what am i hearing from older electronics, like tv or computer moniter, when the power is on but with the sound is completely off?
I’m sure it has something to do with the display or screen as i can hear this in older tvs and moniters (80’s & 90’s) but not the newer ones.
my experience is that not many people i know (that I’ve asked anyway, so a small sample size) have hearing that’s capable of hearing what I’m talking about. if a house is really quiet, i can hear an older tv or monitor that’s been left on in some other room of the house. the term silence is deafening to me is because when everything is silent i could always hear these frequencies and it’d drive me nuts. I’d always have to have some other noise to drown it out (like a fan or radio).
so, what specifically am i hearing and who among you can hear what I’m talking about?
In the old days of CRT monitors and TVs, I could detect a very high pitch “hum” (more like a squeal), especially on start-up.
I think the source originated in the fly back transformer… no longer required in today’s monitors.
There’s plenty of faint, high-frequency noise coming from computers and displays. In my case, moving my cordless mouse generates high-pitched sounds through the computer’s speakers; my guess is this is simply RFI from the mouse’s radio signals getting picked up by the speaker system. Sometimes other noises come through that are related to the computer engaging in different actions, e.g. loading a program or opening a document. All very faint, the only way to hear them is when everything else in the house is quiet - no HVAC, no fridge sound, no TV.
Old CRT based displays emitted noise at the horizontal scan frequency, often because they used this same drive frequency in the EHT transformer, but even if not that, the scan coils could also emit sound.
Modern equipment of course does not use a CRT, so no scan. However they are filled with various DC-DC converters and these operate at quite high frequencies as well. The transformers used will also tend to emit some sound. You can often hear the sound change in character as the power demands in the equipment changes.
The higher the operating frequency the smaller a transformer can be for the same power capability, so there is significant incentive to run at quite high frequencies. This is the core reason you no longer see huge lumps of iron in power supplies. They don’t use mains frequency, but rather rectify the mains and chop it to much higher frequencies.
The mention of transformers brings up the reason they make noise: magnetostriction. Basically, the ferromagnetic core material changes shape by a tiny amount every time the magnetic field in the core gets reversed. This manifests as mechanical vibration of the transformer, which becomes acoustic vibration of the air around it and the chassis to which it is affixed. More electrical power moving through the transformer results in stronger magnetization on each electrical current cycle, resulting in stronger vibrations. I have a TIG welder that uses a very large transformer (which operates at mains frequency); when I push the pedal all the way down to get maximum welding current, the transformer vibrates strongly/loudly enough to shake the floor a little bit.
the difference in what I’m talking about and this is, in your scenario, there’s something involved that’s supposed to make noise (speakers). what I’m talking about I’d hear if you were to just plug a monitor, by itself, into the wall and turn it on.
i notice that too from my welders. it’s especially noticeable in the 240v one’s. the higher the amps, the more noticeable it is.
I’ve got arc & bottle fed mig welders. i like arc welding the most, i guess because I’m a little masochistic or something. i plan to get a plasma cutter. I’d like a tig, for no real good reason other than just because.
mr horsepower, do you suffer from tinnitus? There’s a phenomenon where some tinnitus sufferers can hear ultrasonic frequencies that other people can’t. There’s a particular type of tinnitus where the ringing is caused by muscle spasms in the ear (otoacoustic emission - it can actually be picked up with the right microphone), and if an ultrasonic signal is added to the mix then non-linearities in the ear’s response produce sum-and-difference frequencies (intermodulation distortion). The sum frequency will be an even higher ultrasonic frequency, but the difference frequency could well fall within the normal audible range.
If, however, you haven’t got tinnitus, then it’s probable that you just have ears with a very good frequency response.
The range of human hearing is commonly cited as 20 Hz - 20,000 Hz, but that varies quite a bit by person and by age. 15,000 Hz is less than half an octave down from 20,000 Hz; it’s not surprising that some people can hear 15,000 Hz (and be bothered by it) while other people can’t hear it at all.
Yeah, that’s the electronic whine from the flyback transformer. I don’t know how old you are, but the reason most people can’t hear it is age. When I was a kid, I remember that sound very well. I could always tell when a TV or CRT monitor was on in the room, even if the sound was off. After your teens or so, the ability to hear this sound fades for most adults. I seem to recall there being some sort of cell phone “ring tone” a few years back that some kids would use that most adults couldn’t hear because it operated somewhere in that 15-20k frequency band.
nope, no tinnitus. whenever I’ve thought i heard something left on in the other room, it’s always been the case that it has. almost always though, I’m the only one that hears it. this has happened with much less frequency though as the electronics have changed over the years.
i can still still hear an older monitors on in another room whenever a house is quiet now, just like when i was younger, it just doesn’t happen near as often.
regarding cell phones, and I’m hesitant to even mention this, but i could tell you if my cell phone was powered on or not by just holding against my ear in a perfectly quiet environment. i could do this with my eyes shut and someone else either turning my phone on or off in another room and bringing it in to me to hold against my ear. i was correct 90% of the time with the times i was wrong almost always being that i couldn’t hear it was on (so saying it’s off when it’s on). this was back in the early 2000’s with a nokia 5100 series (if i recall correctly) phone. my phone that replaced it i couldn’t detect anything. haven’t tried it in years since.
*this noise wasn’t a whir but a super faint, yet audible to me click. it didn’t have a fast tempo. it was very faint, once every so many seconds, and rythmic.
**I’m prepared to be called crazy for saying this.
Yeah, it’s the flyback transformer. Some of the cheap ones (or good ones that’re failing) would produce a very audible squeal that anybody close by could hear. Usually a short, sharp whack will temporarily halt the noise, one of the few legitimate uses of “percussive maintenance”. There’s a valid reason why it fixes this specific problem, but I can’t recall what it was (read it many years ago).
I know what you’re talking about, though. In the days of tube tv’s, I could tell when one was on, even if it was showing a blank screen, and had no audio. They’d make a faint noise right on the edge of my hearing.
When I was a kid, I knew when anyone turned our TV, no matter where I was in the house. What I was hearing was the hum (caused by hysteresis, I believe) caused by the flyback transformer at 15,750 Hz (that’s 525 lines at 30 frames/sec). By the time I hit 30, I no longer heard that frequency.
Doesn’t surprise me.
First of all, all modern electronics use “switching” power supplies, and they all make some small amount of audible noise. Secondly, phones have speakers in them, and I wouldn’t be at all surprised to find that they leak a small amount of signal to the speakers even when off (power supply fluctuations becoming audible noise).