Just as an aside, has anyone heard the one about the chemistry professor who did the xenon-voice stunt (I don’t know if he actually used xenon or some other heavy inert gas)and apparently had trouble exhaling the gas. The teacher had to be turned upside-down by several of the students so that the gas would “flow” (my word) out.
This sounds suspiciously like an urban legend, but there’s no punchline, no irony…
Well, the irony is that the chemistry professor didn’t realize the full ramifications of inhaling a gas with a higher density than nitrogen. The punchline is he got turned topsy-turvey by his students. What more do you want?
Whoa, there! I thought the opposite was true. That helium is less dense than air I agree with, but wouldn’t that make the sound travel slower?
I offer three points for my logic:
If sound travels quicker the less dense the medium becomes, then by extension, sound should travel fastest in a vacuum. This is obviously not the case.
Ever notice how far away you can hear things on a foggy day? I’ve always understood that to be due to the fact that the air was very dense and therefore conducted sound well. (I realize this speaks to the quality of sound transmission in a medium, and not the speed of it, but I believe the two are linked.)
Imagine this experiment I saw performed in junior high school: placed on a table there are two rods of equal diameter and length (about five feet apiece), but of different materials (say steel and aluminum); the rods do not touch each other, but each abuts against a bowl of water. At the other end of each rod, there is a heavy ball suspended so that when one draws it back and releases it, the ball swings in an arc and strikes the end of the rod; the vibration created travels down the rod and when it reaches the bowl of water, causes the water to ripple. Now draw back the balls at the end of each rod and release them at the same time; I seem to recall that the bowl of water abutted to the denser rod always rippled first. Since sound is just the passing of a vibration of one molecule to the next – just as in this experiment – wouldn’t the same hold true for the speed of sound?
Do these three exaples show that sound travels faster the denser the medium??? Or am I just thoroughly confused?
Below is something that might help explain why the speed of sound decreases as the density of gas increases. You probably know this, but it bears repeating anyway, gas density increases as temperature decreases in accordance with the “ideal gas law.” I state this here because the linked article addresses your question from a different direction, but the conclusion is still valid.
Also, sound will not travel through a perfect vacuum since sound waves require some medium for propagation. A vacuum is obviously a special condition and must be considered independently of gasses of various densities.
Stark is right, and I was about to post the same thing, sound waves travel a lot faster in denser substances. For instance, the sound barrier’s speed is inversely proportional to the altitude. In the same manner, sound travels A LOT faster in water. But this does not mean that sound travels BETTER in denser substances. In fact, the denser the substance, the more the sound gets dampened. Well, it isn’t as simple as that, but it gives a rough idea. NASA uses water vapor to dampen the freak sound waves of the Space Shuttle when it lifts off. If they didn’t, the Shuttle would shake itself down to pieces…
I’m not sure it’s valid to compare gasses and liquids. While they are both considered fluids, the simple fact is while you can compress a gas, you cannot compress a liquid.
Sound waves travel at a constant speed, regardless of the loudness or softness of a sound. Temperature, however, does affect their speed. And rising temperatures reduce density according to the ideal gas law. At room temperature (70º F) sound travels in air at a speed of 1,129 feet a second. With each rise of one degree Fahrenheit, the speed increases by more than one foot a second.
Hydrogen has a very narrow temperature range where it assume a liquid state, about 11 degrees F. It does not chage states directly between solid and gas.
Are gasses merely an excited liquid? Umm, I s’pose that’s one way to describe them. I think rather than “excited”, I’d call them “warmer” or perhaps more “energetic.” You see, gasses can also have an “excited” state. Think plasma, described thusly: “When a gas is heated by many thousands of degrees, the individual atoms collide with enough violence to knock electrons free, resulting in a collection of positively charged ions and free, negatively charged electrons. The gas is said to be ionized, and when a sizable number of the atoms become ionized, the gas is called a plasma.”
Speed of sound at 20 degrees C:
Air 344m/s
Helium 927m/s
The problem is that you are discussing solids and liquids in this dicussion too, which really confuses the issue.
Here’s a formula:
Speed of sound = SQR(Bulk modulus/density)
The Bulk modulus describes an elastic property of the media. Liquids and solids are very noncompressible. They have bulk moduli that are in the order of billions of Pascals. Gases are quite compressible in comparison and they have much smaller bulk moduli (Air’s bulk modulus is only about 100,000 Pascals). As a result, the speed of sound tends to be much higher in liquids and solids than in gases.
Because gases have a much smaller bulk moduli, density plays a much larger role in determining the speed of sound.
When comparing air and helium, it is correct to say that the speed of sound is higher in helium because it is has a lower density (due to its smaller molecular weight). Extrapolating to compare the speed of sound in helium to the speed of sound in steel based solely on density isn’t accurate.
Since helium is less dense than air, the helium atoms have to be moving faster to achieve the same pressure. Faster atoms means faster sound in a gas.
As far as the OP about the professor having trouble exhaling the gas, I was listening to an NPR (That’s National Public Radio, here in the US, for all you furriners) segment where the host and guest inhaled first Helium, then some dense gas (Not xenon. I believe it was some sulfer compound). The helium effect on the voice disappeared pretty quickly, but the effect of the denser gas lasted for quite a bit longer. The guest attributed this to the higher density of the dense gas. This makes sense if you realize the molecules of the dense gas are moving more slowly, and therefore take longer to dissipate from the lungs.
Of course, the guest mentioned that they were using a pure grade of helium, so don’t try this at home, kids.
Just a clarification: Gasses with less massive particles will transmit sound faster. Gasses where the molecules are farther apart transmit it slower. If temperature and pressure are equal, the average spacing between helium atoms will be the same as the average spacing between oxygen or nitrogen molecules, so the only relevant factor is the molecular mass, and the speed is higher in He. If, on the other hand, you take N2/O2 air and decrease the density by increasing the intermolecular spacings, then the speed will be lower.
I’m not sure how dangerous this makes it, but xenon is an anesthetic. That’s in addition to it displacing the oxygen in the lungs. Any potential user out there should be leery of overusing it.
Not quite a UL. A few years ago, I watched the shows David Letterman did from San Francisco. A demonstrator from the Exploratorium science museum was on one show and demonstrated the apparent voice-lowering effects of inhaling sulfur hexafluoride. Immediately afterwards, he hung upside-down by his knees from a horizontal bar. He explained that he had to do this to get all of the gas out of his lungs. It dense enough that if he didn’t do this, a certain amount would remain in his lungs.
