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Why Does a Screen Re-Draw Make Noises?
grungy asks: "On several computers I have owned, I have noticed an audible noise related to large screen re-draws. A hardware guy once hypothesized that the large memory-move operation was creating electronic 'noise' which was then picked up and audibly amplified by my speaker. I unwired my speaker, removed it from the machine and put it in a different room, and the phenomenon still occurred. At this point I assumed it was something going on/emanating from the monitor itself. Now I have a TiBook laptop with an LCD panel. At quiet moments I can still hear it when I drag windows around. I have tried doing big memcpy's & the like, I don't get the same noise. I've been wondering about this for years. Anybody know what gives?"
Whenever you have a flow of current, you will have a magnetic field generated, and that field reacts with the environment to cause motion. Even though computers are 'digital' doesn't mean that some how they are immune from the all the laws of electromagnetism we use to design analog devices like speakers - it's all the same thing. If nothing else, there is always the earth's magnetosphere to react with like a speaker's coil to it's magnet.
It's the same reason electrical transformers hum, and fluorescent lights buzz.
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The hissy-screechy-screech-screech that you're hearing might also be coming from the power supply. To the extent that it would carry into your audio circuit, electrical noise would easily translated to acoustic noise through your speaker/headset.
However, it's also possible that you have a marginal power supply that operates at switching frequencies that approaches human-audible frequencies; or the actual current draw changes from high-speed memory transfers within the graphic sections (board) has a human-audible frequency component to it that actually emanates from (say) the torroids in the supply.
People with very sensitive high-frequency hearing can sometimes tell the brightness of a television screen just by listening...
This could also happen from other activity -- I once had a 386 PC which, when running DOS, would emanate the tell-tale sound when it was waiting for keyboard input. It was kinda neat, actually -- I could go read other things while waiting for a program to finish its calculation -- and I didn't have to keep looking up at the screen...
LCD actually change physical state, i.e. there is a mechanical change. Enough of those at the same time, and there will be some sound. A little like piezo-electric shriekers, but much smaller, and with much smaller movement, and only a single pulse of movement rather than repeated oscillation.
My Psion 5 used to sing a merry song to me all the time as things changed on screen. Almost everything LCD makes these noises in some quantity. I do have very sensitive ears though, so perhaps not everyone hears them.
YAW.
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I'm amazed at the number of answers from the "Slashdot experts" and yet I don't see the correct one yet.
The noise you hear is actually fairly simple to explain. First of all, people should realize that this is not RF noise coming through the speaker, as you tried to explain. This is a noise generated by the vibration of a system component.
Your graphics card is the culprit. Remember that your hardware is full of clocks(vibrating crystals) and switches(transistors). These microscopic components move or vibrate at very high frequencies. Vibration creates noise, as we all know. But, the vibrations(or frequencies) change when the image on the screen changes. Certain colors and certain movements on the screen create frequecies that are perceptible to human hearing and you hear a slight buzz or high pitched whine form your video card.
If you want to test my answer, try changing the frequencies for your display and you will hear the sound come and go. You will also notice the pitch will change when different frequency setting are used.
Some hardware is less prone to this because of thicker cladding or more secure mountings but, they all do it. It's just that some equipment is louder than others.