Comment by Terretta | Hacker News Reader

Comment by Terretta | Hacker News Reader

I don’t understand the “Electromagnetic Spectrum Infographic” that purports to illustrate a continuum from sound to light where what’s changing is frequency, and says “radio spectrum” has “perceptible sounds”.

Which ones of these things are not like the other ones?

Conventionally, sound waves are a vibration traveling through an object and cannot travel through a vacuum, while light is* a wave of oscillating electromagnetic fields that can travel fine through a vacuum.

In more detail: https://socratic.org/questions/how-are-sound-waves-different...

OK, there’s this on “Sensation of Hearing in Electromagnetic Fields” but not at these frequencies:

“The "hearing" of electromagnetic waves is an established fact. It appears that this takes place by direct stimulation of the nervous system, perhaps in the brain, thus bypassing the ear and much of the associated hearing system. It is a possible, perhaps the most probable, explanation of the reports of hearing meteors and auroras” — https://www.bibliotecapleyades.net/scalar_tech/the_hum/ingal...

So, not so much “hear” as “perceive” perhaps. But I doubt this is what the graphic is suggesting either.

I fear grade school kids reading this PopSci issue could be mislead for a while about why they can hear the radio.

* for interesting values of “is”

To be clear: there is NOTHING at all about sound in this article. The spectrum does not show a continuum from sound to light. It shows the electromagnetic spectrum, which does not contain any sound waves at all. One can encode acoustic information on an EM wave (that is how radios work), but the transmitted signal (from the radio tower to your radio receiver) is an EM wave, not a sound wave. The receiver converts the EM wave to the appropriate sound wave by decoding the information that was encoded into the EM wave, and using that information to drive a speaker.

Sound waves are pressure variations in the air. Thus they require air in order to propagate.

Electromagnetic waves are oscillations of an electric field (and also a magnetic field). These fields exist independent of air (or any other medium), so they can propagate in completely empty space.

These are two completely distinct phenomena. They both involve waves, but that's about all they have in common.

The diagram of the EM spectrum at the top has a very confusing section on the left where ranges of frequencies are labeled 'infrasound', 'perceptible sound' and 'ultrasound'. They have nothing to do with the ability of the corresponding radio waves to be modulated to transmit sound signals, but are literally just marking off the frequencies that correspond to those ranges in sound waves, over a diagram of EM frequencies.

Yea, that's confusing, despite not being wrong. It is a juxtaposition of two completely distinct phenomena on one graphic, which is confusing, for sure. But the whole article is about EM waves, not sound waves.

Sigh.

I didn't discuss the article. I believe I scoped my comment to the "infographic" in the first line, and closed with "But I doubt this is what the graphic is suggesting either."

I also shared the same information as your 2nd (aside: sound doesn't require air, as bone conduction headphones or scuba diving demonstrate, does require "not vacuum") and 3rd paragraph, and made the same point as in your close.

So I agree with you, which is why I objected to the graphic.

> To be clear: there is NOTHING at all about sound in this article. The spectrum does not show a continuum from sound to light.

Except that a portion of the spectrum graphic is labeled “perceptible sound.” That’s what the OP is referencing.

Perhaps the illustrator is pointing out that the frequency range of these EM waves coincides with frequency range of audible sounds? Or perhaps they’re confused. Either way, there’s no mistaking that the graphic is labeled with the words “perceptible sound.”

True, but the article is all about electromagnetism, not acoustics. I'm sure the author of the article was not the same person as the person who made the graphics. They probably ought to have communicated with each other a bit more clearly.

I mean, it's popsci.com...what did you expect? /s

Yeah, the article confuses a lot of physics and doesn't make sense from a wavelength standpoint. Of course, light has similar properties than electromagnetic waves, but sound definitely doesn't have anything to do other than it can be described with wavelengths.

If you approach the terahertz spectrum, however, resonance is getting a little complicated because it can hit the natural vibrations of molecules. I guess that's what the author was trying to write about in terms of potential medical/scanning applications.

As moving molecules physically in order to create terahertz frequencies seems unfeasible (as of now, u know, because lack of vacuum to make it possible), the only possibility is to generate electromagnetic frequencies and/or light based emissions.

Electromagnetic frequencies for transmission on that scale get complicated real fast though, due to interference and basically everything making it worse, from humidity to infrared waves until basically all longer molecules.

Finding a molecule to use this as a transmission medium in something like a chip as a circuit is a whole other question.

Overall I think the article tries to make "this is ohmagerd" point without understanding the basic physical interactions, laws and properties.

Light IS an electromagnetic wave, so to say it has similar properties to EM waves is kind of like saying that pizza has similar properties to food.

Also not sure what you mean by "needs a vacuum to make it possible" seeing as we generate terahertz radiation in air all the time, using a variety of techniques that use off-the-shelf commercial products.

As far as I can tell, the article doesn't say very much that is outright false, although for sure he glosses over some fairly complex concepts with very little detail in a few cases. His statement about "a few dozen meters" of propagation range is outright false, but other than that he more or less got the facts right.

Touche, you're right. I should've worded that differently in the moment.

I was trying to come from the perspective of the author, where he tries to argue about difference of appliances through technological means. E.g. using a laser for communication vs using a wifi like transmitter setup.

Both are EM waves but the practicality of the environment with degradation and radiation and the medium that are common for us (gases in the air) make the approach of how to reach the target vibration (in the sense of targeting a specific antenna length on a specific wavelength) harder when trying to use the sub infrared spectrum.

As I wanted to say, I think finding the right material and medium will be a huge challenge in this regard.

Well, actually it's much easier at lower frequencies. After all, your cell phone operates at around 2.5 GHz (if it is a 4G phone).

But yes, there are huge challenges involved in using higher frequencies for communications. We will eventually use 120 GHz, and then probably 290 GHz, for comms, but it will be a while. The technical challenges are... not trivial.

You might be interested to know that the Japanese television broadcasts of the 2008 Olympic games in Beijing made use of a wireless link operating at 120 GHz. It was really just a demonstration of feasibility, not a fully deployed system. But still, pretty fantastic. And that was 14 years ago...

From what I heard at ESA projects (not being directly involved) the L-band seems to be able to penetrate clouds and static charges in the atmosphere, whereas higher frequencies like the Ku-Band (12-18Ghz) tend to collide too much with rainy clouds and droplets in the air.

That's why I was assuming that higher frequencies than the Ku Band will likely lead to more signal degradation on the way.

I also don't have any calculations or plots in my head for the minima and maxima, so I could be totally wrong about this :D

120Ghz is quite amazing as an achievement though. They probably used a wider band and multiple channels, I would assume?

edit: looked up a little on K (18-27Ghz) and Ka band (26.5-40Ghz), but both seem to be still unfeasible for long range communications.

> looked up a little on K (18-27Ghz) and Ka band (26.5-40Ghz), but both seem to be still unfeasible for long range communications

umm, Ka band is rather popular for satellite communications (very long range by most standards!). Yes things like L band are great for propagation and penetration, but the fundamental motivation for going to higher frequencies is that they can carry more data. This is the same reason you don't get music on AM radio--there's not enough bandwidth for good sound quality.

The atmospheric transmission's dependence on frequency is complicated. There are water vapor resonances above 100 GHz which one would want to avoid. There is frequency-dependent scattering from droplets, and there is a continuum background from water vapor dimers that rises with frequency. It is a very non-trivial situation. Having said that... the 2008 Olympics demonstration had a broadcast range of about 1 km, and people have demonstrated ranges up to several km at higher frequencies, even close to 300 GHz. So although it will never be as good as the range one can obtain at lower frequencies, there's a lot one can do.

Big yikes on that second link. Everything there should be taken with a whole dump truck of salt.

I was told by media Russia’s already putting it to use!

https://foreignpolicy.com/2020/12/05/us-diplomats-havana-syn...

https://www.foxnews.com/world/us-officials-were-targeted-acr...

Sonic attacks? Radio waves? Either way, gives ‘em a headache.

Or more likely: People trained to be paranoid being paranoid.

Yeah if you go on the home page of that article you get this:

https://www.bibliotecapleyades.net/esp_gaia.htm#Additional_I...

Its something about the Gaia theory, which makes even the electromagnetic hearing stuff look sane in comparison.

Credible sources and studies on "The Hum" have different conclusions than what the second link is purporting to be true. Some of them are cited on Wikipedia's entry for "The Hum"[1].

[1] https://en.wikipedia.org/wiki/The_Hum

> different conclusions

Those seem to be wildly different phenomena. The “hearing radar” mentioned frequencies of 1, 3, and 10 GHz(!?!?), while “The Hum” is between 32 Hz and 80 Hz, modulated from 0.5 to 2 Hz.

My including the link was not suggesting the thing reported was true, rather, it was the only thing I quickly found suggesting anyone hearing electromagnetic frequencies of any kind.

In the hum, most thinking is either physical noise, or resonance. That said, one paper linked from Wikipedia does have this:

“Analysis of the largely anecdotal data that are available at the present time suggests that the most probable explanation is that some people have the capability to interpret radio transmissions at certain wavelengths as sound. It is well established in the scientific literature that people can hear electromagnetic energy at certain frequencies and peak power levels.” — https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.51...

However, while this could validate both phenomena as being perceivable by some, I do not think this is why Pop-Sci made their chart as they did. I stand by my objection.

Yes, that electromagnetic spectrum chart is totally weird. They go directly from microwaves to ultrasound. Based on wavelength? No mention of the RF spectrum from UHF, VHF, HF, LF, VLF....

There are lots of uses for terahertz waves, if they become cheap to generate and sense. Lots of things could use high-definition short-range radar. Right now, experimental radars are pushing 300GHz.[1] Most commercial stuff is below 100GHz.

[1] https://elva-1.com/products/a40098

It's just very convenient to convert some time-frequency domain to sound, even if frequency is shifted for easier hearing (e.g. how they slow down black hole mergers).

That's because human ear is very good at hearing small changes in frequencies. E.g. first movies vere silent not because they couldn't play the audio, but because they couldn't synchronize between the video and audio.

Human eye is very forgiving to uneven or hickking-up video playback, but not the ear.

Actually, here's a fun fact: the audio "chirp" that you have heard in association with black hole mergers is not slowed down. The measurement is a gravitational wave, of course, not a sound wave. But when it is played as a sound wave, that's real speed, not slowed. This is actually mostly determined by the frequencies at which the gravitational wave detectors (LIGO and VIRGO) are sensitive, which happen to coincide with acoustic frequencies that your ears can hear.

The low frequencies carry less data so perhaps they mean that the low frequencies cannot be used to transmit perceptible sound.

No that's not it. There is NOTHING about sound in that article. It is ALL about electromagnetic waves, which are NOT sound waves.

Sound waves are pressure variations in the air. Electromagnetic waves do not require air. They are oscillations of an electric field, which can propagate in empty space. Completely different wave phenomena.

The article has nothing at all to do with sound.

Of course, you can send acoustic information on an electromagnetic wave. That's how radio works. But it's not the sound wave that is transmitted by the radio station - it is the EM wave, with information about the sound wave encoded into it.

I presume most people here are aware that sound waves are different from electromagnetic waves.

To transmit audio you have to encode it into electromagnetic radiation, transmit it and then decode it to reconstruct the audio.

You might have noticed that the audio transmitted on lower frequencies or longer wavelength sounds worse. AM (medium wave) sounds pretty bad, long wave sounds even worse. Even lower and you can’t make out speech anymore, it’s no longer possible to usefully transmit sound.

That question of the sound quality is actually a question of the bandwidth used to encode the transmission. It is naturally easier to access a broader bandwidth if the frequency is higher. But there is a point of diminishing returns. You don't get improved audio quality if you use more bandwidth than the original acoustic signal requires. So, yes, AM is not as good as FM... but a THz signal would be no better than an FM signal in transmitting an acoustic signal.

Am, modulated at 10 to 20Khz would sound GREAT! We just don't do that. Typical bandwidth is 5 to 8Khz these days. Earlier AM broadcasts were wider, out to 10+Khz.

I had an AM Stereo modulator for a while that did 10Khz and on the better radios was quite good. Commercial AM, with a few rare exceptions, isn't a good representation of what one can get at those frequencies. Nor are the radios sold today. They are frankly, terrible!! Get any older AM radio, 80's era and older and it's quite the difference.

Shortwave is modulated at about 3.5Khz, and is enough for speech and the gist of other material.

Noise is the primary reason we do not use those wavelengths. Power requirements go up too, if the signal is to reach max distance. There is a compromise in play, bandwidth vs propagation.

Finally, we also use an emphasis curve where higher order frequencies pack a real punch, with lower ones getting a less due to how sensitive we are to noise at various audio frequencies. Higher pitch noise stands right out, with our peak sensitivity around 3.5Khz.

We can, if desired, transmit a pretty great audio signal at the Khz frequency range. We just don't, because a narrow bandwidth is a better use of the spectrum.

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