Why do many microwave ovens hum in an interval of a minor 7th?

Something that I’ve noticed across most of the microwave ovens that I’ve used is that when they hum while cooking food, I can pick out 2 distinct tones. One of them is pretty clearly 60 120 hz, the 2nd harmonic of the AC power frequency. The other is consistently a minor 7th above that (which would be somewhere around 106-108 212-214 hz depending on the exact ratio). What causes this 2nd frequency to be produced?

Edit: after checking against a tone generator, the low frequency is actually 120 hz, double the grid frequency. The question is still the same, just an octave higher.

CADmonkey,

There are two things making noise in the microwave when it’s running. There’s the transformer that’s making the high voltage for the magnetron, which is you 120 Hz noise you’re hearing.

The other noisy thing in the box is the fan. The fan is most likely a three blade metal fan running on a little shaded pole induction motor. That motor is very simple, just one moving part, and two poles. Two poles at 60 Hz gives us 3,600 RPM, which the motor can’t quite reach because as the RPM gets closer to that magic number of 3,600 rpm, the motor draws lesa current and makes less power. This difference between the speed the motor is trying to run (“syncronous speed”) and the actual speed is called the “slip” and is probably around 3%-5%. This gives us a fan speed between 3400 and 3500 rpm.

I don’t know if that is giving you the second noise you’re hearing, or if it’s mixing with the transformer noise to make it. But now you’ve caused a situation where I’m going to be caught holding a guitar tuner to my microwave.

CanadaPlus, (edited )

Wow. This question is a good but very specific observation, and I did not expect an actual answer.

CADmonkey,

I don’t know much about music or why some sounds are more pleasant than others… but I do know about electric motors.

Mango,

He did the math!

wheeldawg,

Can I give a second upvote to just the last sentence?

z00s,

Why do microwaves hum

Because they don’t know the words, obviously

kaupas24,
kaupas24 avatar

50 / 60 hz from the transformer under load?

dirtypirate,

you're hearing the magnatron's power supply

Pilkins,

There is a fan running and then the actual microwave itself. If you lower the power setting, you can hear the fan stay on consistently but hear the microwave shut off and on. If the power is at 100%, the microwave runs the entire time.

lolcatnip,

A few microwaves don’t do that because they’re able to continuously run the magnetron at less than full power by using an inverter instead of a transformer.

dpunked,

Do you also observe this when running the microwave on max power? Usually, when a microwave states its 800 watts but you can adjust it down, it actually only changes the duration that power is supplied. So 400 watts would be 50% of the time the power is supplied at max.

Mr_Blott,

Do people actually use microwaves for cooking or is that just a meme?

OhmsLawn,

Reheating, generally.

KISSmyOS,

30s-1min in a microwave will make old bread soft again
30s will melt cheese
other than that, I use it for reheating food and heating up spiced wine.

CameronDev,

What do you use it for?!

Mr_Blott,

Collecting dust. Popcorn.

Literally nothing is cookable in a microwave without it being disgusting

CameronDev,

Disgusting how?

I mostly use mine for reheating stuff, but cook the occasional two minute noodles or frozen pie. Never had it be disgusting, at worst it can be cold if not done long enough. Cutting the food up and stiring it around halfway through helps.

accideath,

Yup, also primarily use it for reheating food. Sometimes, when I’m lazy and just want sth to eat, I’ll make myself microwave potatoes. And I also use it to defrost frozen vegetables or fruit sometimes.

However, when I was living in a one bedroom apartment, which didn’t have an oven in the kitchen-corner, I had a microwave with integrated oven and with that I was able to bake myself anything that fits into a pizza sized round baking tray. Still love that thing. It’s a 30 y/o hand me down Siemens from my uncle that still works perfectly. It’s also rather large (about twice the size of a normal microwave) and sadly doesn’t fit into my current kitchen…

ElderWendigo,

Maybe you’re just doing it wrong. It’s a poor craftsman that blames their tools. I don’t get mad at the toaster for being bad at boiling water.

Primarily0617,

frozen peas

it strikes me as weird that somebody operating on your level of elitism considers microwave popcorn acceptable

Shalakushka,
Shalakushka avatar

I like how instead of imagining there might be something you don't know about microwaves, you just kind of assumed everyone bought a dust and popcorn machine for no reason. It's such an "am I so out of touch?" moment.

arin,

Op uses it for music

Auk,
Auk avatar

Yes, microwaves are a poor substitute for an oven but they work fine for vegetables that you might otherwise use a steamer to cook. Stuff like broccoli, beans, carrot pieces etc. Corn on the cob works well too, just give it a few minutes in the microwave with the husk still on.

ilinamorato,

Anything that’s ok mushy, really. You can also use it to start cooking something that you finish somewhere else.

clay_pidgin,

Great for reheating leftovers, if it has a good humidity sensor.

CameronDev,

Could it be the motor?

The second harmonic from 60hz is 120, which is close to where your hearing it? For what its worth, i get a peak at 100hz, which for the australian grid would make sense?

I used this app: play.google.com/store/apps/details?id=org.intoorb…

I dont see a 50hz peak, but 100hz was clearly represented.

olsonexi, (edited )

I checked against a tone generator, and realized that the lower frequency was actually 120, not 60, so I’d guess you’re probably hearing that. That puts the higher frequency at 212-214, which I also checked and that range matches what I’m hearing (for 100hz, it should be at around 177-180).

clay_pidgin,

Interesting. I saw mostly 38Hz, 117, and 240. 100W LG microwave on US 60Hz with an audible fan.

CameronDev,

Mine is also LG, not sure what its power output is, but i think its either 1000W or 1200W, yours must take forever to heat anything 😉.

otter,

I honestly have no idea, and never before knew that I do not.

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