Physics Puzzle: No Sound in a Vacuum?
June 9th, 2009
| Categories: Fermi Questions, Music and Audio, Physics Puzzles, Thermodynamics
Sound waves can’t travel in a vacuum; they need a medium, air, to propagate. In introductory physics classes the following demonstration is sometimes given to “prove” this: a ringing bell is placed inside a sealed jar, out of which the air is slowly sucked. As the air is sucked out of the jar the bell’s ringing diminishes and finally fades out completely.
Does this indeed demonstrate what it has set out to do?
EDITED, 14/June/2009: a solution has been posted here.
Sounds like a trick question!
I hesitantly say yes. Demonstrates, but not terribly well. Significantly increasing the pressure of the jar would have a similar effect; there are other factors at play. The pitch of the bell is going to depend on the density of the medium (think how your voice changes when inhaling helium). The resonating bell is going to have difficulty adjusting to the difference in natural pitch mid-ring, and much of its energy will be lost. As usual, I do not have the relevant equation handy, and my explanation is lacking in depth.
A device that strikes the bell repeatedly or, better yet, a function generator outputting to a speaker would make a much better demonstration.
Ok, I’ll grant you a function generator, plus the freedom to tune it to whatever frequency you want prior to the experiment :). Do you feel more comfortable saying yes now?
No, now I’m scared.
Sorry about that, didn’t mean to intimidate you :).
Your intuition is correct, namely, that there are two forces to be reckoned with here: (1) the transmission of sound (or lack of it) due to decreased density, and (2) the amount of sound energy reflected back into the jar due to the difference in density. The name of the game is now estimating which would win out first.
Regarding the helium comment, it is a common misconception that the change in pitch results from a change in density. When you think about it, mountain climbers don’t sound like Mickey Mouse up in the thin air, so the density of the medium is not the parameter of interest. The culprit is the light molecular weight of Helium, which leads to the speed of sound being faster (by the way, the speed of sound doesn’t change when the density is changed).