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I hope many people might feel little awkward to see this question. But I feel this is important. Because in my place where I am living I can get a radio (Audio Receiver) very cheaply. I don't know till what extent they allow the audio frequency ranges. I guess they use ICs for Frequency Demodulators, Frequency Clippers, etc. If the Frequency demodulator doesn't work properly and allows all the frequencies that comes through air then its going to be a big problem to the humans. Can somebody answer which are all the frequency ranges that are available and which causes human hazards?

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personally i wear a tinfoil hat and hence am safe –  Mark Nov 9 '10 at 5:25
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@Mark, There was a study that found the tinfoil hat improved matching of certain frequencies into the brain. These frequencies all fell into reserved by Gov't ranges in the USA. I hope that makes you laugh. –  Kortuk Mar 23 '11 at 7:41
    
Did I read that right? Are you worried that if your receiver breaks just the right way, that the electromagnetic 100MHz FM signal you've just been listening to is suddenly going to 'get out' as a 100MHz acoustic signal in the air? If so, don't worry, that doesn't happen. –  JustJeff Jun 24 '11 at 9:52
    
Possibly related, maybe not, low frequency basically in the heartbeat range 60Hz or so, at high power, can cause pacemakers to not be able to sense their humans heart rate and they may shut off until they are removed from that field. If that human's medical condition is such that they cannot drive their own heart rate without the pacer, they can be at risk. It is rare though, have to hug a transformer or the turbine generator or something like that to get the power level required. And physically dragging the passed out person away from the field fixes it. –  dwelch Jun 24 '11 at 12:03
    
Pacmakers are communicated with using high frequency, and there was a time in the early days where you could scramble or temporarily upset the pacer. This is why you still see stickers on buildings that are warnings to pacer patients that microwaves are in use. Basically the communication now is not just frequency but protocol so generic high frequency fields do not have the protocol to actually reprogram or change the pacer. Nevertheless there is the concern that implanted devices like pacers and icds can be affected by high frequency fields. –  dwelch Jun 24 '11 at 12:06
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2 Answers

up vote 24 down vote accepted

The short answer is that unless you are dealing with professional power levels in the several watts range, RF is very difficult to cause injury with.


Long answer

RF doesn't affect humans directly unless there is a tremendous amount of power. Effects are typically thermal, when a particular chemical bond is struck just-so, it will absorb a photon, moving it slightly. Enough heating will damage cells by denaturing or "cooking" proteins.

Particular wavelengths (2.4 GHz) are well absorbed by water and fat, but absorption is still very diffuse so it would take a tremendous dose to cause enough heating in any one area to cause damage. The FCC safe exposure limit is 1.6 W absorbed per kilogram (as per one source), and 4 W/kg (in following link) for the entire body.

More:

Atomic nuclei can also also respond to RF, allowing for nuclear magnetic resonance spectroscopy (or MRIs), but this is a strictly nuclear effect and has no influence on chemical bonds.

IR and light obviously can cause burns, but only at sufficient power. IR lasers can be particularly hazardous to eyes, as it invisible it will not trigger a blink or aversion reflex, allowing a large, damaging dose to be absorbed before noticing.

Higher energy photons, UV, X-ray, and gamma, are able to ionize atoms when they strike them, causing unexpected chemical reactions to occur which can destroy, damage, or mutate cells.

These are all forms of "radiation", but the layperson couldn't tell what the implications of non-ionizing versus ionizing radiation are, which causes all sorts of unfounded fears of these invisible phantoms that carry our cell phone calls and webpages.

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You mean web pages aren't generated by supernatural entities living inside my computer? –  endolith Nov 8 '10 at 17:41
    
Pretty good summery. I used to work with Electronic Warfare Equipment and we had systems with enough power to be dangerous. It all depends on the power and frequency. The higher the frequency the less power was needed to be dangerous. –  Jim C Nov 10 '10 at 13:16
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The first sentence of your long answer made my hair stand on end. We're not quite sure yet, but there are indications/presumptions that frequent use of mobile phones close to your head may cause some level of brain damage. We're not talking tremendous amounts of power here. –  stevenvh Jun 24 '11 at 8:26
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@steve, we're unsure about many things. I'm unaware of any studies that have published results saying cellphones cause cancer. The WHO had a review of multiple studies and still only came out and said they might increase the risk of cancer. –  Nick T Jun 24 '11 at 12:18
    
fat is a nonpolar molecule and so it doesn't absorb 2.4 GHz very well. That's why on the box of my cheese sticks, it says "do not microwave". I would also point out that the risk category for cell phones is the same as for carpenters and coffee drinkers. –  ajs410 Jun 24 '11 at 21:57
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The question sounds like it's asking: What if a radio "lets through" high frequency (RF) signals into the audio spectrum - will it hurt people to hear audio signals way above (or below) the normal audio spectrum of human hearing? It's not asking about the effects of radio waves on a human.

From an engineering perspective you might translate this as: does the audio amplifier in a radio even try to amplify signals outside the range of human hearing, roughly 20Hz-20kHz? It would be bad if it did, as such behavior can cause components in the radio to run hotter than they would otherwise (wasting battery life and shortening the operating lifespan of the radio) without producing any effect in the audio output that human ears could hear. So a well-designed radio would probably have a lowpass filter before the audio amp to get rid of high-frequency noise, wherever it may come from (not necessarily a broken part upstream in the radio). Whether a very cheap radio would bother with this is questionable.

The next question would be, suppose you tried to drive a speaker with a signal way above the normal audio frequency range - what would happen? Speakers can't respond quickly enough to the signal to generate sounds all that far above audio range. They might be able to reproduce the signal accurately up to 22kHz or maybe 30kHz but much above that and the signal won't be getting made into vibrating air, it'll just dissipate as heat somewhere in the system. You won't get 1MHz audio from a consumer product because the speakers can't reproduce it.

Which is not to say that sound outside the range of human hearing doesn't exist or can't hurt you, just that generally speaking, a consumer radio won't reproduce such sound.

http://en.wikipedia.org/wiki/Sonic_weaponry

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