{What I Love!}
Hello! I’m Scruffy, and today I’m talking about microwave ovens! Really! I know it’s a bit
different course from my usual trivia videos, but near the end of 2017, I saw VSauce host
Michael Stevens and former Mythbuster Adam Savage in their live science tour Brain Candy, and it got
me inspired to try my hand at a science video! So I’m going to show you some trivia about the sound
of your household microwave oven. As fascinating as they can be, I do not encourage tampering with
one yourself unless you’re following maintenance instructions provided with the microwave.
Now, why do microwaves hum? Because they don’t know the words. …But what
do microwaves hum? If you’ve been around a microwave, has it sounded like this?
[microwave oven noise plays] If you live in the Americas or some parts of Asia,
your microwave will resonate at this frequency, around 120 Hz or about halfway between A# and B
an octave below middle C. In other parts of the world, you’ll hear 100 Hz microwaves, or between
G and G# an octave below middle C. But these frequencies are consistent microwave to microwave,
and that’s because microwaves are humming the tune of the electricity being supplied to them from the
main power line. The power lines around the world supply electricity with a frequency of 50 Hz or 60
Hz, depending on where you live. That means 50 or 60 cycles of electricity get pumped into the
microwave per second. Even when the microwave isn’t running, this creates a magnetic field
around the microwave. When voltage is applied while the microwave is running, a hum becomes
audible as the magnetic field starts to cause the enclosed microwave parts to vibrate. The influence
of the magnetic field is strongest twice every cycle of electricity, so, since there are 50 or
60 cycles of electricity per second, the resulting hum has a frequency twice that much. That’s the
lower note in this chord [sings note], that note. The upper note [sings note] is the fan inside the
microwave keeping its components from overheating. The two tones together produce this soothing
chord, a satisfying drone with which you may find spiritual resonance, safe with the knowledge
that your pizza is steadily reheating.
Ahem, so that’s what your microwave hums…
while it’s running. If you hear a hum while the microwave is not running, that’s a sign that a
component of the microwave is malfunctioning. But when it’s just plugged in, inactive, and quiet,
you can generate a hum from its idle magnetic field simply by bringing a microphone close to it.
The magnetic field will influence the microphone, creating a ghost hum of 50 or 60 Hz, straight
from the power line. This is what you’re hearing now: the magnetic field of my microwave creating
a 60 Hz hum as I move my hand into and out of it with this microphone. This phenomenon is called
mains hum. Mains hum occurs whenever magnetic fields from electrical appliances or wiring are
either audible by themselves or picked up by audio equipment like microphones and speakers,
producing a hum of the frequency of the power line. To hear the mains hum, you need either
an appliance that picks up enough electricity to vibrate idly, such as some transformer boxes
or refrigerators, or audio equipment that can be affected by a nearby magnetic field. This
mains hum is the song every electrical appliance knows, it’s the sound that you hear when you have
an unplugged audio jack leading to an amplifier, and to most sound technicians and music producers,
it’s a nuisance. Microphones picking up a low electric droning sound because of grounding
issues or being left near a plugged-in microwave? That has to be stopped to get a clean sound! Some
techniques have developed to counter mains hum, such as humbucking. You may have heard
of a humbucker guitar: in this guitar, two coils are arranged in the pickup in opposite
polarities in the pickup, so that the hum signals between the two are in opposite phase, and thus
cancel out. That’s “bucking” the hum. Another technique is using a band-stop audio filter,
also called a notch filter. This signal filter allows most frequencies to pass but reduces a
select few frequencies, in this case 49-51 Hz or 59-61 Hz are completely reduced. Why filter
out that range instead of just 50 or just 60? Well that's because the electricity supplied
via the main power line actually fluctuates a few thousandths of a Hz depending on the supply
and demand for electricity. If there are more people trying to use electricity than supply
can keep up with, the mains hum gets a little lower in pitch than usual. And if fewer people
than expected are using electricity, the mains hum gets a little higher.
It’s not a change you’re bound to notice with the naked ear, but you can at least hear the
mains hum, and you can hear that it's all around us. Appliances that vibrate with
their supplied electricity all hum this tune, from vending machines to power transformers to
amplifiers. This is the song of our technology and our environment, and we may consider it a
nuisance at times, but when we have nothing else to hear, it can be quite a mellifluous drone.
I’m Scruffy, and thank you very much for watching.