Light, sound, and the elemtromagnetic spectrum

Factual Questions
1

Obviously, light and sound are not the same thing, but when considered solely in terms of frequency bands on the electromagnetic (EM) spectrum, how analogous are they?

My limited understanding of the basic difference between sound waves and most other types of EM waves is that while the latter are self-propagating, the former require a medium (such as air), and that it is the oscillation of the molecules comprising that medium that is in effect in what we call sound, and so sound does not exist in a vacuum. So help me out:

a) Assuming I could will myself into outer space, and further assuming I could survive and function in space without a spacesuit, what happens when I open my mouth and scream? Besides the fact that no one can hear me, and that no sound will be produced in the vacuum of space, what happens to the energy produced by my vocal chords? Could it be said that the wave still exists but quickly “dies” because it cannot propagate, or does the energy dissipate in some other way?

b) When people refer to the “voice band” of the EM spectrum, are they simply referring to the frequency range of the spectrum that also happens to be occupied by human-generated sound or is sound actually considered to be a special case of EM radiation? If you tweaked your radio transmitter to output waves at 440 HZ, would they be audible? And if they were, would the only difference between the sound produced by the radio waves and the sound produced by, say, striking a tuning fork, be the causative agent?

c) Along similar lines, if one had a hypothetical tuning fork that was capable of generating waves whose frequency was within the visible light spectrum, is it correct to say the waves would still not be visible because light is fundamentally different stuff? Inversely, imagine a hypothetical LED which could emit light at extraordinarily low frequencies–would that simply not be possible for light, or would it essentially be a repeat of the radio-transmitter scenario (and it just wouldn’t be called “light”)?

I know there must be a Cecil column on this but can’t find it…

2

[QUOTE=Sequent]
Obviously, light and sound are not the same thing, but when considered solely in terms of frequency bands on the electromagnetic (EM) spectrum, how analogous are they?
[/QUOTE]

While sound waves can be characterized by a frequency, they are not electromagnetic waves. Sound waves are compression and rarefaction waves of particles in a medium, while EM waves are transverse oscillations of electric and magnetic fields.

[QUOTE=Sequent]
My limited understanding of the basic difference between sound waves and most other types of EM waves is that while the latter are self-propagating, the former require a medium (such as air), and that it is the oscillation of the molecules comprising that medium that is in effect in what we call sound, and so sound does not exist in a vacuum.
[/QUOTE]

Again, you’re close, but you must remember that sound waves are not EM waves.

[QUOTE=Sequent]
a) Assuming I could will myself into outer space, and further assuming I could survive and function in space without a spacesuit, what happens when I open my mouth and scream? Besides the fact that no one can hear me, and that no sound will be produced in the vacuum of space, what happens to the energy produced by my vocal chords? Could it be said that the wave still exists but quickly “dies” because it cannot propagate, or does the energy dissipate in some other way?
[/QUOTE]

Assuming you took a nice big gulp of air before willing yourself into outer space, your vocal chords could make waves out of it as you screamed. Those waves would exit your mouth but could not propagate in the vacuum. So they’d just dissipate in space. One you had no more air in your lungs, you could no longer make sound. Even if you could set your chords a-humming without air to rush over them, they could not impart sound waves in the absence of a medium. A tuning fork in space would vibrate for a long time, but it would make no sound.

Your ears work by vibrating in sympathy to input compression waves in air, and do not respond to EM waves ('ceptin maybe x-rays or ionizing radiation, but not as ears qua ears).

Now you’ve got it. Light is fundamentally different stuff.

3

In fact, there’s plenty of EM emissions at frequencies that would be audible if they were audio. That 60Hz hum from a public address speaker or transformer buzz are electromagnetic emissions that get converted to audio that we couldn’t otherwise hear. They are present in many circuits and devices but we are only away of them in certain circumstances.

Audio signals are molecules bumping into each other. If you walk up to a table and nudge it with your hip, you’ve created an audio impulse. Shake the table back and forth and you have a periodic audio signal. You can’t really do anything like that in the electromagnetic spectrum. Unless you were on fire…

4

Don’t be confused that both light and sound are referred to in terms of waves and frequencies; it’s really apples and oranges.

Well an audio wave is just the pattern of motion of the particles, so without the particles the wave ceases to exist. Think of waves at the beach - once the wave runs out of water as it washes up onto the beach it becomes an ex-wave (ignoring the fact that the energy probably continues on a bit through the air molecules).

5

Oh, and if you were going through the motions of screaming in outer space, your vocal cords and other involved muscles would be doing less work (that is, producing less energy) in the first place, since no energy would be lost in the scream. But that’s a fairly small amount of energy, anyway: Much more energy will go into heat, and that’ll occur in vacuum as well.

6

[QUOTE=Chronos]
Oh, and if you were going through the motions of screaming in outer space, your vocal cords and other involved muscles would be doing less work (that is, producing less energy) in the first place, since no energy would be lost in the scream. But that’s a fairly small amount of energy, anyway: Much more energy will go into heat, and that’ll occur in vacuum as well.
[/QUOTE]
It seems to me that with the absence of air in the lungs, attempting to expel air thru the vocal chords would not result in their vibration.

7

[QUOTE=illoe]
Assuming you took a nice big gulp of air before willing yourself into outer space, your vocal chords could make waves out of it as you screamed.
[/QUOTE]

Ah, so…fatal hypothetical flaw if there ever were one: you can’t breathe in space, either, can you? :smack:

So then, to say whatever wave you hypothetically could produce vocally would “dissipate” in the sense of a wave gradually losing amplitude doesn’t really sound like it would fit the scream-in-space scenario, either. If I’m reading Chronos right, it would simply be converted immediately into heat? So rather than propagating at all, my vibrating vocal chords could maybe warm the few cubic centimeters of vacuum in my throat by maybe one or two billionths of a degree K?

OK, so that’s pretty much what I was thinking: light is just fundamentally different stuff. Hence the whole “can behave as a wave but isn’t really a wave,” at least as we think of them.

So the fact that there EM radiation exists at the frequencies corresponding to the “vocal band” (as well as the fact that we call it that) is really completely unrelated to sound and sound waves. And while EM activity can result in sound waves (should an appropriate medium exist), there’s essentially no difference between the mechanics behind that and my bumping into a table and making it squeak?

8

[QUOTE=Sequent]
So the fact that there EM radiation exists at the frequencies corresponding to the “vocal band” (as well as the fact that we call it that) is really completely unrelated to sound and sound waves. And while EM activity can result in sound waves (should an appropriate medium exist), there’s essentially no difference between the mechanics behind that and my bumping into a table and making it squeak?
[/QUOTE]

That’s right - the fact that the frequencies are similiar doesn’t mean that audible sound waves are related to EM waves. EM activity only results in sound waves when that activity is harnessed to make molecules move (e.g. a speaker cone pushing air).

In fact, there are applications where EM fields in the “audio” band are used, although the FCC doesn’t consider anything under 9kHz to be RF. For example, old avalanche beacons transmitted at 2275 Hz. But that doesn’t mean you could hear it - the transmitter was send out EM waves, not sound waves.

Conversely, ultrasound machines sound in the MHz range (although not through air), but that doesn’t mean a radio receiver nearby will pick up the signal.

9

…or, more clearly, whatever energy your vocal chords could produce in space (assuming you could make them vibrate without air) wouldn’t result in even a vanishingly short wave; the energy would be converted directly to heat?

10

[QUOTE=Sequent]
My limited understanding of the basic difference between sound waves and most other types of EM
[/quote]
It’s incorrect to say “most other types of EM.” Sound waves are longitudinal pressure waves in a medium, such as air, water, metal, while EM waves are fluctuations in the electric and magnetic fields. These fields are force fields. That is, an electric field exerts a force and an electric charge and a magnetic field exerts a force on a magnetic pole.

There can also be waves in the surface of water, in a vibrating string, or sheet and all kinds of waves. Most of them are motions of some physical material, except for EM waves.

I have a hunch that by “voice bands” you are speaking of are radio frequency (EM) bands that are used for voice communication such as amateur, police, intra corporate or other entities.

Once again. Sound waves and EM waves are entirely different physical phenomina, one is the motion of a material and the other is fluctuations in electric and magnetic fields.

If you transmitted a 440 Hz EM wave you couldn’t hear it unless it was somehow applied to a transducer. A transducer is a device that converts one physical phenomenon into another. Such as a microphone that converts sound waves into electrical currents or the inverse, a loud speaket that converts electrical currents into sound waves. So, if the 440 Hz EM wave impinged on something that moved in the air in response to the EM wave you would hear a 440 Hz sound wave. You would NOT, however, be hearing the EM wave.

Yes, yes. The exceedingly low frequency LED would really be an MPD (Micropulsation Device). Micropulsations are super low frequency fluctuations in the electric and magnetic fields of the earth. Google “micropulsations” and you’ll find all sorts of scholarly articles about them.

11

[QUOTE=David Simmons]
I have a hunch that by “voice bands” you are speaking of are radio frequency (EM) bands that are used for voice communication such as amateur, police, intra corporate or other entities.
[/QUOTE]

No, I actually was talking about RF at frequencies corresponding to that of audible sound (a frequency band which I’m aware is rarely if ever used in RF), but your next point still speaks to that.

I think one big misconception I had is that any vibration or wave within a certain frequency range will necessarily result in sound.

12

[QUOTE=Sequent]
I think one big misconception I had is that any vibration or wave within a certain frequency range will necessarily result in sound.
[/QUOTE]

…assuming the wave occurs in an appropriate medium. (I’d make for a crappy technical writer :rolleyes: )

13

[QUOTE=Sequent]
No, I actually was talking about RF at frequencies corresponding to that of audible sound (a frequency band which I’m aware is rarely if ever used in RF), but your next point still speaks to that.

I think one big misconception I had is that any vibration or wave within a certain frequency range will necessarily result in sound.
[/QUOTE]
Very low frequency transmitters (a few kHz) and receivers can be used to communicate with submerged submarines around the world. The communication is by the equivalent of Morse code.

On thing that can be confusing is that people have heard AM radio broadcasts via their teeth. Any non-linear device can detect amplitude modulated EM signals and produce sound. The fillings in teeth are non-linear conductors and if slightly loose can produce sound in response to excitation by EM waves.

14

[QUOTE=David Simmons]
So, if the 440 Hz EM wave impinged on something that moved in the air in response to the EM wave you would hear a 440 Hz sound wave. You would NOT, however, be hearing the EM wave.
[/QUOTE]

That makes sense, another conceptual misstep I think I’ve been making: confusing the transducer with the wave. You wouldn’t hear the EM wave, but you might hear a sound wave that resulted from its interaction with the air. The vocals chords are interacting with the air, so you’re not really hearing their movement but instead the result their movement has on the air.

15

[QUOTE=Sequent]

OK, so that’s pretty much what I was thinking: light is just fundamentally different stuff. Hence the whole “can behave as a wave but isn’t really a wave,” at least as we think of them.
[/QUOTE]

Well, light is a wave, and radio signals are waves. Just that the “stuff” that vibrates/oscillates/*waves * in an EM signal isn’'t “stuff.” It’s fields. The mathematics you would use to describe both kinds of waves are very similar, so you can speak of frequencies and wavelengths and all that for both sound and light, but the physical phenomenon are fundamentally distinct.

Here’s an example: The highest sound frequency our ears can detect is about 20,000 Hz (vibrations per second). Sound moves through air at around 350 m/s, so the wavelength (=speed/frequency) of that high-frequency sound wave is about 2 cm. If you could “freeze” the air through which that sound wave was travelling, you would see regions of high density air and low density air with about a 2 cm separation between two high density regions. As the high density regions hit your eardrum, your eardrum vibrates, and you hear that high frequency sound.

Now 20,000 Hz light (EM radiation), on the other hand, is a very low frequency. And since light speed is a heckuva lot faster than sound speed (300,000,000 m/s), that 20,000 Hz frequency corresponds to a very loooong wavelength – about 15 kilometers peak to peak. And for light, the peaks aren’t regions of high-density or compressed air, they are regions of high-amplitude electric fields. As the amplitude of the electric field changes, it generates a magnetic field, which in turn generates an electric field. Air particles are not necessary for these fields to exist (and in fact, can hamper or scatter their propagation) So light can propagate in a vacuum, while sound, which depends on the compression and rarefaction of a medium, cannot.

Finally, you’re right that the “voice band” (at least per Wikipedia") is just a name to identify a particular frequency range. It has nothing to do with whether you could hear light at that frequency, absent a transducer, as David Simmons (and you, I see, on preview) have pointed out.

16

[QUOTE=Sequent]
That makes sense, another conceptual misstep I think I’ve been making: confusing the transducer with the wave. You wouldn’t hear the EM wave, but you might hear a sound wave that resulted from its interaction with the air. The vocals chords are interacting with the air, so you’re not really hearing their movement but instead the result their movement has on the air.
[/QUOTE]
The possibility is remote. There would need to be sufficient isolated electrical charges or magnetic poles floating around in the air so as to bang into enough air particles with enough energy to result in a pressure wave producing audible sound. Sensitive sound detectors would probably have a residual noise signal that would exceed the level of the sound created by an EM wave.

The mean free path in air is very short, 68 nanometers at ordinary pressures. So the charges wouldn’t move very far before they bumped into another particle and the energy they could gain in such a short distance would be low.

With a high enough voltage in the electric field you might break down the air and hear a crackling noise and I suppose, again with a high enough voltage, there might be something akin to corona discharge on a power line. That’s a tad out of my specialty so maybe others can comment.

17

  1. The idea that light comprised vibrations in some medium was pretty popular until around 100 years ago. The medium was called the “ether” or “aether” (the “ae” being a ligature, which I don’t know how to type here).

  2. There are natural phenomena that are electromagnetic in nature but happen at frequencies within the audio range. With an antenna connected directly to headphones or to an amplifier to headphones, you hear them. The “spring chorus” is one such phenomenon, and “whistlers” are another.

  3. The Omega navigation system operates (operated?) at several frequencies near 11,000 Hz, and you can (could?) hear it this way too.

18

[QUOTE=Napier]
1)2)With an antenna connected directly to headphones or to an amplifier to headphones, you hear them. The “spring chorus” is one such phenomenon, and “whistlers” are another.

  1. The Omega navigation system operates (operated?) at several frequencies near 11,000 Hz, and you can (could?) hear it this way too.
    [/QUOTE]
    Er, ah, um, you need a detector, a diode or other nonlinear element, in there someshere too.
19

[QUOTE=David Simmons]
Er, ah, um, you need a detector, a diode or other nonlinear element, in there someshere too.
[/QUOTE]
Am I allowed to retract this claim? If so, I do.

20

>Er, ah, um, you need a detector, a diode or other nonlinear element, in there someshere too.

Er, ah, um, no you’re not. You can’t even spell. Now the whole Internet knows, too. Shame, shame.

On the other hand, why not? Sure, go ahead.