WEBVTT 00:00:00.000 --> 00:00:02.839 So have you ever been sitting alone in a perfectly 00:00:02.839 --> 00:00:05.860 quiet room, maybe late at night, and you just 00:00:05.860 --> 00:00:08.599 start to notice it? Oh, the wine. Yes, exactly. 00:00:08.880 --> 00:00:11.279 Yeah. That incredibly faint high -pitched wine 00:00:11.279 --> 00:00:13.599 coming from your laptop charger plugged into 00:00:13.599 --> 00:00:15.960 the wall. It drives people crazy once they hear 00:00:15.960 --> 00:00:18.500 it. Right. Or maybe you're standing on a subway 00:00:18.500 --> 00:00:21.820 platform and you hear that distinct rising sort 00:00:21.820 --> 00:00:24.640 of alien sounding hum of the train motor as it 00:00:24.640 --> 00:00:27.039 accelerates. Yeah. Or just the relentless buzz 00:00:27.039 --> 00:00:29.519 of a fluorescent lamp over your desk. Exactly. 00:00:29.559 --> 00:00:32.159 And you look at these objects and they are completely 00:00:32.159 --> 00:00:35.250 still. I mean, there are no moving parts, no 00:00:35.250 --> 00:00:39.490 fans, no tiny speakers, and yet they are undeniably 00:00:39.490 --> 00:00:42.539 singing to you. It is a profoundly universal 00:00:42.539 --> 00:00:44.240 experience, really. It's one of those things 00:00:44.240 --> 00:00:46.880 we've all just collectively accepted as, you 00:00:46.880 --> 00:00:48.679 know, background noise. Yeah, we just assume 00:00:48.679 --> 00:00:50.340 it's the sound of electricity doing its job. 00:00:50.420 --> 00:00:52.719 Right. But the thing is, electricity itself is 00:00:52.719 --> 00:00:55.439 entirely silent. Electrons moving through a wire, 00:00:55.479 --> 00:00:58.020 they do not have a voice. Which is just a bizarre 00:00:58.020 --> 00:01:00.880 paradox. Because if electricity makes no sound, 00:01:01.320 --> 00:01:04.420 why is my solid plastic laptop brick whining 00:01:04.420 --> 00:01:07.000 at me from the floor? That is exactly what we're 00:01:07.000 --> 00:01:08.840 going to get into. And that's the mission of 00:01:08.840 --> 00:01:11.760 today's deep dive. We are jumping into this fascinating, 00:01:12.200 --> 00:01:16.459 highly detailed Wikipedia article on electromagnetically 00:01:16.459 --> 00:01:20.480 induced acoustic noise, which is sometimes simply 00:01:20.480 --> 00:01:23.140 called coil wine. Yeah, coil wine is the common 00:01:23.140 --> 00:01:26.180 term. Right. And we are going to figure out exactly 00:01:26.180 --> 00:01:29.140 what invisible forces are grabbing hold of our 00:01:29.140 --> 00:01:31.980 everyday electronics and forcing them to physically 00:01:31.980 --> 00:01:35.489 sing. buzz, and vibrate. We definitely need to 00:01:35.489 --> 00:01:37.849 be very precise right from the start about what 00:01:37.849 --> 00:01:40.230 this phenomenon actually is, though. OK, lay 00:01:40.230 --> 00:01:42.849 it on us. Because when we say noise in this context, 00:01:42.909 --> 00:01:45.890 we aren't talking about, like, electrical signal 00:01:45.890 --> 00:01:48.310 noise. Right, like static on a radio. Exactly. 00:01:48.349 --> 00:01:50.890 We aren't talking about radio frequency interference 00:01:50.890 --> 00:01:53.349 or electromagnetic compatibility. That's the 00:01:53.349 --> 00:01:55.109 stuff engineers spend a lot of time trying to 00:01:55.109 --> 00:01:57.230 filter out of the actual electrical circuits. 00:01:57.329 --> 00:01:59.390 So it's not a signal problem. No. We are talking 00:01:59.390 --> 00:02:02.329 about literal, physical, acoustic sound. This 00:02:02.329 --> 00:02:05.030 is a purely structural phenomenon. Very structural. 00:02:05.349 --> 00:02:08.750 Yes. The solid materials inside your devices 00:02:08.750 --> 00:02:12.789 are physically vibrating violently and rapidly 00:02:12.789 --> 00:02:15.590 under the excitation of electromagnetic forces. 00:02:15.729 --> 00:02:18.150 That is the biggest mind bender for me. You're 00:02:18.150 --> 00:02:20.509 looking at that charger on your floor and inside 00:02:20.509 --> 00:02:23.710 that sealed box the metal is literally flexing 00:02:23.710 --> 00:02:26.569 and moving. It is. It's pushing the air which 00:02:26.569 --> 00:02:29.090 hits your eardrums. Now a crucial distinction 00:02:29.090 --> 00:02:31.490 we need to make right away. based on our sources, 00:02:31.930 --> 00:02:34.409 is about high -voltage power lines. Oh, right. 00:02:34.509 --> 00:02:36.370 The big transmission lines. Yeah, because if 00:02:36.370 --> 00:02:38.270 you've ever walked under those massive lines 00:02:38.270 --> 00:02:41.030 and heard that aggressive, crackling hiss, you 00:02:41.030 --> 00:02:43.069 might logically think, oh, that's just coil wine 00:02:43.069 --> 00:02:45.629 on a massive scale. But it's actually not. Right. 00:02:46.229 --> 00:02:48.650 That hissing is something called corona discharge, 00:02:49.229 --> 00:02:51.469 which is the electrical breakdown of the air 00:02:51.469 --> 00:02:53.669 itself. The voltage is literally tearing the 00:02:53.669 --> 00:02:56.490 air molecules apart. Yeah. It is not magnetism 00:02:56.490 --> 00:02:58.759 vibrating the metal of the wire. the mechanism 00:02:58.759 --> 00:03:00.840 there is entirely different. Got it. So we're 00:03:00.840 --> 00:03:03.400 talking about magnetism here. Exactly. What we 00:03:03.400 --> 00:03:05.400 are focusing on today is sometimes referred to 00:03:05.400 --> 00:03:08.159 in the literature as the reciprocal of microphonics. 00:03:08.520 --> 00:03:11.000 OK. Break that down for us. Well, if you are 00:03:11.000 --> 00:03:13.500 familiar with audio engineering, you know microphonics 00:03:13.500 --> 00:03:16.400 is when mechanical vibrations like tapping your 00:03:16.400 --> 00:03:18.780 finger on a vacuum tube or dropping a pearly 00:03:18.780 --> 00:03:22.500 shielded cable. create an undesired electrical 00:03:22.500 --> 00:03:24.979 noise in the signal. Right, the physical world 00:03:24.979 --> 00:03:28.159 intrudes on the electrical. Precisely. But what 00:03:28.159 --> 00:03:30.500 we're looking at today is the exact opposite. 00:03:30.860 --> 00:03:32.800 The electrical intruding on the physical. Exactly. 00:03:33.280 --> 00:03:35.500 The electrical and magnetic forces are creating 00:03:35.500 --> 00:03:38.159 mechanical vibrations out of thin air. Okay, 00:03:38.159 --> 00:03:39.979 so we have these physical vibrations happening 00:03:39.979 --> 00:03:42.819 inside silent machines. Let's look at the invisible 00:03:42.819 --> 00:03:44.719 hands actually doing the pushing and pulling. 00:03:44.960 --> 00:03:47.240 because there's no tiny hammer inside. Right. 00:03:47.300 --> 00:03:49.780 If you tear open that laptop charger, you aren't 00:03:49.780 --> 00:03:52.219 going to find a tiny hammer hitting a bell. You're 00:03:52.219 --> 00:03:55.319 going to find coils, circuits, and magnets. So 00:03:55.319 --> 00:03:57.879 what are the actual physical forces at play here? 00:03:58.340 --> 00:04:00.960 Well, the primary culprits are what we call Maxwell 00:04:00.960 --> 00:04:04.960 forces or reluctance forces. And these forces 00:04:04.960 --> 00:04:08.219 concentrate incredibly intensely at boundaries 00:04:08.219 --> 00:04:11.520 where there is a massive change in magnetic relativity. 00:04:11.840 --> 00:04:15.460 Okay. Reluctivity. Let's translate that. That's 00:04:15.460 --> 00:04:17.980 basically just how much a material resists a 00:04:17.980 --> 00:04:20.019 magnetic field, right? That's a great way to 00:04:20.019 --> 00:04:22.819 put it, yeah. Think of a magnetic field traveling 00:04:22.819 --> 00:04:25.519 easily through a piece of iron, and then suddenly 00:04:25.519 --> 00:04:28.019 it reaches the edge of the metal and slams into 00:04:28.019 --> 00:04:31.639 a brick wall of air. Because air resists magnetism 00:04:31.639 --> 00:04:34.879 way more than iron does. Exactly. That sudden, 00:04:35.259 --> 00:04:38.060 massive shift in resistance creates immense physical 00:04:38.060 --> 00:04:40.399 stress right at the boundary. So it's a surface 00:04:40.399 --> 00:04:43.279 thing. Right. These are the exact same fundamental 00:04:43.279 --> 00:04:46.399 forces that make two solid magnets violently 00:04:46.399 --> 00:04:48.480 pull together or push apart when you hold them 00:04:48.480 --> 00:04:50.540 close to each other in your hands. Okay, let's 00:04:50.540 --> 00:04:54.500 unpack this. Imagine a solid block of iron inside 00:04:54.500 --> 00:04:57.040 a machine. Okay. The Maxwell forces are acting 00:04:57.040 --> 00:04:59.939 like an invisible tug of war happening right 00:04:59.939 --> 00:05:02.079 at the surface, right at that boundary where 00:05:02.079 --> 00:05:05.410 the metal meets the air. It's yanking aggressively 00:05:05.410 --> 00:05:07.329 on the outside of the metal. That's a perfect 00:05:07.329 --> 00:05:09.649 visual. Yeah, but the sources point out another 00:05:09.649 --> 00:05:12.889 distinct force called magnetostriction So if 00:05:12.889 --> 00:05:14.870 Maxwell forces are pulling from the outside, 00:05:14.970 --> 00:05:16.970 maybe a restriction is like the metal itself 00:05:16.970 --> 00:05:19.870 breathing or squeezing from the inside Wow. Yeah, 00:05:19.990 --> 00:05:23.050 it's an internal force entirely contained within 00:05:23.050 --> 00:05:25.389 the atomic structure of the ferromagnetic material 00:05:25.519 --> 00:05:28.399 So the actual physical shape and volume of the 00:05:28.399 --> 00:05:30.699 magnetic core changes minutely as the magnetic 00:05:30.699 --> 00:05:33.480 field fluctuates through it. Yes. You have the 00:05:33.480 --> 00:05:37.199 Maxwell stress pulling at the surfaces and magnetostriction 00:05:37.199 --> 00:05:39.540 warring within the molecular structure of the 00:05:39.540 --> 00:05:41.779 material itself. That sounds chaotic. And the 00:05:41.779 --> 00:05:43.819 physics don't stop there either. We also have 00:05:43.819 --> 00:05:46.379 to account for Lorentz forces, sometimes called 00:05:46.379 --> 00:05:49.019 Laplace forces. Oh, right, on the wires. Exactly. 00:05:49.439 --> 00:05:52.459 These specifically act on the conductors, the 00:05:52.459 --> 00:05:55.000 actual copper wires themselves, when they are 00:05:55.000 --> 00:05:57.560 plunged into an external magnetic field. So every 00:05:57.560 --> 00:06:00.160 time the alternating current pulses through those 00:06:00.160 --> 00:06:03.500 coiled windings, the wire physically tries to 00:06:03.500 --> 00:06:06.319 jump or twitch like a muscle. Yes. And if there 00:06:06.319 --> 00:06:08.959 is a strong electrical field present, you add 00:06:09.500 --> 00:06:11.879 electrostatic and even piezoelectric forces into 00:06:11.879 --> 00:06:15.180 the mix. It is an absolute chaotic mosh pit of 00:06:15.180 --> 00:06:17.220 physics inside these little devices. It really 00:06:17.220 --> 00:06:20.279 is. But, and this is a key point for you listening, 00:06:20.680 --> 00:06:23.120 to keep in mind all these invisible pushes and 00:06:23.120 --> 00:06:25.500 pulls, all this metallic breathing and wire twitching, 00:06:25.939 --> 00:06:29.300 it only actually matters to us if two very specific 00:06:29.300 --> 00:06:31.439 physical conditions are met. Right, because otherwise 00:06:31.439 --> 00:06:34.649 it's just vibrating silently. Exactly. First, 00:06:35.029 --> 00:06:36.810 the frequency of these vibrations has to land 00:06:36.810 --> 00:06:39.829 right in the sweet spot of human hearing, which 00:06:39.829 --> 00:06:43.110 is roughly between 20 Hz and 20 kHz. Which is 00:06:43.110 --> 00:06:45.209 a pretty wide range, but still. Right. If the 00:06:45.209 --> 00:06:47.870 metal is vibrating at 5 Hz, it might physically 00:06:47.870 --> 00:06:49.810 shake the device on your desk, but you won't 00:06:49.810 --> 00:06:52.399 actually hear a whine. Yeah. And second, the 00:06:52.399 --> 00:06:54.899 surface area radiating that vibration has to 00:06:54.899 --> 00:06:57.060 be large enough to actually push a meaningful 00:06:57.060 --> 00:06:59.379 amount of air to your eardrums. Right. If the 00:06:59.379 --> 00:07:02.160 vibrating part is microscopically small and completely 00:07:02.160 --> 00:07:05.079 isolated, it just remains a silent, invisible 00:07:05.079 --> 00:07:07.279 struggle. And the frequency of the noise you 00:07:07.279 --> 00:07:09.920 end up hearing is directly tied to the nature 00:07:09.920 --> 00:07:12.399 of those electromagnetic forces and the frequency 00:07:12.399 --> 00:07:14.899 of the electromagnetic field itself. Which changes 00:07:14.899 --> 00:07:17.149 depending on the power type, right? Absolutely. 00:07:17.269 --> 00:07:19.529 It changes dramatically depending on whether 00:07:19.529 --> 00:07:23.769 there is a direct current or DC component involved 00:07:23.769 --> 00:07:26.649 or if it is purely alternating current. Okay, 00:07:26.709 --> 00:07:29.029 so these forces are universal. They are happening 00:07:29.029 --> 00:07:31.610 anytime we have alternating currents and magnetic 00:07:31.610 --> 00:07:34.550 fields. Anytime. Let's look at how they manifest 00:07:34.550 --> 00:07:37.189 in everyday objects, specifically the ones that 00:07:37.189 --> 00:07:39.810 just sit there, the static singers, as we could 00:07:39.810 --> 00:07:42.230 call them. Passive components are a great place 00:07:42.230 --> 00:07:44.910 to start with this. Let's look at inductors, 00:07:44.970 --> 00:07:47.449 which are sometimes called reactors or chokes 00:07:47.449 --> 00:07:50.170 in the engineering world. OK, inductors. In these 00:07:50.170 --> 00:07:52.990 devices, the magnetic energy is actually stored 00:07:52.990 --> 00:07:55.870 in an intentional air gap within the magnetic 00:07:55.870 --> 00:07:57.850 circuit. An intentional air gap. So that means 00:07:57.850 --> 00:08:01.029 a massive boundary change. Exactly. Because you 00:08:01.029 --> 00:08:03.790 have that sudden transition from metal to air 00:08:03.790 --> 00:08:07.050 and back to metal again, massive Maxwell forces 00:08:07.050 --> 00:08:10.600 apply right across that gap. trying to pull the 00:08:10.600 --> 00:08:12.560 gap closed. Oh wow, so it's constantly trying 00:08:12.560 --> 00:08:14.899 to crush itself. Yeah, and the resulting noise 00:08:14.899 --> 00:08:17.120 depends heavily on the geometry of the circuit 00:08:17.120 --> 00:08:19.360 and the specific materials used to build it. 00:08:19.529 --> 00:08:21.970 And then you have transformers, like those big 00:08:21.970 --> 00:08:24.110 green utility boxes sitting in your neighborhood, 00:08:24.430 --> 00:08:26.569 or the gray cylinders up on the power poles. 00:08:26.730 --> 00:08:28.910 Oh, transformers are fascinating because they 00:08:28.910 --> 00:08:30.910 get hit from all sides. Yeah, they're getting 00:08:30.910 --> 00:08:33.330 hit with a trio of forces all at once. You have 00:08:33.330 --> 00:08:36.110 Lorenz forces physically shaking the copper windings, 00:08:36.429 --> 00:08:38.789 the Maxwell forces attacking the joints of the 00:08:38.789 --> 00:08:40.610 metal laminations trying to pull them apart. 00:08:40.690 --> 00:08:43.070 And the magnetostriction happening deep inside 00:08:43.070 --> 00:08:46.269 the laminated core. Right, making the whole massive 00:08:46.269 --> 00:08:49.289 block of metal subtly expand and contract. over 00:08:49.289 --> 00:08:52.509 and over again. It's honestly a minor miracle. 00:08:52.590 --> 00:08:54.649 They don't tear themselves apart, let alone just 00:08:54.649 --> 00:08:58.049 hum. It is a profoundly hostile environment for 00:08:58.049 --> 00:09:00.929 a piece of metal to exist in over decades of 00:09:00.929 --> 00:09:02.870 use. No kidding. But this brings us to one of 00:09:02.870 --> 00:09:05.470 the most counterintuitive examples in the literature, 00:09:05.649 --> 00:09:07.950 which is the singing capacitor. OK, wait. Hold 00:09:07.950 --> 00:09:10.769 on. I'm with you on the transformers and the 00:09:10.769 --> 00:09:12.769 inductors. I mean, they have magnetic cores and 00:09:12.769 --> 00:09:16.179 coils of wire. Sure. But a capacitor. A capacitor 00:09:16.179 --> 00:09:19.240 is essentially just two conductive metal plates 00:09:19.240 --> 00:09:22.899 with an insulator, a dielectric, sandwiched in 00:09:22.899 --> 00:09:25.580 between them. There are no magnetic cores. There 00:09:25.580 --> 00:09:28.000 are no coils. There are no moving parts whatsoever. 00:09:28.820 --> 00:09:31.679 So how in the world does a microscopic sandwich 00:09:31.679 --> 00:09:35.179 of metal and plastic make a sound? What's fascinating 00:09:35.179 --> 00:09:37.659 here is that we have to step away from magnetism 00:09:37.659 --> 00:09:40.679 entirely for a moment and look purely at the 00:09:40.679 --> 00:09:42.919 electric field. Okay, pure electricity. Yeah. 00:09:43.070 --> 00:09:45.990 Capacitors are subject to immense electrostatic 00:09:45.990 --> 00:09:49.210 forces. When the voltage or current waveform 00:09:49.210 --> 00:09:51.389 running through the capacitor isn't perfectly 00:09:51.389 --> 00:09:54.009 smooth and constant... Which I imagine is often 00:09:54.009 --> 00:09:56.250 the case. Right. When it contains what we call 00:09:56.250 --> 00:09:59.029 time harmonics, these harmonic electric forces 00:09:59.029 --> 00:10:02.090 appear. Time harmonics. Let's visualize that. 00:10:02.450 --> 00:10:05.269 It's like, imagine an ocean wave that isn't a 00:10:05.269 --> 00:10:08.149 smooth, rolling swell, but instead is choppy, 00:10:08.330 --> 00:10:10.870 stuttering and uneven as it hits the shore. Exactly. 00:10:11.049 --> 00:10:13.509 That choppy electrical flow... creates irregular 00:10:13.509 --> 00:10:15.970 forces. But the real trick here is the reverse 00:10:15.970 --> 00:10:18.129 piezoelectric effect. Oh, piezoelectric, like 00:10:18.129 --> 00:10:20.370 the clicker in a lighter. Right. But in reverse, 00:10:20.629 --> 00:10:23.470 in ferroelectric capacitors, the dielectric insulator 00:10:23.470 --> 00:10:25.730 material literally changes its physical shape 00:10:25.730 --> 00:10:27.970 when an electric field is applied. Are you serious? 00:10:28.330 --> 00:10:32.429 Yeah. As the choppy voltage fluctuates, the material 00:10:32.429 --> 00:10:36.090 rapidly expands and contracts. It acts exactly 00:10:36.090 --> 00:10:39.129 like a tiny, unintended speaker driver pushing 00:10:39.129 --> 00:10:42.100 against the air. Oh. And that is the singing 00:10:42.100 --> 00:10:44.799 capacitor effect. So the stuttering electricity 00:10:44.799 --> 00:10:48.039 is literally squeezing the material like a microscopic 00:10:48.039 --> 00:10:51.240 accordion. Too much. That is incredible. And 00:10:51.240 --> 00:10:53.419 our sources briefly mentioned a few other static 00:10:53.419 --> 00:10:55.799 suspects that suffer from this noise, too. Right. 00:10:55.840 --> 00:10:58.500 Like in medical equipment? Yeah. In MRI machines, 00:10:58.679 --> 00:11:00.840 the receiving coils have something called coil 00:11:00.840 --> 00:11:03.899 noise simply due to their non -zero temperature. 00:11:04.059 --> 00:11:06.639 Which sounds crazy, right? Yeah. The baseline 00:11:06.639 --> 00:11:09.960 heat creates minute thermal agitations that interact 00:11:09.960 --> 00:11:12.200 with the massive magnetic fields of the MRI. 00:11:12.419 --> 00:11:14.340 And then there's the electric trains. Oh, the 00:11:14.340 --> 00:11:16.740 braking resistors. Yes. Because they use these 00:11:16.740 --> 00:11:18.879 massive braking resistors to burn off excess 00:11:18.879 --> 00:11:21.220 electrical power when the overhead lines, the 00:11:21.220 --> 00:11:23.700 catenary can't absorb it. And those giant resistors 00:11:23.700 --> 00:11:26.600 can howl just from the raw sheer power being 00:11:26.600 --> 00:11:29.059 dumped through them. Which makes me wonder if 00:11:29.059 --> 00:11:31.820 these static objects are practically screaming, 00:11:32.220 --> 00:11:34.340 just sitting there bolted to wall or sitting 00:11:34.340 --> 00:11:37.379 on a circuit board. What kind of chaos happens 00:11:37.379 --> 00:11:39.620 when we take these exact same magnetic forces 00:11:39.620 --> 00:11:42.960 and deliberately spin them at thousands of RPMs 00:11:42.960 --> 00:11:45.399 in a motor? Oh, well now we are entering the 00:11:45.399 --> 00:11:48.919 realm of radial and axial flux rotating machines. 00:11:49.220 --> 00:11:51.940 Which includes what, exactly? This includes induction 00:11:51.940 --> 00:11:55.679 motors, switched reluctance motors, and synchronous 00:11:55.679 --> 00:11:58.740 motors. Oh, right. Remember that subway electric 00:11:58.740 --> 00:12:00.759 motor whine you mentioned at the beginning? Yeah, 00:12:00.879 --> 00:12:04.269 that rising alien hum. That is a permanent magnet 00:12:04.269 --> 00:12:07.149 synchronous machine, or PMSM, doing exactly what 00:12:07.149 --> 00:12:10.289 it was designed to do, but with intense acoustic 00:12:10.289 --> 00:12:13.009 consequences. And the primary antagonist here 00:12:13.009 --> 00:12:15.309 seems to be something called torque ripple. Yes, 00:12:15.450 --> 00:12:18.169 torque ripple is a major factor. Now, the main 00:12:18.169 --> 00:12:20.210 electromagnetic torque of a motor is what actually 00:12:20.210 --> 00:12:23.490 spins the wheels. And as a static steady force, 00:12:23.610 --> 00:12:25.389 it doesn't make noise, it just creates motion. 00:12:25.509 --> 00:12:27.529 Right, ideally. But the torque isn't perfectly 00:12:27.529 --> 00:12:30.779 smooth, right? It has harmonic variations, little 00:12:30.779 --> 00:12:33.059 stutters, and microscopic pulses in the rotational 00:12:33.059 --> 00:12:36.759 push. Exactly. And in a permanent magnet machine 00:12:36.759 --> 00:12:40.080 with an open circuit, this is specifically called 00:12:40.080 --> 00:12:43.809 cogging torque. Cogging torque. So these dynamic 00:12:43.809 --> 00:12:46.730 stuttering pulses create torsional vibrations, 00:12:46.870 --> 00:12:48.990 physically twisting the rotor and the stator 00:12:48.990 --> 00:12:51.690 back and forth like someone wringing out a towel. 00:12:52.090 --> 00:12:55.049 Yeah, but while a simple solid cylinder twisting 00:12:55.049 --> 00:12:57.690 back and forth doesn't radiate sound very efficiently 00:12:57.690 --> 00:13:00.730 into the air, the complex outer boundary of a 00:13:00.730 --> 00:13:03.690 motor stator certainly can. Right. Lots of surface 00:13:03.690 --> 00:13:06.429 area. Furthermore, those Maxwell stress forces 00:13:06.429 --> 00:13:08.990 we talked about earlier aren't just pulling in 00:13:08.990 --> 00:13:11.659 one simple direction. pulling all over the place. 00:13:11.919 --> 00:13:13.820 Right. They have radial force harmonics that 00:13:13.820 --> 00:13:16.679 pull outward on the stator and tangential force 00:13:16.679 --> 00:13:18.799 harmonics that pull sideways on the individual 00:13:18.799 --> 00:13:21.740 stator teeth. Oh, wow. So imagine trying to snap 00:13:21.740 --> 00:13:24.240 off the teeth of a plastic comb by pushing them 00:13:24.240 --> 00:13:26.639 sideways over and over. That's exactly what's 00:13:26.639 --> 00:13:28.679 happening. That intense bending moment causes 00:13:28.679 --> 00:13:31.200 the entire outer casing, the yoke of the machine, 00:13:31.600 --> 00:13:33.559 to vibrate outward into the surrounding air. 00:13:33.720 --> 00:13:35.960 But the sources highlight that to get that really 00:13:36.330 --> 00:13:39.289 deafening ear piercing scream from a motor. You 00:13:39.289 --> 00:13:42.450 need more than just vibrations. You do. You need 00:13:42.450 --> 00:13:45.649 a perfect storm. You need resonance. Exactly. 00:13:46.250 --> 00:13:48.769 In radio flux machines, resonance isn't just 00:13:48.769 --> 00:13:52.629 about matching a single frequency, like a singer 00:13:52.629 --> 00:13:55.330 hitting the perfect pitch to shatter a wine glass. 00:13:55.450 --> 00:13:59.169 OK. It requires a very specific simultaneous 00:13:59.169 --> 00:14:02.230 double match. Double match. Condition one. The 00:14:02.230 --> 00:14:04.570 frequency of the exciting Maxwell force must 00:14:04.570 --> 00:14:06.909 perfectly match the natural vibrating frequency 00:14:06.909 --> 00:14:09.590 of the stator or rotor. Which makes sense. But 00:14:09.590 --> 00:14:12.590 condition two, the harmonic wave number, which 00:14:12.590 --> 00:14:15.049 is the physical periodicity of the force distributed 00:14:15.049 --> 00:14:18.309 along the air gap, that must match the structural 00:14:18.309 --> 00:14:20.590 modal shape of the machine. OK, here's where 00:14:20.590 --> 00:14:23.210 it gets really interesting for me. So timing 00:14:23.210 --> 00:14:26.870 is the frequency, like pushing a kid on a swing 00:14:26.870 --> 00:14:29.509 at the exact right second to make them go higher. 00:14:29.590 --> 00:14:31.809 That covers the frequency condition, yes. If 00:14:31.809 --> 00:14:33.769 you push when they are coming backward at you, 00:14:33.850 --> 00:14:35.590 you kill their momentum. You have to push at 00:14:35.590 --> 00:14:37.710 the exact right time. Right. But wave number 00:14:37.710 --> 00:14:40.529 adds a physical shape to that timing. Imagine 00:14:40.529 --> 00:14:44.409 that swing isn't a normal seat, but a giant flexible 00:14:44.409 --> 00:14:47.649 hula hoop suspended in the air. Oh, I see. To 00:14:47.649 --> 00:14:50.370 make a flexible hula hoop warp into an oval or 00:14:50.370 --> 00:14:52.549 an ellipse, I can't just push it in one spot. 00:14:52.549 --> 00:14:54.389 It would just swing away from me. Exactly. I 00:14:54.389 --> 00:14:56.149 had to push it inward on two opposite sides at 00:14:56.149 --> 00:14:59.250 the exact same time. That is it. That physical 00:14:59.250 --> 00:15:02.049 pattern of where you push is the wave number. 00:15:02.909 --> 00:15:05.889 So if the natural bending mode, the structural 00:15:05.889 --> 00:15:08.629 modal shape of the motor's stator, is an ellipse, 00:15:09.149 --> 00:15:12.250 the electromagnetic force wave number must be 00:15:12.250 --> 00:15:15.299 precisely 2. Two pushes, basically. It has to 00:15:15.299 --> 00:15:17.279 be pushing out at two points and pulling in at 00:15:17.279 --> 00:15:19.679 two points at the exact right frequency. And 00:15:19.679 --> 00:15:21.779 when both the timing and the physical shape of 00:15:21.779 --> 00:15:24.159 the force match the metal structure perfectly. 00:15:24.340 --> 00:15:26.720 The maxima of the electromagnetic excitation 00:15:26.720 --> 00:15:29.039 and the maxima of the physical displacement are 00:15:29.039 --> 00:15:31.899 perfectly in phase. The motor essentially amplifies 00:15:31.899 --> 00:15:34.299 its own physical distortion and the vibrations 00:15:34.299 --> 00:15:37.120 just explode in magnitudes. They absolutely explode. 00:15:37.240 --> 00:15:39.940 There is a brilliant experimental illustration 00:15:39.940 --> 00:15:42.740 of this in the sources using a simple tuning 00:15:42.740 --> 00:15:45.090 fork. that really brings this to life. Oh, I 00:15:45.090 --> 00:15:47.230 love that example. Yeah, imagine a heavy iron 00:15:47.230 --> 00:15:50.210 tuning fork. But one of the prongs is wound tightly 00:15:50.210 --> 00:15:53.330 with a coil fed by a variable frequency power 00:15:53.330 --> 00:15:56.330 supply. Right, which creates a variable magnetic 00:15:56.330 --> 00:15:58.669 flux density in the empty airspace between the 00:15:58.669 --> 00:16:01.370 two prongs. Generating dynamic magnetic forces 00:16:01.370 --> 00:16:04.049 trying to pull them together at twice the frequency 00:16:04.049 --> 00:16:06.440 of the electrical supply. And as you slowly dial 00:16:06.440 --> 00:16:09.200 up the frequency of that coil, sending faster 00:16:09.200 --> 00:16:11.519 and faster alternating current through the wire, 00:16:12.179 --> 00:16:14.600 nothing dramatic happens at first. The fork might 00:16:14.600 --> 00:16:18.200 hum slightly, but the moment that exciting magnetic 00:16:18.200 --> 00:16:20.720 force hits the fundamental natural mode of the 00:16:20.720 --> 00:16:23.620 tuning fork, which in this specific historical 00:16:23.620 --> 00:16:26.259 experiment is right around 400 Hertz. The two 00:16:26.259 --> 00:16:30.960 prongs suddenly lock in. Yes. A massive, spontaneous, 00:16:31.320 --> 00:16:33.860 and violently loud acoustic resonance occurs, 00:16:34.259 --> 00:16:36.860 driven entirely by an invisible magnetic field 00:16:36.860 --> 00:16:39.440 jumping across the gap. It clearly demonstrates 00:16:39.440 --> 00:16:42.100 how violently a solid piece of iron can be forced 00:16:42.100 --> 00:16:44.659 to move by magnetism alone. It's quite the party 00:16:44.659 --> 00:16:47.889 trick. It really is. OK, so we know exactly how 00:16:47.889 --> 00:16:50.389 this perfect storm forms. We've got the tug of 00:16:50.389 --> 00:16:52.889 war of Maxwell forces, the internal breathing 00:16:52.889 --> 00:16:55.490 of magnet restriction, the twisting of torque 00:16:55.490 --> 00:16:58.830 ripple, and the swing set physics of double match 00:16:58.830 --> 00:17:01.190 resonance. We've covered a lot of physics. Yeah. 00:17:01.450 --> 00:17:03.909 So how do engineers actually predict this before 00:17:03.909 --> 00:17:05.970 they build a motor? And more importantly, how 00:17:05.970 --> 00:17:08.529 do they stop our devices from driving us absolutely 00:17:08.529 --> 00:17:12.059 crazy? This brings us to the realm of numerical 00:17:12.059 --> 00:17:15.000 simulation and multi -physics modeling. Which 00:17:15.000 --> 00:17:18.740 sounds intense. It is. Engineers don't just build 00:17:18.740 --> 00:17:21.740 a massive multi -million dollar industrial motor, 00:17:22.240 --> 00:17:24.460 plug it in and cross their fingers that it won't 00:17:24.460 --> 00:17:27.410 scream. I would hope not. They calculate the 00:17:27.410 --> 00:17:29.750 electromagnetically induced noise beforehand 00:17:29.750 --> 00:17:32.450 in a highly complex three -step process. Okay, 00:17:32.609 --> 00:17:34.809 what are the steps? First, they calculate the 00:17:34.809 --> 00:17:37.450 electromagnetic forces. Second, they calculate 00:17:37.450 --> 00:17:39.990 how those forces will cause magnetic vibrations 00:17:39.990 --> 00:17:42.549 in the structure. And third, they calculate how 00:17:42.549 --> 00:17:44.630 those structural vibrations will translate into 00:17:44.630 --> 00:17:47.230 acoustic noise radiated into the air. What's 00:17:47.230 --> 00:17:49.069 really interesting is that the sources call this 00:17:49.069 --> 00:17:52.819 a weakly coupled problem. Which honestly sounds 00:17:52.819 --> 00:17:55.019 like a bad thing, like the math is falling apart. 00:17:55.259 --> 00:17:57.259 It does sound like a flaw, but it actually makes 00:17:57.259 --> 00:17:59.259 the simulation possible. Right, it means it's 00:17:59.259 --> 00:18:01.819 a one -way street. While the magnetic forces 00:18:01.819 --> 00:18:04.559 cause the massive steel frame to physically deform, 00:18:05.119 --> 00:18:07.660 that physical deformation is microscopic enough 00:18:07.660 --> 00:18:10.480 that it doesn't bounce back and significantly 00:18:10.480 --> 00:18:13.480 alter the overall magnetic field itself. Exactly. 00:18:13.720 --> 00:18:16.140 The arrow of cause and effect mostly points in 00:18:16.140 --> 00:18:18.950 one direction. And to do this, Engineers use 00:18:18.950 --> 00:18:21.750 massive multi -physics simulations. Specialized 00:18:21.750 --> 00:18:24.029 software, yeah. It's not enough to just model 00:18:24.029 --> 00:18:27.049 the electricity. The software has to calculate 00:18:27.049 --> 00:18:29.809 the invisible magnetic fields, figure out exactly 00:18:29.809 --> 00:18:32.829 how those fields physically warp the steel, and 00:18:32.829 --> 00:18:35.509 then calculate how much air that warped steel 00:18:35.509 --> 00:18:38.210 pushes into your eardrum. It's an insane three 00:18:38.210 --> 00:18:41.140 -step chain reaction. But even with perfect software 00:18:41.140 --> 00:18:43.680 modeling that chain reaction, the engineer has 00:18:43.680 --> 00:18:45.680 to understand what they are actually diagnosing 00:18:45.680 --> 00:18:47.859 in the real world. Like is the machine broken 00:18:47.859 --> 00:18:50.180 or is it supposed to sound like that? Precisely. 00:18:50.400 --> 00:18:53.819 Are you modeling a healthy machine working perfectly 00:18:53.819 --> 00:18:57.319 or a faulty one? Because even a perfectly healthy 00:18:57.319 --> 00:19:00.460 factory fresh machine will generate harmonic 00:19:00.460 --> 00:19:02.720 electromagnetic forces. Because of the power 00:19:02.720 --> 00:19:05.279 supply, right? Yeah, this can come from pulse 00:19:05.279 --> 00:19:09.559 width modulation or PWM supplies. And PWM is 00:19:09.559 --> 00:19:11.900 basically a technique that uses a rapid flickering 00:19:11.900 --> 00:19:14.619 switch turning the power on and off thousands 00:19:14.619 --> 00:19:17.319 of times a second to control the overall power 00:19:17.319 --> 00:19:20.000 output. But that flickering introduces harsh 00:19:20.000 --> 00:19:22.720 high -frequency harmonics into the magnetic field. 00:19:22.880 --> 00:19:25.519 So it's healthy but noisy. Right. Healthy noise 00:19:25.519 --> 00:19:27.640 can also come from slotting effects, which is 00:19:27.640 --> 00:19:30.180 the physical gaps between the stator teeth causing 00:19:30.180 --> 00:19:32.759 sudden magnetic fluctuations every time the rotor 00:19:32.759 --> 00:19:35.380 passes by one of the gaps. So those are the unavoidable 00:19:35.380 --> 00:19:38.559 baseline noises of just doing business with electricity. 00:19:39.339 --> 00:19:41.720 But what if the machine is faulty? That introduces 00:19:41.720 --> 00:19:43.950 a whole new level of chaos into the Because you 00:19:43.950 --> 00:19:46.269 might have an uneven air gap because of a microscopic 00:19:46.269 --> 00:19:48.769 manufacturing defect, right? Yeah, or you could 00:19:48.769 --> 00:19:50.789 have short circuits, missing magnetic wedges, 00:19:50.849 --> 00:19:53.349 or even partial demagnetization of the permanent 00:19:53.349 --> 00:19:55.730 magnets from overheating. And one of the most 00:19:55.730 --> 00:19:57.990 severe faults an engineer has to hunt for is 00:19:57.990 --> 00:20:02.240 called unbalanced magnetic pull. or UMP. UMP 00:20:02.240 --> 00:20:05.559 is terrifying for a motor. It's basically the 00:20:05.559 --> 00:20:08.140 electromagnetic equivalent of an unbalanced washing 00:20:08.140 --> 00:20:10.700 machine, where all the heavy wet towels get stuck 00:20:10.700 --> 00:20:12.640 on one side during the spin cycle. That is a 00:20:12.640 --> 00:20:16.200 perfect analogy. It's a mechanical rotating unbalance 00:20:16.200 --> 00:20:20.220 driven purely by invisible forces. Yes. If the 00:20:20.220 --> 00:20:22.839 electromagnetic forces inside the air gap aren't 00:20:22.839 --> 00:20:25.420 perfectly symmetrical around the entire circle, 00:20:26.039 --> 00:20:29.839 a non -zero magnetic force constantly yanks the 00:20:29.710 --> 00:20:32.269 heavy spinning rotor to one side. And that can 00:20:32.269 --> 00:20:34.930 literally bend the thick steel rotor shaft itself 00:20:34.930 --> 00:20:37.730 while it's spinning at high speed, creating massive 00:20:37.730 --> 00:20:40.210 highly destructive vibrations and a tremendous 00:20:40.210 --> 00:20:42.589 amount of low frequency noise. It will eventually 00:20:42.589 --> 00:20:45.950 tear the bearings completely apart. Yikes. So 00:20:45.950 --> 00:20:47.930 the engineers have run the massive multi -physics 00:20:47.930 --> 00:20:50.190 simulations. They've diagnosed the source as 00:20:50.190 --> 00:20:52.990 a healthy switching noise or a terrifying unbalanced 00:20:52.990 --> 00:20:55.559 pull. How do they actually silence the wine? 00:20:55.700 --> 00:20:58.259 Well, what are the NVH noise, vibration, and 00:20:58.259 --> 00:21:00.500 harshness mitigation techniques? If we connect 00:21:00.500 --> 00:21:02.920 this to the bigger picture, engineers essentially 00:21:02.920 --> 00:21:06.180 have three overarching strategic choices to solve 00:21:06.180 --> 00:21:08.460 this puzzle. Okay, strategy one. Strategy one. 00:21:09.039 --> 00:21:11.539 They can attempt to reduce the magnitude of the 00:21:11.539 --> 00:21:14.019 electromagnetic excitations right at the source, 00:21:14.319 --> 00:21:16.980 regardless of how the structure behaves. Basically, 00:21:17.380 --> 00:21:19.859 stop the magnets from pulling so hard. Make the 00:21:19.859 --> 00:21:22.940 invisible hands weaker. Exactly. Strategy two. 00:21:23.079 --> 00:21:26.099 They can stiffen or dampen the physical structure 00:21:26.099 --> 00:21:28.660 of the motor to reduce its physical response, 00:21:29.119 --> 00:21:31.700 basically making it too rigid to vibrate, regardless 00:21:31.700 --> 00:21:33.819 of how hard the magnets are pulling. Okay, and 00:21:33.819 --> 00:21:36.160 strategy three. They can surgically alter the 00:21:36.160 --> 00:21:38.539 design to avoid that perfect storm we talked 00:21:38.539 --> 00:21:41.420 about, ensuring the resonances between the magnetic 00:21:41.420 --> 00:21:43.559 excitations and the structural bending modes 00:21:43.559 --> 00:21:46.400 never ever match up. Which practically involves 00:21:46.400 --> 00:21:48.599 some incredibly clever engineering tricks. Oh, 00:21:48.599 --> 00:21:51.279 absolutely. They might carefully choose very 00:21:51.279 --> 00:21:54.490 specific combinations of of slots and pulls inside 00:21:54.490 --> 00:21:57.230 the motor so the patterns never perfectly align. 00:21:57.369 --> 00:22:00.109 Or they might actually build the stator or the 00:22:00.109 --> 00:22:03.019 rotor with a slight diagonal skew. Oh, so the 00:22:03.019 --> 00:22:05.799 magnetic forces don't hit all at once like a 00:22:05.799 --> 00:22:07.900 hammer along the entire length of the machine, 00:22:07.920 --> 00:22:10.480 but rather roll across it smoothly. Right. They 00:22:10.480 --> 00:22:13.640 use pole shaping, flux barriers, or, and this 00:22:13.640 --> 00:22:17.319 is my favorite, they even inject specific mathematically 00:22:17.319 --> 00:22:20.900 calculated harmonic currents into the power supply 00:22:20.900 --> 00:22:23.400 that are designed to perfectly cancel out the 00:22:23.400 --> 00:22:26.539 vibrating frequencies. It is literal active noise 00:22:26.539 --> 00:22:29.480 cancellation, but done with raw electrical current 00:22:29.480 --> 00:22:32.859 inside a spinning motor. delicate, brilliant 00:22:32.859 --> 00:22:35.079 balancing act of electromagnetics, structural 00:22:35.079 --> 00:22:37.779 mechanics, and acoustics. It really is. So what 00:22:37.779 --> 00:22:39.700 does this all mean for you and your everyday 00:22:39.700 --> 00:22:42.480 electronics? Well, sometimes the fix is incredibly 00:22:42.480 --> 00:22:45.140 low tech compared to injecting canceling currents. 00:22:45.180 --> 00:22:48.440 Oh, very much so. For simple coil noise on a 00:22:48.440 --> 00:22:51.019 computer graphics card or a circuit board, engineers 00:22:51.019 --> 00:22:53.400 might just change the physical shape of the copper 00:22:53.400 --> 00:22:55.900 coil to a figure eight instead of a standard 00:22:55.900 --> 00:22:58.220 cylinder, which alters how the magnetic field 00:22:58.220 --> 00:23:00.380 distributes itself into the air. That's a classic 00:23:00.380 --> 00:23:02.809 trick. Or, and I found this detail from the sources 00:23:02.809 --> 00:23:04.789 completely relatable, they just dump glue on 00:23:04.789 --> 00:23:07.890 it. They really do. They literally add a thick 00:23:07.890 --> 00:23:10.950 layer of adhesive or potting compound over the 00:23:10.950 --> 00:23:13.730 vibrating coils and televisions and power supplies 00:23:13.730 --> 00:23:17.069 to physically lock the wires in place and increase 00:23:17.069 --> 00:23:20.190 the structural damping. It is a brute force solution, 00:23:20.589 --> 00:23:23.109 certainly not as elegant as altering the wave 00:23:23.109 --> 00:23:26.410 number, but it is highly effective at minimizing 00:23:26.410 --> 00:23:29.549 the structure borne noise in cheap consumer electronics. 00:23:29.990 --> 00:23:32.130 But there's a catch. Always a catch. The text 00:23:32.130 --> 00:23:34.849 points out that this glue degrades over time 00:23:34.849 --> 00:23:37.049 due to the constant heat in the electronics and 00:23:37.049 --> 00:23:39.009 just general aging. Yeah, the thermal cycling 00:23:39.009 --> 00:23:40.970 breaks it down. Right. The grip of the adhesive 00:23:40.970 --> 00:23:43.509 loosens, the copper coils suddenly have room 00:23:43.509 --> 00:23:46.049 to wiggle and vibrate again, and the sound level 00:23:46.049 --> 00:23:48.569 slowly increases year after year. Which is exactly 00:23:48.569 --> 00:23:51.829 why older TVs, ancient power bricks, and aging 00:23:51.829 --> 00:23:53.910 electronics get louder and start whining more 00:23:53.910 --> 00:23:55.690 aggressively as they get older. The glue has 00:23:55.690 --> 00:23:57.789 given up the ghost, and the Lorentz forces are 00:23:57.789 --> 00:24:00.809 back in charge. It is a perfect everyday example 00:24:00.809 --> 00:24:04.130 of how the physical reality of materials eventually 00:24:04.130 --> 00:24:06.950 yields to the relentless, invisible nature of 00:24:06.950 --> 00:24:10.299 electromagnetic forces. That is profound. I mean, 00:24:10.299 --> 00:24:12.720 we've journeyed from the microscopic boundary 00:24:12.720 --> 00:24:15.420 layers where Maxwell forces play tug of war with 00:24:15.420 --> 00:24:20.680 the air to the internal atomic breathing of magnetostriction. 00:24:20.819 --> 00:24:23.680 All the way to the singing pharaoh electric capacitors 00:24:23.680 --> 00:24:26.819 and the howling resonance of subway motors. We've 00:24:26.819 --> 00:24:28.980 seen how the invisible forces of electricity 00:24:28.980 --> 00:24:32.640 and magnetism demand a physical, violent toll 00:24:32.640 --> 00:24:35.660 from the materials that carry them, shaking and 00:24:35.660 --> 00:24:37.839 bending them into acoustic submission. It really 00:24:37.960 --> 00:24:40.140 frames how you look at the built environment 00:24:40.140 --> 00:24:43.019 around you. It really does. Every powered device, 00:24:43.180 --> 00:24:45.700 no matter how silent it appears, is a site of 00:24:45.700 --> 00:24:48.450 constant microscopic structural tension. And 00:24:48.450 --> 00:24:49.910 this raises an important question, something 00:24:49.910 --> 00:24:52.150 we should all ponder as we wrap up. OK, later 00:24:52.150 --> 00:24:54.390 on this. We spent this entire deep dive looking 00:24:54.390 --> 00:24:56.849 at how engineers are desperately trying to silence 00:24:56.849 --> 00:24:58.750 these electromagnetically induced vibrations 00:24:58.750 --> 00:25:01.470 to damp them with glue or cancel them with software. 00:25:01.769 --> 00:25:03.930 Right. But think about the sheer amount of mechanical 00:25:03.930 --> 00:25:06.470 energy that represents across the globe. From 00:25:06.470 --> 00:25:08.809 the humming transformers on the power grid to 00:25:08.809 --> 00:25:10.950 the whining chargers in millions of bedrooms, 00:25:11.509 --> 00:25:14.190 are we just bleeding energy through sound? Wow, 00:25:14.450 --> 00:25:17.099 that's, I never thought of that. Could future 00:25:17.099 --> 00:25:20.299 engineering look beyond just acoustic mitigation? 00:25:20.799 --> 00:25:23.880 Could we find a way to harvest those ambient 00:25:23.880 --> 00:25:27.279 microscopic structural vibrations and turn that 00:25:27.279 --> 00:25:30.880 annoying physical wine back into usable electrical 00:25:30.880 --> 00:25:33.099 power? Turning the very physical struggle of 00:25:33.099 --> 00:25:36.440 the machine into an asset. Exactly. A fascinating 00:25:36.440 --> 00:25:38.700 proposition. That is incredible to think about. 00:25:38.809 --> 00:25:40.690 Well, until the engineers figure that one out, 00:25:40.990 --> 00:25:42.990 the next time you are sitting in that quiet room 00:25:42.990 --> 00:25:45.150 and hear that faint alien hum coming from your 00:25:45.150 --> 00:25:47.509 charger, you'll know exactly what it is. It's 00:25:47.509 --> 00:25:50.470 not just random noise. No. It's the invisible 00:25:50.470 --> 00:25:52.750 forces of the universe, physically wrestling 00:25:52.750 --> 00:25:56.190 with the metal inside the plastic box. Thank 00:25:56.190 --> 00:25:58.410 you for joining us on this deep dive. Go ahead 00:25:58.410 --> 00:26:00.269 and listen closely to the subtle hums of the 00:26:00.269 --> 00:26:02.710 technology around you today. You might just hear 00:26:02.710 --> 00:26:03.549 the physics at work.