What is the frequency range of buzzing electronics?
January 26, 2023 11:09 AM   Subscribe

I can hear high-frequency sounds from some electronics (as discussed here). Does anyone know what frequencies these electronics are buzzing at?

Since having Bell's Palsy, I can hear high-frequency things that are very annoying. I wish I knew what frequencies they were at so that I could try to mediate them somewhat. Maybe I could buy a detector that goes at that range to identify and verify what they are. In some ideal world, I would ask my workplace to change things in my workspace, so documentation will be useful.

I'm thinking of getting a high-frequency hearing test. Those are in the 9-20 kHz range (I have normal hearing at and below 8 kHz). Before I pay for that, I wonder if I can figure out (if you can help me figure out) the actual frequency.

The most obvious example is my wired smoke detectors which I can clearly hear buzz when they are plugged in, but not when they are operating simply on battery power. Note, this is NOT that the smoke detectors are triggered, but a background buzzing in their idle state.
posted by lab.beetle to Health & Fitness (13 answers total) 1 user marked this as a favorite
It’s going to vary by device, and if you can’t find the spec anywhere, you’ll need an oscilloscope or audio spectrum analyzer.

As a fellow sufferer who could hear a CRT tube 15kHz carrier frequency from 4 rooms away, my condolences.
posted by hwyengr at 11:15 AM on January 26 [3 favorites]

Response by poster: Maybe I should expand my question slightly to include: Is there a reasonably priced audio spectrum analyzer that anyone has used before?
posted by lab.beetle at 11:22 AM on January 26

You could try just installing a freebie spectrum analyzer app on your phone. Phone microphones are pretty rubbish but at high frequencies so are human ears, even palsy-sensitized ones; any near-ultrasonic noise that's loud enough to bother you will probably at least show as some kind of peak on a phone-microphone-based spectrum analyzer, albeit at a lower reported level than actually exists.

Finding ambient noises that the spectrum analyzer can hear but you can't will tell you how far the upper reaches of your own hearing extend. Plugging in a directional mic, or adding some kind of improvised directional funnel to the existing phone mic, might also be useful for tracking down exactly which device the insanity-making electric crickets are living in.
posted by flabdablet at 11:47 AM on January 26 [2 favorites]

The most common tone of electronics is the 60-cycle hum one hears briefly at any concert when musicians plug (or unplug) their microphones & electric guitars into the amplifiers. This buzz can also emanate from a lot of audio equipment when there's some shielding or grounding troubles. It's the frequency of alternating current (AC).

A similar sound can be heard from big, older transformers; electrical engineers call this 'singing' rather than buzzing.
posted by Rash at 11:51 AM on January 26

Switched-mode power supplies are supposed to be designed in such a way that the frequencies they generate are inaudible. So either you're hearing frequencies above 20 kHz or you're hearing subharmonics as some other part of the power supply resonates at a fraction of the switching frequency. For example, if the switching frequency were 30 kHz, the 15 kHz subharmonic is within human hearing, and a boost to your hearing in the upper frequency range would make you more sensitive to it than the average person your age. But switched-mode power supplies are commodity parts, you're not really going to know the switching frequency of any given power supply, and the resonance of any component in a power supply will depend on a bunch of other variables you can't control. Even given two otherwise identical power supplies from the same manufacturer, one of them could have just a bit more solder in the right spot than the other (or epoxy, depending on the design), making it less vulnerable to resonant vibrations on any particular frequency.

I don't think you can count on finding a standard frequency to test for. If you know a particular device bothers you, you can use a spectrum analyzer to try to figure out what frequency it's resonating on, or you could literally just replace it with another device from a different manufacturing run and see if the problem goes away.
posted by fedward at 11:53 AM on January 26 [3 favorites]

The kinds of buzz that Bell's Palsy will sensitize people to are way way higher-pitched than 60Hz mains hum. They'll most likely be above 10kHz, and their source will be coil whine from squegging due to regulation instability in shit-grade switching power supplies.
posted by flabdablet at 11:58 AM on January 26 [7 favorites]

High frequency noise that children, asthmatics, and people like you (probably among other categories) can hear from electronic components is usually attributed to flyback transformers.

The audible frequency from older televisions and other CRT displays is ~15 kHz, but modern displays use flyback transformers operating at 30+ kHz. The linked Wikipedia article points out that flyback transformers are extensively used in low voltage applications too, and I suppose that explains how power bricks were suddenly able to shrink so much a decade or two back, but I didn’t see a specified range of frequencies for those.

Inability to hear those 15 kHz sounds is generally attributed to damage to the inner ear and simple aging, but your experience suggests something very different.

Because the nerve which is compromised in Bell's palsy, the seventh cranial nerve, also ennervates the stapedius muscle, which serves to dampen "vibrations passed to the cochlea via the oval window".

Which means that tensing the stapedius muscle is probably what protects many of us from hearing those high frequencies and suffering the damage they might do, as well.

But having to have the stapedius muscle tensed all the time is probably a source of stress to the inner ear and the body generally, and also reduces a person's ability to hear other, lower frequency sounds, and is therefore a cause of worse hearing in general.
posted by jamjam at 1:24 PM on January 26

integer multiples of 60hz
posted by j_curiouser at 2:10 PM on January 26

modern displays use flyback transformers operating at 30+ kHz.

Modern displays are LCD or OLED flat panels and just don't have the flyback transformers that were needed to drive CRT displays.

What a lot of modern electronics does have is switching power supplies. These things are built around oscillators that run at frequencies well in excess of the highest a human ear can detect, typically somewhere between 40kHz and 150kHz. They contain ferrite-cored transformers, often including an air gap in the core to discourage core saturation, and these will sing at the switching oscillator frequency.

If all's working well then that can only ever make noise that humans can't hear, though god only knows what we're inflicting on all the mice and bats. But there are certain operating conditions, especially in half-assed designs, that cause the switching action to stop and start in bursts. This is squegging, and it makes the transformers sing at sub-harmonics of the primary switching frequency as well.

For example, if a switching power supply designed to run at 50kHz has a poorly stabilized regulation loop that always makes its transformer operate with duty cycles that vary in a four-step repeating pattern, then it will sing at a totally audible 12.5kHz as well as at the primary 50kHz rate. Sometimes this will even shake the ferrite core enough to break the glue that holds its halves together, at which point the singing will get really loud. Some truly shitty little supplies contain transformers with cores that were never glued together to begin with and those just shriek.

The exact frequencies at which a bad switching power supply will squeg depend sensitively on both its designed switching frequency and the instantaneous level of load it's being required to supply, and neither of these numbers is in any way standardized.

It's fairly rare, though, for squegging to happen much below 10kHz and there are an awful lot of people (especially rich, old people) who can't even hear that high, so persuading the boss that the noise driving you nuts actually exists and isn't just tinnitus can often take far more effort than it should.
posted by flabdablet at 2:38 PM on January 26 [4 favorites]

persuading the boss that the noise driving you nuts actually exists

<chatfilter>He wasn't my boss, he was the office mate who walked in as I was crawling under my desk, and the G5 nap chirp was real, TYVM.</chatfilter>

To make this comment productive: one other crazy source for whines that I've encountered is the backlight in a couple older devices we have. One of our TV remotes lights up when you lift it, and it hisses when it lights up, but I think even that noise is still an artifact of high frequency switching in the power circuitry for the backlight despite the fact the remote is battery powered. Some of our LED light bulbs also whine, which is going to bother me for a couple days now that I've thought about it, before my brain's noise filter does me the favor of forgetting to notice it.
posted by fedward at 3:27 PM on January 26

Modern displays are LCD or OLED flat panels and just don't have the flyback transformers that were needed to drive CRT displays.

My direct experience of "modern computer displays" is entirely restricted to laptops, tablets, and phones, but here is the passage from the Wikipedia article I relied on for that part of my comment:
The flyback (vertical portion of the sawtooth wave) can be a potential problem to the flyback transformer if the energy has nowhere to go: the faster a magnetic field collapses, the greater the induced voltage, which, if not controlled, can flash over the transformer terminals. The high frequency used permits the use of a much smaller transformer. In television sets, this high frequency is about 15 kilohertz (15.625 kHz for PAL, 15.734 kHz for NTSC), and vibrations from the transformer core caused by magnetostriction can often be heard as a high-pitched whine. In modern computer displays, the frequency can vary over a wide range, from about 30 kHz to 150 kHz.
posted by jamjam at 4:00 PM on January 26

Yes, it's about time that article got an update.

CRT-based personal computer displays from the era of the original IBM PC until flat panels won the entire market did indeed run at a variety of horizontal refresh rates well above the 15kHz television standard, but in 2023 it's a real stretch to describe any such device as a "modern computer display".

CRT flyback transformer circuitry is also nowhere near as prone to squegging as that of a typical switching power supply, mainly because if it did squeg it would cause unacceptable horizontal display shimmer. So if a CRT flyback transformer is going to emit a whine, it will do so at the actual horizontal line rate rather than some subharmonic of it, which in turn means that no human is going to be bothered by a CRT computer monitor with a 20kHz+ horizontal refresh rate.

If there's an audible whine coming out of a flat panel display then it will probably be due to squegging in the power supply for its fluorescent backlight; flat panels just don't have horizontal scanning and don't need flyback transformers. But again, we're living in the future now, LED backlights already outnumber fluorescents by quite a wide margin, and LED backlights don't need a dedicated high voltage power supply.

There does exist a switching power supply architecture referred to as a flyback converter, but there also exist a lot of other switching power supply architectures, all of them containing magnetics that are physically capable of acting as transducers, and all of them capable of squegging given poor design.

So power supply squeal is by no means exclusive to flyback transformers or flyback converters. If you're looking at any circuit board and you see a little inductor mounted near a power MOSFET and a bunch of low-ESR electrolytic capacitors, you're looking at a potential source of coil whine. There are motherboards and graphics cards that are notorious for it.
posted by flabdablet at 9:43 PM on January 26

Seconding the idea of starting with a free phone/tablet spectrum analyzer app. You could also try adding a measurement microphone designed to work with phones & etc. - like this cheap one from Dayton that uses the 3.5 mm jack or one of these more expensive ones that will work with a Lightning or USB-C port.

Either way, my main concern would be that the specific tones bothering you will be overwhelmed by your general environmental noise, especially if you're measuring a "normal" distance away from the source. So you might have to consider/plan for getting your mic (external or built in) much closer to your smoke detectors & other devices to better pinpoint the actual frequencies.
posted by soundguy99 at 7:55 AM on January 27 [1 favorite]

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