A few weeks ago, my wife was out walking the dog, and she sent me four or five photos of small orange boxes planted all around our neighborhood. (OK, I’ll bite!) They had little cards on them explaining that they were geophones, and a QR code on them that lead to a website with all the details. Munich was getting a large-scale seismic survey to map out our underground water, with the aim of using it for geothermal heat and power in the near future.

How do you map up to five kilometers under the earth? You pound the ground, sending shockwaves downward, and then listen for their reflections. At the boundaries between different layers, the change in the speed of sound in the different media cause reflections. Calculating the time it took for a given reflection to reach you lets you figure out how deep the layer boundary is.

The seismic survey procedure goes like this: geophones are set out at roughly 20 m intervals in lines spaced around 300 m apart that run roughly north-south, while “vibrotrucks” drive a roughly east-west course, creating mini-earthquakes every 20 meters along the way. Covering a surface of 1,000 km^2 with over 120,000 sample locations and exciting them 86,000 times is going to take a while. Lucky for me, they started in my part of town.

Hot Water

Because the earth is essentially a ball of hot molten metal with a thin and crispy outer crust, and because there is radioactive decay going on even within the crusty bit, temperatures rise as you dig down: roughly 30 °C per kilometer. And Munich has this fantastic source of water that’s folded under the earth in a layer that dates back to the Jurassic, which makes it both low in dissolved minerals and sitting just around 3 km down: at 100 °C. This turns out to be the sweet spot in terms of difficulty of drilling and heat gained from doing so.

Blessed with hot water underground, the question is how to best use it. The plan at the moment is to situate a number of geothermal plants around the city, drill relatively large bore holes at each site that reach down 1 km to 2 km, and then drill diagonally down after that to spider out into a larger source of water. This “extended reach drilling” pulls from a larger heat source, prevents cool spots, and has only recently become technologically feasible.

Which brings us to the GIGA-M study. A 3D map of the underground will help plan out where to put the geothermal plants, in which directions the runners will need to be drilled, and generally how to best coordinate the resource. There were a number of individual smaller surveys done over the last 20 years, and Munich and the surroundings already have a number of geothermal plants, but this survey aims to fill in all of the gaps.

The Hunter and His Prey

It was a Thursday morning when my wife thought she heard some pounding and humming outside. She was wrong, but it got us to look at the online map, and while they weren’t in our neighborhood just yet, they were probably within easy driving distance. I threw my camera and tripod in the car and headed out.

Out in the country, just outside of Grossdingharting (you can’t make these names up!) I spotted a plume of dust rising up from a field. Was it just a farmer tilling? Or was it a vibrotruck? I drove through the woods on a logging road, and when I came out, I was nearly face-to-face with the beasts.

They were loud, and I felt the rumble when I stepped out of the car, so I ran up and asked if I could film it. They said “sure”, and I set up the tripod. The first video I shot, apparently I hadn’t screwed the camera down hard enough in the tripod, and it vibrated loose. So I ran another 20 m further and tightened everything down.

I’m not going to lie: it was exceedingly loud and very exciting. I drove back home and couldn’t wait to review the footage.

The next morning, I heard the same sound outside my own house. They were driving vibrotrucks around in the field about a block up the street. I grabbed the camera and ran out barefoot to get footage. And then Saturday morning, while cooking blueberry pancakes for the family, they drove up my street. Am I stalking the vibrotrucks, or are they stalking me?

An engineer asked if they could stand in our front yard, so I took my chance to bombard him with questions. He seemed stoked to talk about it.

Geophones and Vibrotrucks

When you stand next to one of these things, first it shakes, and then you can hear a bassy tone, and then it rises and stops. They’re obviously doing a frequency sweep, much like you would use a sonar chirp to help disentangle the multiple reflections from one another.

The sweep means that even if they receive two reflections at once, they will hear two different pitches because one signal traveled further than the other, and comes from the earlier, lower-pitch part of the impulse. It also makes for a very easy to find signature. Correlating these times of flight across the entire array of geophones gives a 3D map of the underground layer discontinuities.

You can see from a cleaned up version of the audio that they’re sweeping from something like 6 Hz up to maybe 96 Hz.  The lower end sounds like distinct hammering, and the top end a humming. Here is another video, with filtered audio. Because you feel the vibrations through your feet and in your chest, the live experience is a little bit like this with more noise. Play on good headphones or with a subwoofer for maximum effect.

Of course, the time of flight matters. I asked if each geohpone had a GPS timebase, and the friendly engineer told me that they individual geophones don’t – too expensive – but that when they install one, they connect it to a laptop with GPS that records the location and sets the geophone’s clock. When they harvest them, they record the location again, dump the time to verify that the clocks haven’t drifted too much, and then pull down all of the recorded timestamped data.

Why do they drive around in pairs? It’s simply because they make twice the vibration. The two trucks are actually synced together in phase, so they emit as one. The trucks have a GPS-disciplined clock inside, so they know exactly when they start each cycle and can derive the time-of-flight to all of the receiving geophones. The rest is math.

He also mentioned that it was too bad that I only got to experience the smaller “urban” vibrotrucks like the one outside my house. I kept my mouth shut – nobody needs to know that I’m a vibrotruck stalker.

If you used a scientific calculator in the 1970s or 1980s, there was a fair chance that it worked differently from almost every calculator you see today. Instead of typing:

2 + 3 =

you entered:

2 ENTER 3 +

There wasn’t even an equals key. Hewlett-Packard made this system — Reverse Polish Notation, or RPN — practically synonymous with serious scientific calculators until other players like TI and Casio got serious. Once you got used to it, ordinary algebraic calculators could feel annoyingly clumsy.

Today, RPN calculators look like a nearly extinct species. HP left the calculator market, licensing the HP calculator line to Moravia Consulting. Old HP-15Cs, 16Cs, 32Ss, 42Ss, and 48s have become collectibles. But RPN isn’t dead. You can still buy new hardware, build your own, or turn almost any computer or phone into a very capable RPN machine. There are reasons some of us still want to.

But Why Polish?

The name goes back to Polish logician [Jan Łukasiewicz], who devised a notation in which operators precede their operands. Instead of writing:

A + B

you can write:

+ A B

The big advantage is that parentheses aren’t required. The structure of the expression tells you exactly what operates on what. Reverse Polish notation simply puts the operator at the other end:

A B +

[Łukasiewicz] wasn’t designing calculators, of course, but the same idea turned out to be extremely convenient for computers and calculators. Your software doesn’t have to remember what operation is in progress. Each operator is ready to go and can simply work on the operands that you’ve already read.

RPN isn’t exactly the way people calculate with pencil and paper, and it certainly wasn’t derived from the slide rule, but there is a similarity in the way you work. With a slide rule, you generally establish some value, operate on it, and continue from the result. When doing a long-hand calculation, you often calculate a subexpression, write down the answer, and use that answer in the next step. You will probably start with the inner parenthesis and work outward, just like someone with an RPN calculator does. RPN formalizes that process with a stack.

Suppose you want:

(3 + 4) × (5 + 6)

On a conventional calculator, you either need parentheses, or you have to calculate one result and remember it. On an RPN calculator:

3 ENTER

4 +

5 ENTER

6 +

×

The first + leaves 7 on the stack. The second leaves 11 above it. The multiply consumes both and leaves 77.

Notice what’s missing: parentheses, an equals key, and any need to tell the calculator about precedence. This isn’t much of a win for a five-key calculation. It becomes more apparent with something like computing the value of a bunch of parallel resistors:

R=1/(1/R1+1/R2+1/R3…)

An RPN user can calculate each reciprocal, add it to the running result on the stack, and finally take the reciprocal. Intermediate answers stay in the calculator naturally instead of being stuffed into memory registers or enclosed in increasingly impressive collections of parentheses.

Is RPN better? Calculator users have been arguing about that for half a century. But once RPN gets wired into your fingers, it can be surprisingly hard to give up.

RPN Before HP

Although RPN and HP are nearly inseparable in popular memory, HP didn’t invent the RPN calculator. That distinction generally goes to the Friden EC-130, introduced in 1964. It was a 44-pound transistorized desktop calculator costing $2,150, and its CRT actually displayed all four levels of its RPN stack at once. More than 18,000 were eventually made. You can even simulate it in Verilog, if you like.

HP entered the business in 1968 with the HP 9100A, a programmable desktop scientific calculator. It weighed about 40 pounds and sold for $4,900, but it provided logarithms, trigonometry, hyperbolic functions, coordinate conversion, programming, and RPN operation. HP famously called it a calculator rather than a computer partly because customers could often buy calculators without getting corporate computer departments involved.

Then came the machine that changed everything: 1972’s HP-35, the first HP pocket scientific calculator. From there, RPN became a defining characteristic of HP calculators for decades.

There were financial calculators, programmer’s calculators, scientific calculators, graphing calculators, and eventually the RPL machines such as the HP-28 and HP-48, which generalized the stack idea into an entire programming environment.

Eventually, though, algebraic entry won the mass market. Even HP began producing machines with algebraic modes, and some later calculators let you choose either system.

So did RPN die out? Are there any RPN machines left in 2026? Quite a few, as it turns out. The real question is do you want a calculator that pretends, in some way, to be your favorite old-fashioned RPN calculator, or do you want a more modern device that happens to use (or, at least, is able to use) RPN?

RPN You Can Still Hold In Your Hand

The strange survivor is the HP-12C. Introduced in 1981 as a financial calculator, it remains available new more than four decades later. Depending on the retailer, expect something around $50 to $70. The 12C Platinum, which can operate in either RPN or algebraic mode, is also still available for roughly $80 to $110.

Then there is the HP Prime G2, generally around $150 to $170. The Prime is a graphing calculator with CAS, programming, symbolic mathematics, touch screen, and textbook-style input — but buried among all of that is an RPN entry mode. It isn’t an old-fashioned HP RPN calculator, but it certainly qualifies.

The real center of new RPN hardware today, however, is SwissMicros. SwissMicros started by producing modern implementations of classic HP designs and has since moved well beyond simple reproduction. Its current machines include the DM15L, modeled after the HP-15C; the DM16L programmer’s calculator; DM41L, DM41C, and DM41X machines inspired by the HP-41 family; and the DM32, which occupies roughly the territory once covered by the HP-32SII.

At the high end is the DM42n, a modern descendant of the HP-42S idea. It has a 400×240 display, USB-C, programmable operation, matrices, complex numbers, equation solving, numerical integration, and 34-digit decimal arithmetic. It currently sells for a bit more than $300.

Perhaps more interesting is the R47, at about the same price. Instead of recreating a particular HP calculator, it grew out of the WP34S/WP43 community projects and attempts to be a modern enthusiast’s RPN calculator. It has an eight-level stack, matrices and vectors, complex arithmetic, base-N and bit operations, statistics, units, financial calculations, equation solving, integration, programming, and even built-in electrical engineering functions. Its firmware is still officially considered beta. This is probably the closest thing today to somebody asking, “What calculator would an obsessive HP engineer build if we started over?”

SwissMicros calculators aren’t cheap. Depending on the model, you’re generally looking at roughly $180 to more than $300 after currency conversion. But they are real, currently manufactured calculators rather than 40-year-old collectibles.

Heat Up The Soldering Iron

There is also a suitably hacker-friendly route. [Alex Garza’s] PAXER calculator projects reproduce machines such as the HP-15C, HP-16C, and HP-41C using an ATmega328. They’re available in kit form, use through-hole components, and aren’t just empty cases containing old calculator electronics.

The ATmega scans the keyboard, drives the display, and emulates the original machine. The design adds such modern amenities as continuous memory, an LCD backlight, real-time clock, and considerably higher execution speed than the originals. Kits and assembled versions are typically under $100.

There is also the 10LC (and related models), which approaches the problem from the other direction: take inexpensive ESP32-based M5Stack Cardputer hardware, add calculator firmware and key labels, and turn the whole thing into a pocket RPN calculator. Figure roughly $50-$60.

It isn’t going to make a calculator collector forget an HP-42S keyboard, but it does show how little hardware is actually required to build an extraordinarily capable calculator today.

There’s An App For That

CalcTastic is a modern RPN phone app.If physical keys aren’t mandatory, RPN is actually thriving. You can get a host of RPN calculators on your Android or iPhone. There are many options for desktop computers, too. You can generally find these on your platform’s program repositories.

One of the best places to start is Free42, [Thomas Okken’s] free, clean-room implementation of the HP-42S. It runs on Android, iOS, Windows, macOS, and Linux. Plus42, from the same author, takes that foundation and expands it with equations, units, directories, plotting, financial functions, and other capabilities that the original HP-42S never had.

Android users also have CalcTastic, which can switch between algebraic and RPN modes and provides scientific calculation, complex numbers, fractions, statistics, conversions, and — in the paid version — programmer functions. The basic version is free, and the Plus version is only a few dollars.

For the HP-48 crowd, there is Droid48 on Android and several HP-48 implementations on iOS. If your idea of a calculator includes directories, symbolic objects, programs, lists, and an RPL command line, the phone in your pocket can impersonate a 48 far faster than the original hardware ever could. If you prefer the HP41C, go41C is the one I like.

Apple users have an especially large selection. i41CX recreates and extends the HP-41 environment. PCalc isn’t primarily an RPN calculator, but has long offered a very good RPN mode. There are also modern RPN calculators including MathU, RPN Calc, RPN Calculator 48, and a growing number of simulations of individual classic HP machines.

Real HP

Even HP’s current flagship environment is available in software. HP Prime Pro runs on Android and iOS and supports optional RPN input.

Desktop users aren’t left out either. HP provides a free HP Prime Virtual Calculator for Windows, and there has also been a macOS version. Most unusually, HP once produced an actual native Linux version. The Linux release was packaged as an AppImage and survives as a 2019 technical preview — build 2.1.14288 in the copy sitting on my Linux machine. It isn’t the Windows version hidden inside Wine; it’s a native HP build, and it still works remarkably well. Unfortunately, it predates one particularly interesting addition to the Prime: Python programming.

There is a way to get that on Linux, although HP doesn’t make it easy. I found that the December 2024 Windows release of the Virtual Calculator, version 2.2.15212, runs under Wine using the Soda 9 runner in Bottles. That version includes the Prime’s Python app and reports MicroPython 1.9.4. The catch is that you don’t want to accept its offer to upgrade. Updating to the current Windows version causes the simulator to stop working under the same setup, and Soda 11 doesn’t appear to fare any better. So there is a slightly absurd sweet spot where an older Windows Prime under an older Wine runner provides a more up-to-date Prime on Linux than HP’s own native Linux build.

Between Free42, Plus42, Prime, HP-48 emulators and innumerable smaller projects, software may actually be the easiest way to use RPN today.

The Used Option

Of course, another source is available: millions of old calculators are already out there. If you’ve always wanted an HP-11C, 15C, 16C, 28S, 32SII, 41CX, 42S, or 48GX, auction sites and used-equipment dealers will happily provide one. Sometimes you can still find a bargain, especially if you’re willing to buy a cosmetically ugly machine or something outside the most desirable models. The problem is that the best old calculators aren’t merely used calculators anymore. They’re collectibles.

A calculator that originally lived in an engineer’s shirt pocket may now be a pristine example with its case, manuals, and box — and priced accordingly. Even fairly ordinary examples of desirable models can sell for enough money that carrying one around every day starts to feel irresponsible. My treasured HP41C, for example, works fine, but is already scuffed up enough that I rarely risk using it at my desk these days.

That creates a slightly absurd situation. A perfectly functional 35- or 40-year-old calculator may cost considerably more than an astonishingly capable modern computer because one is being priced as a collectible and the other as a tool.

On the other hand, battered calculators exist. If you don’t care about scratches, engraved names, missing battery doors, or someone’s old asset-control sticker, those are often exactly the machines to buy. You’re going to use it, after all.

ENTER Isn’t Dead

RPN plainly lost the calculator wars. Walk into an office supply store, and almost every calculator on the shelf expects conventional algebraic input. But it didn’t disappear.

You can still buy an HP-12C. You can buy sophisticated new RPN machines from SwissMicros. You can solder together your own HP-inspired calculator around an 8-bit AVR. You can turn an ESP32 gadget into one. Or you can install an app and have an HP-42S, HP-48, or modern RPN calculator in your pocket for anywhere from nothing to a few dollars.

That’s a surprising amount of life for an input system the mainstream calculator industry decided we didn’t want decades ago. Then again, people who like RPN tend to really like RPN.

If 2 ENTER 3 + looks more natural to you than 2 + 3 =, apparently somebody is still willing to sell you a calculator. If you want to see how easy it can be to parse RPN in a program, we’ve done that.

If you tear into old TVs or recording equipment, you may see shields made from some exotic-looking metal. Old timers will tell you it’s called mu metal, and its purpose is to — sort of — shield things from magnetic fields. The qualification is important. Unlike a conductive RF shield, mu metal doesn’t really stop a magnetic field. Instead, it gives magnetic flux an easier path to follow around whatever you’re trying to protect.

What’s In The Metal?

Mu metal belongs to a family of soft magnetic nickel-iron alloys. A typical modern formulation is about 80% nickel and 15% iron, with molybdenum and a few other elements making up most of the remainder. What makes it useful is its extremely high magnetic permeability. Commercial material can have relative permeability around 100,000 or more, and some specialty alloys can reach even higher.

You can think about reluctance as the magnetic equivalent of resistance. Put a high-permeability shell around something sensitive, and magnetic flux would much rather travel through the shell than through the space inside it, just like current tends to take the path of least resistance.

This works particularly well for DC and low-frequency fields, exactly where your usual copper or aluminum EMI shield isn’t much help.

You May Have Seen It Before

Classic applications included shielding CRTs, tape heads, transformers, photomultipliers, and sensitive analog instruments. Put a transformer too close to the wrong part of an old television or audio amplifier and 60 Hz magnetic fields could cause very visible — or audible — trouble. The disappearance of CRTs and magnetic tape might make mu metal sound like another material destined for the antique electronics cabinet.

However, mu metal is still around. Modern applications include magnetometers, precision current sensors, electron microscopes, scientific instruments, and experiments that require extremely low magnetic fields. Commercial multi-layer mu-metal chambers are still sold for creating near-zero-field environments; with suitable construction and degaussing, some claim attenuation of static and low-frequency fields by factors approaching a million.

Quantum and cryogenic instrumentation have also created some 21st-century magnetic shielding problems. Ordinary mu metal loses performance at very low temperatures, so related nickel-iron alloys are made specifically for operation at liquid-nitrogen and liquid-helium temperatures.

Don’t Bend It

There are a couple of catches. First, mu metal gets much of its impressive permeability from its metallurgical structure. Machining, stamping, welding, or even bending it can introduce stresses and seriously degrade its magnetic properties. High-performance shields are therefore commonly formed first and then hydrogen annealed to restore their permeability.

So buying a sheet of wonderfully permeable material and folding it into a box isn’t necessarily the recipe for a wonderfully permeable box.

The second surprise is saturation. Mu metal is superb with weak fields but isn’t necessarily what you want closest to a powerful magnet. Its saturation induction is only around 0.75 tesla. In strong fields, manufacturers recommend combining it with a lower-permeability material having higher saturation capability, letting that outer layer tame the field before the mu metal handles what’s left.

History

British scientists Willoughby S. Smith and Henry J. Garnett patented mu metal in 1923 for inductive loading of submarine telegraph cables for a British company that built the Atlantic undersea telegraph cables. The seawater surrounding these cables added capacitance, requiring inductance to compensate. This was first done by wrapping the conductors with a helical wrapping of metal tape or wire of high magnetic permeability, which confined the magnetic field.

Mu-metal was invented to directly compete with permalloy, the first high-permeability alloy used for cable compensation, but it belonged to competitor Western Electric. Mu-metal was developed by adding copper to permalloy to improve ductility. Each 1.6 km of cable needed about 80 km fine mu-metal wire so there was a great demand for the alloy.

Other Tricks

Mu metal isn’t the only way to fight magnetic interference, as you can see in [FesZ’s] video below. Ordinary steel and other high-saturation magnetic alloys can redirect stronger fields. At higher frequencies, conductive copper or aluminum shields become effective through induced eddy currents. Ferrite is good at high frequencies, too, but is not very ductile nor is it very conductive. When you really need a quiet magnetic environment, active compensation coils can measure the ambient field and generate an opposing one.

But if the problem is a weak DC or low-frequency magnetic field, the basic trick hasn’t changed much. You just give the magnetic flux an easier path. Sometimes the old material in that 50-year-old television can be at home in a quantum computer, too.

People have lived in space stations for decades now, but something is wrong with them. Where are the big rotating wheels? You know the ones. They show up in old paintings of the future and, perhaps most memorably, in 2001: A Space Odyssey. Spin a great wheel in space, and people can stroll around inside with something that feels suspiciously like gravity. It seems like an obvious idea.

It is also an old idea. Much older than actual spaceflight, in fact. But to find the beginning of the space station, we have to go back to a time when powered aircraft were still several decades in the future.

A Moon Made of Bricks

In 1869, Edward Everett Hale published The Brick Moon in The Atlantic Monthly. The moon in question wasn’t natural. Hale imagined building a 200-foot-diameter sphere made from bricks and putting it into orbit as a navigation aid. Sailors could sight it and use its known orbit to determine their longitude. There was only one small problem: the thing was accidentally launched with people aboard.

That makes The Brick Moon generally regarded as not only the first fictional artificial satellite, but also the first fictional space station. Hale followed it in 1870 with Life on the Brick Moon, describing how the accidental colonists got along up there.

Hale didn’t have rockets. He proposed flinging the thing into the sky with giant flywheels, but, then again, it was 1869, so we’re inclined to cut him some slack. As the 19th century turned into the 20th, however, people started doing the math.

Konstantin Tsiolkovsky is best remembered for putting rocket flight on a sound theoretical footing. The Russian schoolteacher wrote extensively about orbital flight and space habitation, envisioning people living in orbit long before anyone had demonstrated that a liquid-fueled rocket actually worked. His ideas included rotating habitats to provide artificial gravity.

Hermann Oberth’s 1923 book Die Rakete zu den Planetenräumen — The Rocket into Planetary Space — Oberth went beyond fiction and seriously considered a permanently inhabited station. He envisioned it being periodically supplied by smaller rockets, serving as an observation and communications platform, and even acting as a jumping-off point for trips farther into space. He also suggested spinning the station to give the crew artificial gravity.

Enter The Wheel

But the space station that looks like the space station in your head probably comes from a different Hermann. Herman Potočnik was an Austro-Hungarian army officer and engineer who wrote under the name Hermann Noordung. In 1929, he published Das Problem der Befahrung des Weltraums, translated by NASA many years later as The Problem of Space Travel: The Rocket Motor.

Noordung didn’t merely say, “We should have a space station.” He drew one. His station consisted of several components, but the memorable one was the Wohnrad — literally the habitation wheel. Living quarters occupied a rotating ring connected to a central hub. Rotation provided artificial gravity, while other portions of the complex could remain weightless. He considered power, communications, observing Earth, astronomy, docking, and the practical business of living in orbit. NASA calls his work one of the first detailed technical designs for a space station.

If you have seen Wernher von Braun’s famous wheel station from the 1950s, you may notice something. Von Braun certainly knew Noordung’s work — he had cited it years earlier — and the family resemblance between Noordung’s 1929 Wohnrad and the wheel station that von Braun and Willy Ley presented to American readers in Collier’s in 1952 is hard to miss. The only place we have found the articles is the reprints in Horizons, by the AIAA (start with page 46).

Whatever the exact family tree, von Braun was the man who put the wheel-shaped station into American popular culture. In 1952 he described a 250-foot-class rotating station in Collier’s, accompanied by some gorgeous Chesley Bonestell artwork. A few years later he took the idea to television with Walt Disney.

If you’ve never seen these, they are worth your time. Disney’s 1955 Man in Space and Man and the Moon let von Braun explain a remarkably detailed vision of rockets, orbital stations, and trips to the Moon to a mass television audience. A generation of kids grew up expecting this stuff. What happened?

Gravity, More Or Less

The attraction of the wheel is simple. You can’t really make gravity, at least not without bringing along a planet-sized lump of mass, but acceleration will do nicely. Stand on the inside of a rotating ring, and the floor keeps accelerating you toward the axis. In your rotating frame, it feels as though something is pushing you outward against the floor.

The acceleration is a=ω²r where (r) is the radius and (ω) is the angular velocity. Sadly, the numbers explain part of the problem.

If you want one Earth gravity at the floor, you need to either spin fast or have a large ring. For example, at 1 RPM the ring has to be 1.79 km in diameter. Speed up to 2 RPM, and you can get away with 447 meters. At 4 RPM, you are down to 112 meters.

Seems like you could just keep going faster, but there’s a problem. Four RPM doesn’t sound very fast until you are inside the thing moving your head around.

Humans can adapt to rotation, but increasing the speed makes Coriolis effects increasingly noticeable. Move your head, climb a ladder toward the hub, throw something, or even walk spinward instead of anti-spinward, and the results aren’t quite what your inner ear expects. NASA artificial-gravity studies have often used approximately four RPM as an important practical region for human tolerance, although this isn’t a hard physical limit and training matters.

There’s also a gravity gradient. Your feet are farther from the axis than your head, so they weigh slightly more. Make the radius large enough, and you won’t notice. Make the station small enough, and things get strange quickly. So bigger is better except when it comes to cost, of course.

There is another oddity that movies sometimes get wrong. The outside circumference of the wheel is the floor. “Down” is away from the hub. Imagine a tire in space. You aren’t walking around on one of the flat sidewalls with the axle beside you. You are walking around the inside of the tread. So when a movie gives you a nice conventional room with a picture window on what looks like the outer wall, stop and think about where gravity ought to be pointing. Depending on the geometry, that window would probably be underfoot.

Stanley Kubrick got this wonderfully right in 2001: A Space Odyssey. Both Space Station V and the rotating centrifuge aboard Discovery make “down” follow the rotation. The famous jogging sequence works precisely because the circular wall of the set becomes the floor as the camera watches.

Next Time

Next time, I’ll look at early attempts to make a space station ranging from TASSEL, MTSS, and MARS to real Soviet and U.S. stations that had varying degrees of success. Spoiler alert: none of them are going to rotate for gravity.

Of course, people didn’t just imagine space stations. They also imagined moon bases, both fictional and actual.

If you don’t actually need one, you’d be forgiven for thinking a hearing aid just makes everything louder for the wearer. Especially since there are plenty of shady products out there which will do exactly that for just four easy payments of $29.99. But the reality is considerably more complex, as a proper hearing aid needs to be capable of selectively enhancing certain frequencies while squashing down others.

The technical challenges involved in pulling that off in a device small enough to fit inside the human ear and run off of a tiny battery are considerable — and while there’s undoubtedly been some degree of artificial price inflation going on over the years, there’s a reason proper hearing aids have been so much more expensive than their “As Seen on TV” counterparts. These same challenges are also why DIY and open source hardware hearing aids have struggled to gain much traction.

But over the last few years the situation has changed. In 2022 the United States Food and Drug Administration (FDA) established the framework by which hearing aids could be sold over the counter (OTC). Although they’re generally less capable than their prescription counterparts and not suitable for individuals with profound hearing loss, the wide commercial availability of OTC hearing aids has kicked off a competition between manufacturers to deliver more affordable devices.

That competition entered a new phase earlier this month when the FDA granted approval for Samsung’s Galaxy Earbuds to fall under the same category. This follows a similar decision made about Apple’s AirPods back in 2024. The two biggest players in the smartphone market being able to offer their earbuds as OTC hearing aids represents a unique value proposition. Not only are they priced for mass market consumption, but many individuals who would be interested in purchasing an OTC hearing aid will already own them and need only to enable the feature with a software update.

Given how different the situation is today than even just five years ago it’s worth asking just what qualifies as a over-the-counter hearing aid, and how the shifting definition of these devices can inform the community’s efforts to develop open hardware solutions.

What is a Hearing Aid, Anyway?

We’ve already covered what a hearing aid isn’t, but before we go too much further it’s a good idea to clarify what exactly a hearing aid does in comparison to a simple audio amplifier, which in the industry are officially known as Personal Sound Amplification Products (PSAPs). This is a topic we’ve touched on previously here at Hackaday, but the short version is that since the 1980s or so, the frequencies that a hearing aid amplifies have been tailored to match the specific auditory deficiencies of the wearer.

Traditionally, getting a hearing aid prescribed would require going to an audiologist and getting an audiogram. This chart plots the results of a hearing test, and shows how well the patient can hear various frequencies. With this data, it’s possible to quantify the severity of a patient’s hearing loss and determine where a hearing aid could improve the situation. This information could then be programmed into a hearing aid to provide a bespoke amplification profile that takes into account the needs of the wearer.

It’s worth noting that this is only in the context of relatively modern digital hearing aids, essentially those created after the introduction of solid state electronics. Prior to that point, hearing aids were closer to what we would now consider PSAPs and amplified incoming audio indiscriminately.

Defining a Middle Ground

With this in mind, the primary difference between a prescription hearing aid and an OTC model is that the audiologist is cut out of the loop. That means there’s no audiogram, and in turn no data to program the hearing aid’s filters with. The specifics of this does vary from model to model, and both Samsung and Apple offer some basic audio testing tools that can help the user fine-tune their experience. But at this point, no OTC hearing aid product is capable of diagnosing the wearer’s hearing to the same level as a comprehensive hearing test performed by an audiologist

That being the case, it might seem like an OTC hearing aid is just a PSAP. But in their ruling, the FDA established the formal requirements for a device that falls somewhere in the middle. The full document, Medical Devices; Ear, Nose, and Throat Devices; Establishing Over-the-Counter Hearing Aids, comes in at around 200 pages if you’re looking for some light reading.

Fortunately, the agency provides a boiled-down list of what features a device must have to meet the definition of an OTC hearing aid. Some of the requirements have to do with the packaging and marketing of the product, which of course are important for a medical device, but it’s the technical parameters that we’re interested in.

The FDA specifies that all OTC hearing aids must use air-conduction, that is, operate with a speaker inserted into the ear canal. The alternative would be bone conduction, which is generally used in cases with more profound hearing loss. The devices must also allow users to adjust, at least to some degree, the output to match their specific needs.

The intention with this second requirement goes deeper than a simple volume control as you’d have in an PSAP. In lieu of the sort of tailored output a prescription hearing aid would offer, the OTC device can offer a selection of pre-defined audio filter profiles which the user could flip through. At the risk of oversimplifying things, it’s a bit like the audio presets for different genres of music that modern headphones and earbuds usually offer in that the user can cycle through different profiles to find what best matches the current environment.

When combined with the acknowledgement that OTC hearing aids can utilize wireless technology to communicate with another device, it’s not hard to see how the smartphone fits into the equation. While a stand-alone device is capable of meeting the requirements necessary for OTC hearing aid classification, being able to connect to the user’s phone to easily switch between audio profiles on the fly makes for a greatly improved user experience. Additionally, by playing tones through the earbuds, software on the phone can perform a hearing test on the user — the results of which can be used to fine-tune the output beyond what’s possible with simple presets.

Hackers, Take Note

Of course, anyone looking to build an open source hearing aid probably isn’t looking for FDA approval. The goal is likely to make the technology cheaper and more accessible, especially in areas where getting a commercially produced hearing aid may be difficult. As such, the new over-the-counter classification may not seem terribly important for folks like us.

But that doesn’t mean we can’t pick up some tips from what companies like Samsung, Apple, and Jabra are doing. By bringing the user’s smartphone into the mix, they have made hearing aids more accessible and easier to operate. Instead of trying to pack all of the functionality directly into the wearable, using a smartphone to do some of that heavy lifting opens up a lot of new possibilities. There are naturally trade-offs when switching a hardware problem for a software one, especially when dealing with mobile operating systems, but it may be a compromise worth making if it means getting this sort of assistive tech to more people that need it.