Why your silent electronics sing

0:00You're listening to a podcast right now, driving, working out, walking the dog. If you're into podcasts, chances are you have something to say too. With RSS.com, starting your own is free and easy. Upload an episode, and we distribute it to Apple podcasts, Spotify, Amazon Music, and hundreds more. Track your listeners, see where they're from, and start earning from ads like this. Even with just 10 listeners a month. If you've been thinking about starting a podcast, this is your sign. Start free at RSS.com. Have you ever been sitting alone in a perfectly quiet room, maybe late at night, and you just start to notice it? Oh, the wine. Yes, exactly. Yeah. That incredibly faint, high-pitched wine coming from your laptop charger, plugged into the wall. It drives people crazy once they hear it. Right. Or maybe you're standing on a subway platform, and you hear that distinct rising sort of alien-sounding hum of the train motor as it accelerates. Yeah.

1:00Or just the relentless buzz of a fluorescent lamp over your desk. Exactly. And you look at these objects, and they are completely still. I mean, there are no moving parts, no fans, no tiny speakers. And yet, they're undeniably singing to you. It is a profoundly universal experience, really. It's one of those things we've all just collectively accepted as, you know, background noise. Yeah, we just assume it's the sound of electricity doing its job, right? But the thing is, electricity itself is entirely silent. Electrons moving through a wire, they do not have a voice. Which is just a bizarre paradox. Because if electricity makes no sound, why is my solid plastic laptop brick whining at me from the floor? That is exactly what we're going to get into. And that's the mission of today's deep dive. We are jumping into this fascinating, highly-detailed Wikipedia article on electro-magnetically-induced acoustic noise, which is sometimes simply called coil wine. Yeah, coil wine is the common term. Right. And we are going to figure out exactly what invisible forces are grabbing hold of our everyday

2:03electronics and forcing them to physically sing buzz and vibrate. We definitely need to be very precise right from the start about what this phenomenon actually is, though. Okay, lay it on us. Because when we say noise in this context, we aren't talking about like electrical signal noise. It's static on a radio. Exactly. We aren't talking about radio-frequency interference or electromagnetic compatibility. That's the stuff engineers spend a lot of time trying to filter out of the actual electrical circuits. So it's not a signal problem? No. We are talking about literal physical acoustic sound. This is a purely structural phenomenon. Structural. Yes. The solid materials inside your devices are physically vibrating, violently and rapidly under the excitation of electromagnetic forces. That is the biggest mind-bender for me. You're looking at that charger on your floor and inside that sealed box, the metal is literally flexing and moving. It is. It's pushing the air, which hits your eardrums. Now a crucial distinction we need to make right away, based on our sources, is about high

3:06voltage power lines. Oh, right. The big transmission lines. Yeah. Because if you've ever walked under those massive lines and heard that aggressive crackling hiss, you might logically think, oh, that's just coil wine on a massive scale. But it's actually not. Right. That hissing is something called corona discharge, which is, it's the electrical breakdown of the air itself. The voltage is literally tearing the air molecules apart. Yeah. It is not magnetism vibrating the metal of the wire. The mechanism there is entirely different. Got it. So we're talking about magnetism here. Exactly. What we are focusing on today is sometimes referred to in the literature as the reciprocal of microphonics. Okay. Break that down for us. Well, if you are familiar with audio engineering, you know microphonics is when mechanical vibrations like tapping your finger on a vacuum tube or dropping a pearly shielded cable. Create an undesired electrical noise in the signal. Right. The physical world intrudes on the electrical precisely. But what we're looking at today is the exact opposite. The electrical and shooting on the physical.

4:07Exactly. The electrical and magnetic forces are creating mechanical vibrations out of thin air. Okay. So we have these physical vibrations happening inside silent machines. Let's look at the invisible hands actually doing the pushing and pulling. Because there's no tiny hammer inside. Right. If you tear open that laptop charger, you aren't going to find a tiny hammer hitting a bell. You're going to find coils, circuits, and magnets. So what are the actual physical forces that play here? Well, the primary culprits are what we call Maxwell forces or reluctance forces. I'm a 12 forces. Got it. And these forces concentrate incredibly intensely at boundaries where there is a massive change in magnetic reluctivity. Okay. Reluctivity. Let's translate that. That's basically just how much a material resists a magnetic field, right? That's a great way to put it. Yeah. Think of a magnetic field traveling easily through a piece of iron. And then suddenly it reaches the edge of the metal and slams into a brick wall of air. Because air resists magnetism way more than iron does.

5:07Exactly. That sudden massive shift in resistance creates immense physical stress right at the boundary. So it's a surface thing. Right. Exactly. Same fundamental forces that make two solid magnets violently pull together or push apart when you hold them close to each other in your hands. Okay. Let's unpack this. Imagine a solid block of iron inside a machine. Okay. The Maxwell forces are acting like an invisible tug of war happening right at the surface, right at that boundary where the metal meets the air. It's yanking aggressively on the outside of the metal. That's a perfect visual. Yeah. This is a point out another distinct force called magnetostriction. So if Maxwell forces are pulling from the outside. Magnetostriction is like the metal itself breathing or squeezing from the inside. Wow. Yeah. It's an internal force entirely contained within the atomic structure of the ferromagnetic material. So the actual physical shape and volume of the magnetic core changes minutely as the magnetic field fluctuates through it. Yes. You have the Maxwell stress pulling at the surfaces.

6:10Magnetostriction warring within the molecular structure of the material itself. That sounds chaotic. And the physics don't stop there either. We also have to account for Lorentz forces, sometimes called Laplace forces. Oh, right. On the wires. Exactly. These specifically act on the conductors, the actual copper wires themselves, when they are plunged into an external magnetic field. So every time the alternating current pulses through those coiled windings, the wire physically tries to jump or twitch like a muscle. Yes. And if there is a strong electrical field present, you add electrostatic and even piezoelectric forces into the mix. It is an absolute peiotic mosh pit of physics inside these little devices. It really is. But and this is a key point for you listening to keep in mind all these invisible pushes and pulls all this metallic breathing and wire twitching. It only actually matters to us if two very specific physical conditions are met. Right. Because otherwise it's just vibrating silently. Exactly. The frequency that these vibrations has to land right in the sweet spot of human hearing,

7:14which is roughly between 20 hertz and 20 kilohertz, which is a pretty wide range, but still. Right. If the metal is vibrating at five hertz, it might physically shake the device on your desk, but you won't actually hear a whine. Yeah. And second, the surface area radiating that vibration has to be large enough to actually push a meaningful amount of air to your eardrums. Right. If the vibrating part is microscopically small and completely isolated, it just remains a silent, invisible struggle. And the frequency of the noise you end up hearing is directly tied to the nature of those electromagnetic forces and the frequency of the electromagnetic field itself, which changes depending on the power type, right? Absolutely. It changes dramatically depending on whether there is a direct current or DC component involved, or if it is purely alternating current. Okay. So these forces are universal. They're happening anytime we have alternating currents and magnetic fields anytime. Let's look at how they manifest in everyday objects, specifically the ones that just sit there. The static singers, as we could call them, passive components are a great place to start

8:16with this. Let's look at inductors, which are sometimes called reactors or chokes in the engineering world. Okay. Inductive. In these devices, the magnetic energy is actually stored in an intentional air gap within the magnetic circuit. An intentional air gap. So that means a massive boundary change. Exactly. Because you have that sudden transition from metal to air and back to metal again, massive Maxwell forces apply right across that gap, desperately trying to pull the gap close. Oh, wow. So it's constantly trying to crush itself. Yeah. And the resulting noise depends heavily on the geometry of the circuit and the specific materials used to build it. And then you have transformers, like those big green utility boxes sitting in your neighborhood, or the gray cylinders up on the power poles. Oh, transformers are fascinating because they get hit from all sides. Yeah. They're getting hit with a trio of forces all at once. Yeah. You have a Lorenz forces physically shaking the copper windings, the Maxwell forces attacking the joints of the metal laminations, trying to pull them apart. And the magnetistriction happening deep inside the laminated core, right, making the whole

9:19massive block of metal subtly expanded, contract over and over again. It's honestly a minor miracle. They don't tear themselves apart, let alone just hum. It is a profoundly hostile environment for a piece of metal to exist in over decades of use. No kidding. But this brings us to one of the most counterintuitive examples in the literature, which is the singing capacitor. Okay. Wait, hold on. I'm with you on the transformers and the inductors. I mean, they have magnetic cores and coils of wire. Sure. But a capacitor, a capacitor is essentially just two conductive metal plates with an insulator, a dielectric sandwiched in between them. That's right. There are no coils. There are no moving parts whatsoever. So how in the world does a microscopic sandwich of metal and plastic make a sound? What's fascinating here is that we have to step away from magnetism entirely for a moment and look purely at the electric field. Okay. Pure electricity. Yeah. Capacitors are subject to immense electrostatic forces.

10:21When the voltage or current waveform running through the capacitor isn't perfectly smooth and constant, which I imagine is off in the case. Right. When it contains what we call time harmonics, these harmonic electric forces appear. Time harmonics, let's visualize that. It's like, imagine an ocean wave that isn't a smooth rolling swell, but instead is choppy stuttering in uneven as it hits the shore. Exactly. That choppy electrical flow creates irregular forces. But the real trick here is the reverse piezoelectric effect. Oh, piezoelectric, like the clicker and a lighter. Right. In ferroelectric capacitors, the dielectric insulator material literally changes its physical shape when an electric field is applied. Are you serious? Yeah. As the choppy voltage fluctuates, the material rapidly expands and contracts. It acts exactly like a tiny, unintended speaker driver pushing against the air. Oh. And that is the singing capacitor effect. So the stuttering electricity is literally squeezing the material like a microscopic accordion.

11:23Three much. That is incredible. And our sources briefly mentioned a few other static suspects that suffer from this noise too. Right. Like in medical equipment. Yeah. In MRI machines, the receiving coils have something called coil noise, simply due to their non-zero temperature. Which sounds crazy, right? Yeah. The baseline heat creates minute, thermal agitations that interact with the massive magnetic fields of the MRI. And then there's the electric trains. Oh, the braking resistors. Because they use these massive braking resistors to burn off excess electrical power when the overhead lines, the catenary can't absorb it. And those giant resistors can howl just from the raw, sheer power being dumped through them. Which makes me wonder if these static objects are practically screaming just sitting there bolted to a wall or sitting on a circuit board. What kind of chaos happens when we take these exact same magnetic forces and deliberately spin them at thousands of RPMs in a motor? Oh, well, now we're entering the realm of radial and axial flux rotating machines.

12:23Which includes what? Exactly. This includes induction motors, switched reluctance motors and synchronous motors. No, right. Remember that subway electric motor wine you mentioned at the beginning? Yeah. That rising alien hum. That is a permanent magnet synchronous machine or PMSM doing exactly what it was designed to do, but with intense acoustic consequences. And the primary antagonist here seems to be something called torque ripple. Yes, torque ripple is a major factor. Now, the main electromagnetic torque of a motor is what actually spins the wheels. And as a static steady force, it doesn't make noise. It just creates motion. Right. Ideally. But the torque isn't perfectly smooth, right? It has harmonic variations, little stutters and microscopic pulses in the rotational push. Exactly. And in a permanent magnet machine with an open circuit, this is specifically called cogging torque. So these dynamic stuttering pulses create torsional vibrations, physically twisting the rotor in the stator back and forth like someone ringing out a towel.

13:26Yeah. But while a simple solid cylinder twisting back and forth doesn't radiate sound very efficiently into the air, the complex outer bounder of a motor stator's, you're listening to a podcast right now driving, working out, walking the dog. If you're into podcasts, chances are you have something to say too. With rss.com, starting your own is free and easy. Upload an episode and we distribute it to Apple podcasts, Spotify, Amazon music and hundreds more. Track your listeners, see where they're from and start earning from ads like this. Even with just 10 listeners a month. If you've been thinking about starting a podcast, this is your sign. Start free at rss.com. You're listening to a podcast right now driving, working out, walking the dog. If you're into podcasts, chances are you have something to say too. With rss.com, starting your own podcast is free and easy. Upload an episode and we distribute it to Apple podcasts, Spotify, Amazon music and more.

14:28Track your listeners, see where they're from and start earning from ads just like this. If you've been thinking about starting a podcast, this is your sign. Start your new podcast for free today at rss.com. Certainly can. Right. Lots of surface area. For those Maxwell stress forces we talked about earlier aren't just pulling in one simple direction. They're pulling all over the place. Right. They have radial force harmonics that pull outward on the stator and tangential force harmonics that pull sideways on the individual stator teeth. Oh, wow. So imagine trying to snap off the teeth of a plastic comb by pushing them sideways over and over. That's exactly what's happening. That intense bending moment causes the entire outer casing, the yoke of the machine, to vibrate outward into the surrounding air. The source is highlight that to get that really deafening ear piercing scream from a motor, you need more than just vibrations. You do. You need a perfect storm. You need resonance. Exactly. In radial flux machines, resonance isn't just about matching a single frequency, like a

15:30singer hitting the perfect pitch to shatter a wine glass. Okay. It requires a very specific simultaneous double match, double match, condition one. The frequency of the exciting Maxwell force must perfectly match the natural vibrating frequency of the stator or rotor, which makes sense. But condition two, the harmonic wave number, which is the physical periodicity of the force distributed along the air gap, that must match the structural modal shape of the machine. Okay. Here's where it gets really interesting for me. So timing is the frequency, like pushing a kid on a swing at the exact right second to make them go higher. That covers the frequency condition. Yes. If you push when they are coming backward at you, you kill their momentum. You have to push at the exact right time. But wave number adds a physical shape to that timing. Imagine that swing isn't a normal seat, but a giant, flexible hula hoop suspended in the air. Oh, I see. To make a flexible hula hoop warp into an oval or an ellipse, I can't just push it in one spot.

16:31It would just swing away from me. Exactly. I had to push it inward on two opposite sides at the exact same time. That is it. That physical pattern of where you push is the wave number. So if the natural bending mode, the structural modal shape of the motor's stator is an ellipse, the electromagnetic force wave number must be precisely two. Two pushes basically. It has to be pushing out at two points and pulling in at two points at the exact right frequency. And when both the timing and the physical shape of the force match the metal structure perfectly. The maxima of the electromagnetic excitation and the maxima of the physical displacement are perfectly in phase. The motor essentially amplifies its own physical distortion and the vibrations just explode in magnitudes. It absolutely explode. There is a brilliant experimental illustration of this in the sources using a simple tuning fork that really brings this to life. Oh, I love that example. Yeah, imagine a heavy iron tuning fork. But one of the prongs is wound tightly with a coil fed by a variable frequency power supply.

17:32Right. Which creates a variable magnetic flux density in the empty airspace between the two prongs. Generating dynamic magnetic forces, trying to pull them together at twice the frequency of the electrical supply. And as you slowly dial up the frequency of that coil, sending faster and faster alternating current through the wire, nothing dramatic happens at first. The fork might hum slightly, but the moment that exciting magnetic force hits the fundamental natural mode of the tuning fork, which in this specific historical experiment is right around 400 hertz. The two prongs suddenly lock in. Yes. A massive, spontaneous and violently loud acoustic resonance occurs driven entirely by an invisible magnetic field jumping across the gap. It clearly demonstrates how violently a solid piece of iron can be forced to move by magnetism alone. It's quite the party trick. It really is. Okay. So we know exactly how this perfect storm forms. We've got the tug of war of Maxwell forces, the internal breathing of magnetistriction,

18:32the twisting of torque ripple, and the swing set physics of double match resonance. We've covered a lot of physics. Yeah. So how do engineers actually predict this before they build a motor? And more importantly, how do they stop our devices from driving us absolutely crazy? This brings us to the realm of numerical simulation and multi-physics modeling. Which sounds intense. It is. Engineers don't just build a massive multi-million dollar industrial motor, plug it in, and cross their fingers that it won't scream. I would hope not. They calculate the electromagnetically induced noise beforehand in a highly complex three-step process. Okay. What are the steps? First, they calculate the electromagnetic forces. Second, they calculate how those forces will cause magnetic vibrations in the structure. And third, they calculate how those structural vibrations will translate into acoustic noise radiated into the air. What's really interesting is that the sources call this a weekly coupled problem. Which honestly sounds like a bad thing like the math is falling apart.

19:33It does sound like a flaw, but it actually makes the simulation possible. Right. It means it's a one-way street. While the magnetic forces cause the massive steel frame to physically deform, that physical deformation is microscopic enough that it doesn't bounce back and significantly alter the overall magnetic field itself. Exactly. Error of cause and effect mostly points in one direction. And to do this, engineers use massive, multi-physics simulations. Specialized software, yeah. It's not enough to just model the electricity. The software has to calculate the invisible magnetic fields, figure out exactly how those fields physically warp the steel, and then calculate how much air that warps steel pushes into your eardrum. It's an insane three-step chain reaction. But even with perfect software modeling that chain reaction, the engineer has to understand what they are actually diagnosing in the real world. Like is the machine broken or is it supposed to sound like that? Precisely. Are you modeling a healthy machine working perfectly or a faulty one?

20:34Because even a perfectly healthy factory fresh machine will generate harmonic electromagnetic forces. Because of the power supply, right? Yeah. This can come from pulse width modulation or PWM supplies. At PWM is basically a technique that uses a rapid flickering switch, turning the power on and off thousands of times a second to control the overall power output. But that flickering introduces harsh, high-frequency harmonics into the magnetic field. So it's healthy, but noisy. Right. Healthy noise can also come from slotting effects, which is the physical gaps between the state or teeth causing sudden magnetic fluctuations every time the rotor passes by one of the gaps. So those are the unavoidable baseline noises of just doing business with electricity. But what if the machine is faulty? Oh, that introduces a whole new level of chaos into the mat. Because you might have an uneven air gap because of a microscopic manufacturing defect, right? Yeah. Or you could have short circuits, missing magnetic wedges, or even partial demagnetization of the permanent magnets from overheating. And one of the most severe faults an engineer has to hunt for is called unbalanced magnetic

21:37pull. Or UMP. UMP is terrifying for a motor. It's basically the electromagnetic equivalent of an unbalanced washing machine, where all the heavy wet towels get stuck on one side during the spin cycle. That is a perfect analogy. It's a mechanical rotating unbalanced, but driven purely by invisible forces. Yes. If the electromagnetic forces inside the air gap aren't perfectly symmetrical around the entire circle, a non-zero net magnetic force constantly yanks the heavy spinning rotor to one side. And that can literally bend the thick steel rotor shaft itself, while it's spinning at high speed, creating massive, highly destructive vibrations, and a tremendous amount of low frequency noise. It will eventually tear the bearings completely apart. Yikes. So the engineers have run the massive multi-physics simulations. They've diagnosed the source as a healthy switching noise or a terrifying unbalanced pull. How do they actually silence the line? Well, what are the NVH noise, vibration, and harshness mitigation techniques?

22:38If we connect this to the bigger picture, engineers essentially have three overarching strategic choices to solve this puzzle. OK, strategy one. Strategy one. They can attempt to reduce the magnitude of the electromagnetic excitations right at the source, regardless of how the structure behaves. Basically, stop the magnets from pulling so hard. Make the invisible hands weaker. Exactly. Strategy two. They can stiffen or dampen the physical structure of the motor to reduce its physical response, basically making it too rigid to vibrate, regardless of how hard the magnets are pulling. And strategy three. They can surgically alter the design to avoid that perfect storm we talked about, ensuring the resonances between the magnetic excitations and the structural bending modes never ever match up. Which practically involves some incredibly clever engineering tricks. Oh, absolutely. They might carefully choose very specific combinations of slots and poles inside the motor so the patterns never perfectly align. Or they might actually build the stator or the rotor with a slight diagonal skew.

23:40Oh, so the magnetic forces don't hit all at once like a hammer along the entire length of the machine, but rather roll across it smoothly. Right. They use pole shaping, flux barriers, or, and this is my favorite, they even inject specific, mathematically calculated harmonic currents into the power supply that are designed to perfectly cancel out the vibrating frequencies. It is literal active noise cancellation, but done with raw electrical current inside a spinning motor. It is a delicate, brilliant, balancing act of electromagnetic, structural mechanics, and acoustics. It really is. So what does this all mean for you and your everyday electronics? Well, sometimes the fix is incredibly low tech compared to injecting canceling currents. Oh, very much so. For simple coil noise on a computer graphics card or a circuit board, engineers might just change the physical shape of the copper coil to a figure eight instead of a standard cylinder, which alters how the magnetic field distributes itself into the air. That's a classic trick. For, and I found this detail from the sources completely relatable, they just dumbed glue

24:42on it. They really do. They literally add a thick layer of adhesive or potting compound over the vibrating coils and televisions and power supplies to physically lock the wires in place and increase the structural damping. It is a brute force solution, certainly not as elegant as altering the wave number, but it is highly effective at minimizing the structure-borne noise in cheap consumer electronics. But there's a catch. Always a catch. The text points out that this glue degrades over time due to the constant heat in the electronics and just general aging. Yeah, the thermal cycling breaks it down. Right. The grip of the adhesive loosens, the copper coils suddenly have room to wiggle and vibrate again and the sound levels slowly increases year after year. Which is exactly why older TVs, ancient power bricks and aging electronics get louder and start whining more aggressively as they get older. The glue has given up the ghost and the Lorentz forces are back in charge. It is a perfect, everyday example of how the physical reality of materials eventually yields to the relentless, invisible nature of electromagnetic forces.

25:47That is profound. I mean, we've journeyed from the microscopic boundary layers where Maxwell forces played tug of war with the air to the internal atomic breathing of magnetostriction. All the way to the singing, pharaoh electric capacitors and the howling resonance of subway motors. We've seen how the invisible forces of electricity and magnetism demand a physical, violent toll from the materials that carry them, shaking and bending them into acoustic submission. It really reframes how you look at the built environment around you. It really does. Every powered device, no matter how silent it appears, is a site of constant microscopic structural tension. And this raises an important question, something we should all ponder as we wrap up. Okay, later on this. We spend this entire deep dive looking at how engineers are desperately trying to silence these electromagnetically induced vibrations to damp them with glue or cancel them with software. Right. But think about the sheer amount of mechanical energy that represents across the globe. From the humming transformers on the power grid to the whining chargers and millions of

26:49bedrooms, are we just bleeding energy through sound? Wow. I never thought of that. Could future engineering look beyond just acoustic mitigation? Could we find a way to harvest those ambient microscopic structural vibrations and turn that annoying physical wine back into usable electrical power? Turning the very physical struggle of the machine into an asset. Exactly. A fascinating proposition. That is incredible to think about. Well, until the engineers figure that one out, the next time you're sitting in that quiet room and hear that faint alien hum coming from your charger, you'll know exactly what it is. It's not just random noise. No. The invisible forces of the universe physically wrestling with the metal inside the plastic box. Thank you for joining us on this deep dive. Go ahead and listen closely to the subtle hums of the technology around you today. You might just hear the physics at work. The sun shining birds are singing and all feels right in the world. Until the season changes and suddenly you lose your motivation to get out of bed.

27:53In fact, one in five people experience some form of depression no matter the season or time of year. At the American Psychiatric Association Foundation, our vision is to build a mentally healthy nation for all because we want you to live your best life and be your best you all year round. Please visit mentallyhealthination.org to learn more.