RF vs. audio of the same frequency
RF vs. audio of the same frequency
From an article by an engineer at Cisco Systems:
An RF signal can have the same frequency as a sound wave, and most people can hear a 5 kHz audio tone. No one can hear a 5 kHz RF signal.
Why not?
4 Answers
4
The audio tone is compression waves traveling through air that your ears can pick up. The RF signal is waves in the electromagnetic field that you ears have no way of picking up.
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Powerful ELF-EMF might rattle your fillings.
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– amI
Sep 14 '18 at 21:41
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@aml Extremely Low Frequency to Extremely Medium Frequency? :P
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– immibis
Sep 17 '18 at 3:31
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The only e.m. signals that we can receive are in the optical spectrum (through our eyes)
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– Gianluca Conte
Sep 17 '18 at 8:07
RF signals are electromagnetic (EM) waves. We do not have any sensors for 5 kHz EM waves.
We do have EM sensors though, our eyes. They can sense EM waves from $4×10^14$ Hz (red light) to $8×10^14$ Hz (violet light). If strong enough we can also feel infrared radiation as heat.
We can also feel (as heat) powerful EM radiation at lower frequencies, but if you feel that then the field is dangerously strong and you should step out of that (radar) beam.
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I think this reminder that we can sense EM with our eyes, helped clarify it for me. Also, that RF can be sensed by us as heat - when testing RF circuits at work we must take care not to make contact or we can be burned by the RF.
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– user167734
Sep 15 '18 at 9:33
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we must take care not to make contact Not only that. Think of how a microwave oven works, it uses EM signals (around 2.5 GHz) and it can heat up water. Some (military) radars use similarly powerful signals but then these are not contained safely inside a shielded cage. If you happen to stand in front of such a transmitter, you will be "cooked" just like food in a microwave. That was btw how the inventor of the microwave oven thought of the idea, he stood too close to some RF transmitter and noticed things got hot.
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– Bimpelrekkie
Sep 15 '18 at 15:10
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I can feel powerful EM radiation from the sun most days. I think you might be overstating the danger. +1 for the sensors.
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– craq
Sep 15 '18 at 20:29
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Although IR being much higher freq has a low skin depth, it is caterogenic and our eyes are much more sensitive to UHF. However far IR 5~25um is well known to have healing properties other than the eyes.
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– Sunnyskyguy EE75
Sep 16 '18 at 6:25
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But wouldn't an RF signal at 5K produce sound if input to a speaker or resonant chamber capable of handling its energy? If "handling its energy" means, converting it to sounds then yes. But then you're simply converting EM waves into sound waves. You're still not hearing the EM waves, you're hearing soundwaves.
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– Bimpelrekkie
Sep 17 '18 at 16:08
Our body is a dielectric (insulator) with salts (conductive ions) so, although we cannot detect EM waves, the absorption of electric fields is generally proportional to the frequency.
Conversely, electric fields can be tolerated with increased levels as the frequency is reduced.
Example bass woofer audio at 60 Hz with 100 mV into the speaker coil is loud enough to be clearly heard and 100 Vpp might rattle something on the walls.
While a 100 V/m 50 or 60 Hz electric field does nothing to us as not only are we tiny compared to the wavelength in xx km the impedance of our 100 pF fingertip is about 50 MΩ, but the salt and an arc can reduce a wire contact to 50 kΩ easily.
You can easily detect 50~100 Vpp just by touching a 10:1 scope probe without touching the earth ground, which then shunts the electric field to ground.
This means we can conduct it easily, but not absorb it as a high impedance electric field. We are low impedance as a dielectric but as an antenna impedance of our body is inversely proportional to the super long EM wavelength of line frequency at the speed of light so it can be detected by a 10M scope probe but not absorbed.
Sound pressures on the other hand in the air are pressure waves and are easily detected by the cilia hairs in our ears, which have progressive different lengths acting as resonators. Below 20 Hz we generally feel the vibrations more than hear them.
Both RF impedances then reduces with increasing surface area into capacitors below antenna wavelengths, but in effect, we act as a weak coupling capacitor to low frequency so there is no energy absorption. It just passed through us. At higher radio and TV frequency at the sub-millivolt signal levels, we can act as an antenna without the sensation except for possibly better reception. However, our energy SAR absorption acceptable rate is a function of frequency and watts/cm3 for a given volume of flesh with a certain "skin depth".
Back in the 1970s our company designed and made 50 W and 100 W VHF and UHF transmitters. Even with the lid open for fine-tuning, and some low stray leakage, the tech's eyes would get bloodshot after a day's work on the production line. So the lid was redesigned with a tuning hole for a plastic screwdriver.
We had all the US military handbooks in our library for aerospace design, so after graduation in the late 1970s, this is how I first learned about human susceptibility to RF spectrum levels.
My first design project there as a young graduate was for a five-channel Doppler tracking Rx using US Navy transmitters around the western hemisphere with a Tx power about 1 megawatt suitable for 100 baud submarine communication all using carriers synchronized like GPS using nuclear clocks (Cesium). All I used was a 2 m (polar bear proof) whip antenna in the Beaufort Sea on an ice flow to track weather and ice movement in the 1970s.
This is an interesting question because I used to wonder the same thing (no, I'm saying it's an interesting question because of my former curiosity).
You're confusing electromagnetic radiation (something radio produces) with pressure waves (something sound produces). Our ears cannot adjust to electromagnetic waves and they are certain not sensitive to changes in electromagnetic waves.
Another way to look at it is that electromagnetic waves don't have nearly enough force to cause the ear drum to vibrate... whereas sound waves do.
If you want to get on a very quantum level about this, think about how strong gluons are.
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You can hear RF signals indirectly, though, when they cause mechanical vibrations.
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– 200_success
Sep 16 '18 at 2:32