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.
When I got back into SLA resin printing recently, I knew that I’d inevitably have to deal with the agony of failed prints and of course resin spills. This moment eventually came, and I felt motivated to treat mistakes as teaching moments on aspects like how to properly prepare an SLA build plate in terms of angles and supports or how to deal with failed print aftermaths.
Before moving on to the disaster, I’d like to first start with a look at the resin print of the previous article, which contained a number of fairly small parts. These I had oriented and supported almost fully using the automatic methods provided by the ChituBox slicer software, and worked about 90% as I had hoped, while leaving plenty of room for improvement as well.
Overall, preparing an SLA build plate in the slicer isn’t quite the same as for an FDM printer, mostly due to one phrase that strikes fear in the heart of anyone who has ever done resin printing: “peeling forces”.
Last time we found out the idea of space stations is surprisingly old. By the 1950s, everyone knew we’d be working in beautiful space stations that rotated like a wheel to give us the illusion of gravity. Of course, that didn’t happen. But we did get some practical space stations, even before the current crop. The road to get there, though, was predictably bumpy.
Convair: From TASSEL to MARS
Convair had been studying multi-person orbital stations under Krafft Ehricke since the late 1950s. One result was TASSEL, an acronym for the Three Astronaut Space System Experimental Laboratory. Proposed in 1960, TASSEL was a three-man laboratory intended for an Atlas-Centaur launch into a roughly 200-nautical-mile orbit and missions lasting two or three weeks.
Around the same time, the Air Force asked contractors for proposals for a Military Test Space Station, or MTSS. Convair was one of five companies selected for the study in 1960. The surviving record suggests that Convair’s TASSEL work fed directly into its MTSS proposal.
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.
Once upon a time, telephones were primarily point-to-point communications systems. There were options for three-way and conference calls out there, but by and large the plain old telephone system was about connecting one handset to another for a direct conversation. For this reason, the telephone was seldom used for mass emergency communications, because it was simply not fit for broadcasting a message to a wide number of people.
However, technology has since changed. Our modern phones are all connected to a big digital over-the-air network, and large swathes of them can be addressed all at once if so needed. This has led to the development of emergency warning systems that use the cellular network, with Cell Broadcast being the most notable iteration.
Trains and the railways they run on are a great way to move lots of stuff, or lots of people, a long way. Steel wheels on steel rail can shift great loads at good speeds and railways remain a backbone of logistics for this very reason. The only problem is that they require a great deal of initial investment to build and plenty of maintenance to keep them functional over time.
These concerns can make a railway a difficult proposition when it comes to getting large amounts of goods in and out of remote areas. It’s a problem that Australia faces, with settlements far off the beaten track that are nevertheless in need of high-throughput freight connections. And if you can’t go rail, you go road… in a big way.
You may have a 4K television. Perhaps you have even bought an 8K screen, despite the shortage of things worth watching in 8K. A 16K display is, today, a rarity. But even when those eventually become commonplace, yours probably will not cover 14,900 square meters, rise 73 meters into the air, or wrap over your head and behind your peripheral vision.
That is approximately what happens inside Sphere in Las Vegas. The venue’s interior display is quoted as having a resolution of 16K by 16K and an area of 160,000 square feet, or about 3.7 acres. Unlike most enormous movie screens, it is not illuminated by a projector. The entire surface is a direct-view LED display: an immense, curved video wall assembled from tens of thousands of smaller pieces.
After seeing The Wizard of Oz at Sphere, however, the most interesting part was not simply the screen’s size. It was how thoroughly the screen could disguise itself.
Where Did The Theater Go?
Radio City or the Sphere? (It is the Sphere; photo courtesy [DP])Before the presentation began, the auditorium appeared to have a conventional architectural ceiling. Great orange ribs curved over the seating, while ventilation grilles, suspended loudspeakers, lighting fixtures, curtains, and video monitors completed the illusion. It looked like the Radio City Music Hall’s proscenium. Then the show started — and the apparent theater completely disappeared. The speakers, the TVs, even the stage.
The obvious first conclusion was that the LED surface must be optically transparent, allowing the audience to see the real roof behind it until the pixels illuminated. That explanation was attractive because Sphere’s audio system really is installed behind the display, and the surface must allow sound through it.
It was also, apparently, wrong. The only explanation that makes sense is that the ceiling, ribs, grilles, speakers, and monitors were already being displayed by the screen. It was like a holodeck impersonating a physical theater interior. When the Oz material began, the system simply replaced one complete visual environment with another.
That’s what happens when a display fills nearly all of your useful visual field. A normal screen announces itself with a bezel, a wall, or at least a clearly visible edge. Sphere’s display extends upward and around the audience, removing many of those references. Give the image credible perspective, texture, shadows, and familiar architectural details, and the brain accepts the pixels as a room.
The same effect makes the Oz landscapes seem less like scenes displayed in front of the audience and more like places into which the auditorium has been inserted. Of course, there are more special effects. For The Wizard of Oz, there is wind and smoke, along with paper leaves, flower petals, and foam-rubber apples that fall from the sky. All of this makes it even more immersive.
