.16 ounces, or 10.16 troy ounces, per cubic inch), a 21-metre-on-a-side cube of gold would weigh 178,737 tonnes. So I suppose you wouldn't have to worry too much about someone stealing it.

(Unless you are very wealthy, you probably can't buy a large enough lump of gold - especially at today's outrageous prices - to really appreciate its density. At current prices, one kilogram of gold would cost you more than $US51,000. Tungsten, however, is 99.7% as dense as gold - I'm sure counterfeiters have gilded tungsten for profit many times - and it's much more affordable, though still expensive. The good people of RGB Research {here on eBay US, here on eBay UK, here on eBay Australia} have their one-kilo tungsten cylinders on sale again for a mere $US220 plus rather pricey delivery. If you can afford one, and have the slightest interest in science toys, I urge you to buy one; my own tungsten cylinder is one of my most treasured possessions. And one of the most durable, too; if the house burns down the tungsten cylinder, like my Bathsheba Grossman Metatrino, will be sitting intact in the ashes.)

My most-favourite way-to-visualise-the-quantity-of-something is that if you breathe on an ordinary marble, the thickness of the layer of condensation from your breath on the marble is approximately to scale with the thickness of the atmosphere on the earth.

(And another one, that doesn't really make anything much easier to understand but is prime stoned-party-talk, is that a human is about as much bigger than an atom as a galaxy is bigger than a human.)

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

Posted in Psycho Science, Science. 7 Comments »

Tag! What is it?

9 May 2012 — dan

A reader writes:

How do the little rectangular anti-theft tags work?

I get how the big anti-theft stickers work. They've got an obvious square spiral antenna that I presume collects enough microwatts from an incoming signal to run a little transmitter that sends another signal out.

But the little tags don't have any circuitry inside. I cut one open, and there are just some tabs of springy metal in there - two pieces next to each other, and a smaller piece separated from the other two by a clear plastic membrane.

The metal sticks to a magnet, but that's the end of my ability to figure out what it does.

Is there invisible nanotechnology in these things, or something? Hey, maybe they're a placebo!

Kim

Magneto-acoustic security tag innards

If they're a placebo, the alarm systems in shops seem to really believe that it works.

What you're looking at there (here's a more elegant cutaway picture on Wikipedia) is called a magneto-acoustic, or acousto-magnetic, tag. Which is one of those things that doesn't really sound as if it ought to work, but does.

The first two of the three tabs inside are, I think, a couple of pieces of amorphous metal - which is quite an exotic material to be stuck to commonplace consumer items just to stop people stealing them. Amorphous metal is, in a way, the opposite of nanotechnology; it's metallic glass, special because it lacks the microscopic crystal structure of normal metals.

The third tab is a piece of less exotic, medium-coercivity metal. When that third piece is magnetised, the two other strips, which are sitting loose in their little plastic coffin, become quite easily moved by external magnetic fields. (They're amorphous metal because that's already unusually easy for external fields to move.)

The security gateways as you leave the store emit a pulsed magnetic field up in the tens of kilohertz, at the resonant frequency of the amorphous-metal strips. When next to their mildly-magnetised buddy, this quite tiny field causes the amorphous-metal tags to buzz, and to continue to buzz for a very brief moment after each pulse of the external field. This very brief "ringing" period causes a tiny change in the magnetic field of the third strip, which an antenna in the security gateway, very implausibly, detects. And off go the sirens.

The thingy at the checkout that deactivates the tags is a degaussing coil. It more-or-less demagnetises the third strip, which both reduces the magnetic sensitivity of the other two strips, and removes the field which the other two strips modulate. So now the sirens don't go off.

I am entirely unable to think about any security system without immediately trying to figure out ways to defeat it. (I try to avoid airports nowadays. They make me feel like Jackie Chan in a deckchair factory.)

One obvious but impractical way to defeat magneto-acoustic tags would be to degauss them yourself; I don't know how strong the degausser needs to be to achieve this, though. You might be able to pinch stuff if you just smuggled a CRT-screen degaussing wand into the shop, and found somewhere to plug it in.

Swiping your own rare-earth magnet across the tag would, if anything, probably make it work better (by more strongly magnetising the third strip), but I wonder if leaving a magnet or three stuck to the tag, in a Halbach array if you're fancy, might silence it. Just chopping it bodily off with a potato peeler would probably do the job too, of course, but where's the fun in that?

(If you can magnetise tags yourself with a ten-cent eBay magnet, then you could pry them off things you've bought, reactivate them, and attach them inconspicuously to things which other people may innocently carry into shops. You could, is all I'm saying.)

The square-antenna type of tag, by the way, is also pretty simple. It doesn't actually have anything fairly describable as a transmitter in it, but is rather a tuned circuit that resonates somewhere in the low megahertz. This makes it detectable, if a nearby transmitter/receiver combo rapidly sweeps its output through the relevant frequency range and looks to see if something is managing to suck up some energy at the appropriate frequency.

This kind of tag is deactivated by, essentially, blowing out the capacitor essential to their resonance with a higher-powered signal. I think a shoplifter could probably defeat these tags by just dragging a knife across them a couple of times, though, breaking the circuit. I haven't actually tried this, though, because it'd mean missing out on all of the fun of a good old-fashioned armed robbery.

Perhaps someone who's worked in retail since fancy security tags came into vogue will enlighten us in the comments.

I would also like to hear from anybody who's successfully used the "just lob the item high over the security gate and into the hands of your partner in crime" technique.

Posted in Psycho Science, Science. 15 Comments »
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