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October 25, 2005 8:58 PM   Subscribe

The Acoustics of Gases. The high-pitched version of your voice that you hear when you inhale helium represents a cool principle. Unlike the speed of light, the speed of sound is quite sensitive to the composition of the medium that it travels through. A gas such as helium that is much less dense than the air we normally breathe and hear in will actually cause the speed of sound to increase, a phenomenon that we perceive as a Chipmunks-like change in pitch. A couple of scientists have used this relationship between sound frequency and gas composition to invent an acoustic device for monitoring the gas composition of air. Of course , if you're a canary this is nothing new.
posted by derangedlarid (23 comments total)

 
Nitrous oxide makes your voice deeper, and everything funnier.
posted by Balisong at 9:00 PM on October 25, 2005


"Unlike the speed of light, the speed of sound is quite sensitive to the composition of the medium that it travels through."

Um...you might want to brush up on your physics, there. :)
posted by Ethereal Bligh at 9:07 PM on October 25, 2005



posted by Heywood Mogroot at 9:29 PM on October 25, 2005


Hey, isn't that the original jacket cover illustration on Newton's Optiks? I love that book, especially the chapter "Any Colour You Like". That rocks!
posted by Ethereal Bligh at 9:47 PM on October 25, 2005


The speed of light varies depending on the medium in which it propagates; this gives rise to refraction in optical materials. Different frequency components of light slow down (refract) differently, as shown in the prism graphic above. In some exotic media, light has even been slowed to a crawl in some recent experiments.
posted by rcarr at 10:04 PM on October 25, 2005


Also in fiction: See "Slow Glass" by Bob Shaw.
posted by Justinian at 11:39 PM on October 25, 2005


Justinian
The story you're thinking of is "Light of Other Days" by Shaw.
posted by Sangermaine at 12:01 AM on October 26, 2005


While we're picking on the physics, I'd like to pick on the linked article: the speed of sound in a gas isn't really dependent on the density. What really matters is the molecular structure of a gas, which determines its adiabatic index. Monatomic gases like helium have a higher speed of sound than diatomic gases like oxygen and nitrogen. The speed of sound in hydrogen is same as in air, because they are both diatomic gases, even though the hydrogen gas is about 14 times less dense.

Despite the linked article's claim, I think this device can really only tell you the number of atoms in each molecule of the gas it's analyzing, not the molecular weight.
posted by yarmond at 12:15 AM on October 26, 2005


Actually, I take that back. I forgot that the speed of sound depends on the gas-specific gas constant, not just the universal (molar) gas constant. The molecular weight of the gas does influence the speed of sound, and hydrogen will have a different speed of sound than air. That said, the effect of the adiabatic index is still important, so there will still be some cases where you can't tell one gas from another.
posted by yarmond at 12:31 AM on October 26, 2005




The behavior of light in a prism in no way limits our understanding to Newtonian ideas.

It is worth considering why it is that light is never at rest. This is partly due to the fact that unlike an acoustic pressure wave in a material substance where the material substance determines the isotropic frame surrounding the sound wave, for light no such definite material medium exists.
So, I stand by my misbegotten statement.

yarmond, thank you for commenting on the actual post. Your criticisms are interesting, and in light of your revised opinion, the strange device may have a purpose afterall.
posted by derangedlarid at 1:33 AM on October 26, 2005


Hey, isn't that the original jacket cover illustration on Newton's Optiks? I love that book, especially the chapter "Any Colour You Like". That rocks!

Don't you mean the original album cover to the album Dark Side of the Moon by Pink Floyd?
posted by alexst at 1:53 AM on October 26, 2005


"The behavior of light in a prism in no way limits our understanding to Newtonian ideas."

Who said that it did?

"...for light no such definite material medium exists.
So, I stand by my misbegotten statement."


Your statement is still wrong. If you're going to compare a light wave with a pressure wave, then the context is the speed of propogation through a medium. In both cases, the speed varies by medium. However, it is simply misleading to compare a light wave with a pressure wave in the first place for the very reason that you say: there is no aether. A light wave is not a pressure wave.

"Don't you mean the original album cover to the album Dark Side of the Moon by Pink Floyd?"

Ah, no one gets my humor.
posted by Ethereal Bligh at 3:09 AM on October 26, 2005


Hmm. Also:

"It is worth considering why it is that light is never at rest. This is partly due to the fact that unlike an acoustic pressure wave in a material substance where the material substance determines the isotropic frame surrounding the sound wave, for light no such definite material medium exists."

Light is "never at rest" because there is no aether? No. That's simply not true. The two things are unrelated.
posted by Ethereal Bligh at 3:17 AM on October 26, 2005


Don't you mean the original album cover to the album Dark Side of the Moon by Pink Floyd?
posted by alexst at 1:53 AM PST on October 26 [!]


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posted by intermod at 5:25 AM on October 26, 2005


On the subject of Shaw - I think he wrote more than one story involving slow glass (or maybe expanded one into a novel or something).
posted by edd at 5:38 AM on October 26, 2005


thanks for the clarification, ethereal bligh, i see your point.
posted by derangedlarid at 8:55 AM on October 26, 2005


... will actually cause the speed of sound to increase, a phenomenon that we perceive as a Chipmunks-like change in pitch...

OK, for the record, it's not that sound travelling faster sounds higher-pitched. It's that a change in the speed of sound changes the resonant frequency of your vocal chords, and THAT makes your voice higher-pitched. That's buried in the linked article, but the FPP kind of glossed over the mechanism.
posted by rkent at 8:56 AM on October 26, 2005


Sounds to me like this is all just another brick in the wall.

I'd go on, but I'm on the run . . . is that my alarm clock ticking.
posted by tzelig at 8:59 AM on October 26, 2005


thanks rkent: Helium is lighter than air, so your vocal cords can vibrate faster, so your voice sounds higher-pitched.
posted by Lord Kinbote at 9:32 AM on October 26, 2005


I learned all I ever wanted to know about the acoustic properties of propelled gases from my Dad. You could hear him let one fly from the other end of the house.
posted by Ber at 11:29 AM on October 26, 2005


I was under the impression, and I'm probably wrong, that refraction was caused because light did not change it's speed in different mediums. It changes it's path so that it doesn't slow down.

How wrong am I?
posted by lumpenprole at 12:28 PM on October 26, 2005


lumpenprole: pretty wrong. You're probably a bit confused by the fact that light always takes the quickest route from A to B. However, if it is made to slow down by entering a medium it may decide to take the quickest route from A to C and not bother with B at all (which is your refraction).
posted by edd at 1:51 PM on October 26, 2005


I remember an experiment in my high school physics class where our teacher took us out to some adjacent railroad tracks. He walked about 1/4 mile down the tracks while we were to put our ears on the track itself.
He hit the track with a hammer. We heard it through the rail about a second before we heard it through the air.

This is because the iron in a RR rail is denser than air, and thus vibrations travel faster through them.

Light doesn't follow those rules, tho.. Or do they?
posted by Balisong at 7:49 PM on October 26, 2005


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