On a related subject, I’ve always wondered how radio stations broadcast silence. How do they overwrite white noise with, essentially, nothing at all?
White noise on a non-station frequency is the result of your stereo amplifier amplifying nothing (well, the random bits of RF that hit the receiving antenna at the right frequency). That’s why communications radios have squelching circuitry – to cut off that white noise when there’s no transmitted signal. The transmitted signal, then, is just a carrier frequency; the “real” frequency is a result of either modulating the amplitude (AM) or the frequency (FM) of the carrier. If you modulate to an inaudible frequency, then you hear silence.
Short version: the transmitted signal is stronger than the white noise.
Clarke’s device in that story works the same way modern “sound cancelling” headphones work – by creating an “anti-acoustic” wave to cancel the existing sound wave. Since these devices work without any of the ill effects Clarke has in his story, I conclude that Clarke either missed or else suppressed for the sake of the story some salient facts.
BTW, HalfManHalfWit, you can “create darkness” by making an interference pattern, or by adjusting one arm of an interfrometer to completely cancel. With light, making the two waves coherent is not trivial, and is sometimes impossible, but sound waves are MUCH more forgiving. Making an intereference pattern using ultrasound is a piece of cake – you can use two separate transmitters, whereas in optics you have to use two coupled sources or a single split one.
All that said, I’ve got no personal experience with human-hearing-range sound cancelling headphones, so I’m not aware of their capabilities and limitations, bt people who use them tell me they work great.
If we could just get hold of some vibranium…
I want one that goes to eleven.
Wasn’t this a plot in an episode of Profiler? Some lady who had seen her grandfather die in a woodworking accident kept hearing him scream, and kidnapped a scientist to make her a silence-creating or sound-deadening apparatus, not realizing that the scream was in her head?
Ah yes, found it: Shattered Silence.
Noise cancelling headphones work based on an extremely simple principle: a microphone just outside the earphone and an amplifier which inverts the signal so that the earphone generates a signal which cancels the original sound. There is no “predicting” or anything of the sort and it does not care whether the sound comes from 3 or 300 feet away. It is really very simple.
Well, yes, with any superposing theory you can find a way to cancel out waves, but you can’t create a general-purpose ‘darkness’ or ‘silence’ source; there’s no ‘dark bulb’ in the same way as there is no ‘quietspeaker’.