So 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. Or 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 are 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 electromagnetically
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 electronics 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. OK, lay
it on us. Because when we say noise in this context,
we aren't talking about, like, electrical signal
noise. Right, like 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. Very 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 -voltage 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 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.
OK. 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 intruding on the physical. Exactly.
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 at play here?
Well, the primary culprits are what we call Maxwell
forces or reluctance forces. And these forces
concentrate incredibly intensely at boundaries
where there is a massive change in magnetic relativity.
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. Exactly. That sudden,
massive shift in resistance creates immense physical
stress right at the boundary. So it's a surface
thing. Right. These are the exact 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, but the sources point out another
distinct force called magnetostriction So if
Maxwell forces are pulling from the outside,
maybe a restriction 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 and magnetostriction
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 chaotic 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. First,
the frequency of these vibrations has to land
right in the sweet spot of human hearing, which
is roughly between 20 Hz and 20 kHz. Which is
a pretty wide range, but still. Right. If the
metal is vibrating at 5 Hz, 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 are 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 with this. Let's look at inductors,
which are sometimes called reactors or chokes
in the engineering world. OK, inductors. 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. trying to pull the
gap closed. 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. You have
Lorenz forces physically shaking the copper windings,
the Maxwell forces attacking the joints of the
metal laminations trying to pull them apart.
And the magnetostriction happening deep inside
the laminated core. Right, making the whole massive
block of metal subtly expand and 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. OK, 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. There are no magnetic cores. 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. When the voltage or current waveform
running through the capacitor isn't perfectly
smooth and constant... Which I imagine is often
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 and 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 in a lighter. Right. But in reverse,
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. Too 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. Yes. 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 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 are entering the
realm of radial and axial flux rotating machines.
Which includes what, exactly? This includes induction
motors, switched reluctance motors, and synchronous
motors. Oh, right. Remember that subway electric
motor whine 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. Cogging torque. So these dynamic
stuttering pulses create torsional vibrations,
physically twisting the rotor and the stator
back and forth like someone wringing out a towel.
Yeah, but while a simple solid cylinder twisting
back and forth doesn't radiate sound very efficiently
into the air, the complex outer boundary of a
motor stator certainly can. Right. Lots of surface
area. Furthermore, those Maxwell stress forces
we talked about earlier aren't just pulling in
one simple direction. 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.
But the sources 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 radio flux machines, resonance isn't just
about matching a single frequency, like a singer
hitting the perfect pitch to shatter a wine glass.
OK. 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. OK, 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. Right. 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.
It 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 2. 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. They 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. Right, 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. OK, so we know exactly how
this perfect storm forms. We've got the tug of
war of Maxwell forces, the internal breathing
of magnet restriction, the 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 weakly coupled problem. Which honestly sounds
like a bad thing, like the math is falling apart.
It 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.
The arrow 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 warped 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? Because 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. And 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 stator 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? That introduces
a whole new level of chaos into the 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 pull. 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 unbalance
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 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 wine?
Well, what are the NVH noise, vibration, and
harshness mitigation techniques? If we connect
this to the bigger picture, engineers essentially
have three overarching strategic choices to solve
this puzzle. Okay, 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. Okay, 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 of slots and pulls inside
the motor so the patterns never perfectly align.
Or they might actually build the stator or the
rotor with a slight diagonal skew. Oh, 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. delicate, brilliant
balancing act of electromagnetics, 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. Or, and I found this detail from the sources
completely relatable, they just dump glue on
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 level
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. That is profound. I mean,
we've journeyed from the microscopic boundary
layers where Maxwell forces play 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
frames 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. OK, later
on this. We spent 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 in millions of bedrooms,
are we just bleeding energy through sound? Wow,
that's, 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 are 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. It's 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.