Experiment 11
The Speed of Sound

General Introduction

In case you are a little more interested in some of the "why"s of this lab (thank you, thank you, I love it when people are curious about things), read the "Theoretical Ramblings" section (from the link just below). If not, that's fine; by now you should have read my thoughts on what the purpose of these labs is. In any case, it might be a good idea to take a look at the sections on what's important and what things to watch out for; both from a theoretical point of view and from a "how do we get this done right/quickly/at-all" point of view, they should hopefully help.

Theoretical Ramblings (feel free to skip this)

What's Important in This Lab

• There is a relationship between the wavelength of a wave, its frequency, and the speed with which it moves.
• The speed of propagation (fancy word for "motion") doesn't depend on how strong the wave is (to an approximation, of course; see the theory section), but only on the type of material. In other words, one of the characteristics of a substance is what speed waves of various types go through it. Air has one characteristic speed for sound waves, and aluminium has another.
• When you come to an interface (see the theory section for more detailed info about interfaces and reflections and the like), or rather when a wave comes to an interface, its wavelength and speed may change and probably will, but not its frequency. (Why this is, is one of the post-lab questions...)
• You definitely don't want to get a job as an aluminium-rod-puller. (If you enjoyed that section of this lab then you should seek professional help or go to the circus more often.)
• By varying the length of a tube (or alternately, by putting varying holes in the side to disturb the vibrations in a particular way), you can vary the resonant tones that make the tube vibrate. That's the basic idea behind all the wind instruments, and pipe organs and the like.

Hints and Tips for making this lab a better experience

• Make sure to record the uncertainty associated with each type of measurement, along with some note about what gave rise to that uncertainty. You'll need it for doing the data analysis, and you need it to be complete...

Part I (of the lab)

• Strike the tuning forks with the rubber mallet, or on your thigh or something relatively soft. Not that you'll hurt the tuning forks or anything, but if you hit them on something hard they'll have all sorts of annoying high-frequency harmonics that sound really bad and interfere. You may have heard these high squeals when Douglass hit the tuning fork on the wooden railing in the lab lecture.
• Hold the tuning fork as close as possible to the end of the tube, and oriented so that a line from one tip of the fork to the other tip goes down the length of the tube. This way, the vibrations in the tube are stronger and truer. (Note that this is true, but is a very small effect for reasons that I didn't want to stop the lab lecture to go into, but if you're interested then ask me.)
• Hold the tuning fork only at/near the base, and if you can, try to hold it with just two fingers and as loosely as possible, since then the vibrations will last as long as possible. And don't worry about re-striking the tuning fork several times while listening for antinodes; in fact, keeping the fork nice and loud will help.
• Try to pick a tuning fork that's a different frequency from the one used by the lab team right next to you. Ever notice that adjacent checkout registers at the supermarket have different-pitch beeps? That helps the cashiers identify their own machine's beeps and warbles from the general confusion.
• Make sure to record the frequency of the tuning forks! Most of the ones in the lab seem to be tuned to the (ghastly; ask me for details) C-512 scaling, but there may be a few A-440-standard ones floating around too, so don't take it for granted which frequencies yours are.

Part II

• Take the time and care to make sure the metal rod is clamped as close to the center as you can get it. This is pretty important, since if it's off-center then you won't get standing waves at all, and your dust will just tend to sit there.
• Also, make sure the metal disc doesn't touch the tube -- should be obvious why not.
• Don't use too much cork dust; just a little line of it is fine. If there's too much in there, not enough of it will move to give you anything to measure!
• This part is just really annoying and difficult. You have to get the cloth really wet (as in, soaking) with the alcohol before the frictional effect starts to work well at all. You may want to bring some hand lotion along for afterward if your skin is sensitive.
• As you pull along the rod, you may hear various unpleasant squeals. These are not what you want; they're just the cloth binding then slipping on the rod. Get the cloth wetter. You'll know it when you hear the right tone; it makes the other squeals seem nice.
• You'll have to pull the rod fairly strongly; don't expect to just rub or "caress" it and get anything out at all.
• The lab manual says that the dust should move along the tube, to form in little piles/clumps at the nodes. That's all well and good if yours does that, but more likely it won't move enough to get good readings that way. Here's my suggestion:
  • Put your line of dust, and try to make it straight along the bottom of the tube. Then turn the tube a little in its frame, being careful not to disturb where it is relative to the rod and disc. This way, the cork dust is now a few degrees up a slope from the bottom of the tube. The vibrations will indeed tend to push the dust along the tube a little bit, but they'll also just jiggle the dust enough to overcome the natural friction, and so the dust that gets jiggled more will tend to move perpendicular to the tube, down toward the bottom. So you should end up with what looks much like a sine wave, and that can help you figure out where the nodes and antinodes are.

Things to Watch Out For (safety-type issues)

• Not much to seriously worry about; this is a pretty safe lab.
• Don't try to eat the cork dust or anything idiotic like that; I don't think you'd get sick, but I'm sure it couldn't taste good.
• Make sure nobody's behind you before you start pulling on the rod, in case your grip slips.
• Make sure your lab partner holds the base steady (but doesn't hold or touch the tube itself) while you're pulling on the rod.

Things to Do for Fun that Aren't related directly to the lab

• Strike the tuning fork(s) strongly and press the bottom end of the handle/base against your forehead. It's neat -- the sound seems to come from everywhere at once, since you hear it through bone conduction only so your ears can't get a direction fix.