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Why do many microwave ovens hum in an interval of a minor 7th? (lemmy.wtf)

submitted 9 months ago* (last edited 9 months ago) by [email protected] to c/askscience

42 comments fedilink hide all child comments

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.

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[–] CADmonkey 60 points 9 months ago (3 children)

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.

[–] Mango 10 points 9 months ago

He did the math!

[–] [email protected] 9 points 9 months ago

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

[–] [email protected] 2 points 9 months ago* (last edited 9 months ago) (1 children)

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

[–] CADmonkey 2 points 9 months ago

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

[–] Pilkins 25 points 9 months ago (1 children)

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.

[–] [email protected] 6 points 9 months ago

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.

[–] z00s 16 points 9 months ago

Why do microwaves hum

Because they don't know the words, obviously

[–] [email protected] 13 points 9 months ago

you're hearing the magnatron's power supply

[–] [email protected] 12 points 9 months ago* (last edited 9 months ago) (2 children)

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: https://play.google.com/store/apps/details?id=org.intoorbit.spectrum

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

[–] [email protected] 9 points 9 months ago* (last edited 9 months ago)

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).

[–] [email protected] 1 points 9 months ago (1 children)

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

[–] [email protected] 2 points 9 months ago

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 😉.

[–] [email protected] 6 points 9 months ago

50 / 60 hz from the transformer under load?

[–] [email protected] 4 points 9 months ago

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.

[–] [email protected] 3 points 9 months ago

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

[+] Mr_Blott -12 points 9 months ago (6 children)

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

[–] [email protected] 14 points 9 months ago

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 12 points 9 months ago

Op uses it for music

[–] [email protected] 11 points 9 months ago (1 children)

What do you use it for?!

[+] Mr_Blott -21 points 9 months ago (4 children)

Collecting dust. Popcorn.

Literally nothing is cookable in a microwave without it being disgusting

[–] [email protected] 18 points 9 months ago (1 children)

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.

[+] Mr_Blott -7 points 9 months ago (1 children)

I've been trying to think of things that a microwave is better for, and could only think of melting butter. Though a small saucepan does that too.

A fan oven or a frying pan or a grill all cook stuff 10x better than a microwave

A craftsman wouldn't use the worst tool for the job, after all

[–] [email protected] 9 points 9 months ago (2 children)

Microwaves are fantastic cooking tools, and I'm pretty confident you're not using yours to its potential. Defrosting, reheating, steaming, boiling. Does it all in half the time with half the mess. All those settings on the keypad do something good. Most people just wack a few numbers in and let the microwave literally cremate the food on full power. Those reheat and defrost settings apply microwave then switch to low or no power, leaving the applied heat to radiate internally before repeating. Different densities and starting temperatures are accounted for.

Obviously you wouldn't cook a stir fry or a steak in a microwave. Potatoes before roasting though? Dumplings? Frozens? Yes please Mike.

[–] HerrBeter 5 points 9 months ago* (last edited 9 months ago) (2 children)

Mashed potatoes :

Dice small, 10-15min

Rice:

1,5dl water to 1dl rice, 10-12min

I could go on. It's a big water heater

[–] SpaceNoodle 3 points 9 months ago (1 children)

Why have I not been microwaving rice? Does this affect the texture at all?

[–] HerrBeter 1 points 9 months ago (1 children)

Only if you don't turn down the power, you want full effect to get in to temp and then turn it down to keep temperature steady

[–] SpaceNoodle 1 points 9 months ago (1 children)

So, not what you said, then

[–] HerrBeter 1 points 9 months ago

I don't think I specified, but much like cooking rice on a stove top you won't run it at the same temperature. The microwave will use less energy than a traditional stove top, not sure about induction ones. And after learning to boil things with it, I do it all the time.

Just get it to boiling then 450 then if needed 300

[–] [email protected] 1 points 9 months ago* (last edited 9 months ago)

I've never seen someone use Deciliters before! Love it. Disclaimer: I live in either the USA, Myanmar, or Liberia.

[–] ilinamorato 2 points 9 months ago

It does passable baked potatoes too, but if you microwave it for about half the time and then toss it in some oil, kosher salt, and pepper and put it in a hot oven, you get great baked potatoes in far less time than in the oven alone. You're basically boiling the middle and baking the outside. Great combo.

[–] [email protected] 13 points 9 months ago* (last edited 9 months ago)

frozen peas

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

[–] [email protected] 5 points 9 months ago (1 children)

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 2 points 9 months ago

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…

[–] SpaceNoodle 1 points 9 months ago (1 children)

What do you think is happening to the popcorn when you put it in the microwave?

Anyway, microwave popcorn is disgusting.

[–] Mr_Blott -1 points 9 months ago (1 children)

How do you recommend cooking popcorn, Mr Blumenthal?

[–] SpaceNoodle 1 points 9 months ago* (last edited 9 months ago) (1 children)

In a popcorn popper ... or a pot.

[–] Mr_Blott 0 points 9 months ago

Right tools for the job, eh

[–] OhmsLawn 8 points 9 months ago

Reheating, generally.

[–] [email protected] 3 points 9 months ago (2 children)

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 3 points 9 months ago* (last edited 9 months ago)

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

[–] [email protected] 2 points 9 months ago

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