Science Buddies: "Ask an Expert"

Last year, I did a project entitled: The Science of Sound; Temperature and Frequency. For this experiment, I changed the temperature of water in a wine glass and struck it with a pendulum, recording the pitch that was created. I then used Audacity to measure the frequency. At any rate, this year I'm going to do a project in which I keep the independent variable the same (temperature) and change the dependent variable to thr amplitude of the transmitted sound waves, again using a wine glass. I'm just not quite sure how to do that. I first thought of coating the glass with a coffee filter and using grape juice as my liquid (I could then see how high the wave got), but then someone brought up the fact that the coffee filter would absorb the juice automatically. I then thought of using a vertically-placed popscicle stick or dowell in the glass, since it's more dense and wouldn't absorb the juice as much. Do you have any other ideas? Also, should I consider this a continuation, or a whole new project?

Thanks,
hurleyan

Hello hurleyan,

What an unusual idea? Did you see much change of frequency?

You were probably able to measure a small change in frequency since you used a microphone to convert the vibration to an electrical signal and then used a computer to measure the frequency. A visual estimate of the surface wave height is likely to be more difficult. Suppose the wave height changed as much as the frequency. How much would the wave height change? Could you see that small change?

Also, you will be looking at the wave on the surface of the glass. It will be affected by the diameter of the glass and other geometric factors. I would not say that this clever experiment won't work, but it will be tough. Here are a couple of idea to simplify or improve it.

• Use an phonograph pickup cartridge to sens the wave motion. Get a stereo cartridge and hook it up to measure vertical motion (using the sum or difference of the two channels). Place a styrofoam disc on the surface of the wine, and place the phono cartridge to touch the top of the styrofoam.
• You seem to be thinking of measuring the surface wave. How about measuring the "body wave" by placing the phono cartridge in contact with the side of the glass?
• Use a straight tube instead of a wine glass; for example, a piece of PVC plumbing pipe or a metal tube.

You might benefit from studying the various types of wave created in this experiment. When you strike the glass you create: a vibration of the whole volume of liquid, a vibration of the glass, a wave on the surface of the liquid, and a sound wave in the air around the glass. Each kind of wave will have different characteristics. You don't have to observe or study all the different waves, but you'll benefit from considering them all, and you can make your project discussion more interesting by doing so.

I'd say this is a new experiment.

Good luck, WW.

Thank you so much! The only thing that I'm think might be a problem is getting the materials. Where would I get a phonograph pickup cartridge or a stereo cartridge? Do you recommend that I do both the body wave and the surface wave, or both? Also, once I got those cartridges, how exactly would I measure that? I was also thinking of doing a project where I use different pre-recorded pitches as my independent variable, and again measure the waves. Do you think that would be easier? Either way, I'd like to use a vocal pitch, since my singing was the basis for the project, both last year and this year. Also, to answer your questions, I didn't see much variation in the pitches, nor did I see any real pattern in the data.

Thanks,
hurleyan

I'm happy to hear that my comments were useful.

I assume that in your previous experiment you used a microphone tied into your computer to record the sound. To use the phonograph cartridge, you would just replace the microphone with the cartridge. Radio Shack lists a cartridge at http://www.radioshack.com/product/index ... absetBasic. This cartridge probably would work simply substituted for your microphone. If it doesn't, you might need a "phono preamplifier." again from Radio Shack, etc.
You will have to connect the cartridge to a cable so you can plug it in. The wire and plug are also available from Radio Shack and other electronic parts suppliers, but you could also cut the cable off an abandoned set of earphones since they have the same plug.

More problematic is hooking it up if you have no experience in electronic wiring. Connecting the cable to the cartridge is usually done by connectors that clip on the cartridge and have wires already attached to them, or the connectors are designed to be easy to solder wires onto them. There are too many options to discuss them all, so rummage around, look at used-stuff stores, and see what you can find. Given what you come up with, I'll provide more details. Again, if you can find an old phonograph, you could remove the cartridge and the wiring from the turntable arm, and it might be cheaper than just buying a new cartridge.

Looking at the various wave types should be interesting. One should always start out with limited goals and just expand the project if you have time. I think I would start out by placing the cartridge in contact with the side of the wine glass (or tube) and map out the response to different pitches. If you get this well characterized, then move on to observing the wave at the fluid surface.

I'm not surprised that you saw little change in pitch with varying temperature. The speed of sound in a liquid does change with temperature, but likely changed very little over the temperature range you achieved. On the other hand, the container of liquid has its response determined by its size, weight, the rigidity of the container, and other factors. It will respond most easily at certain frequencies and not so well at others.

Many parts of your setup may vary in how they respond to different pitches. You will need to consider all these. For example, if you sing the pitch to excite the liquid vibration, can you sing exactly the same volume at each different pitch? How might you standardize it?

I recommend that you read about sound in liquids and about resonance. You might start at http://en.wikipedia.org/wiki/Underwater_acoustics and http://en.wikipedia.org/wiki/Acoustic_resonance.

I hope you can make progress with these comments, and again, good luck.

Alright, so I've officially decided that I'm doing the project that has to do with temperature. You've been a great help and cleared a lot up for me. At any rate, I still have one question (if you haven't figured out by now, I ask a lot of questions): If I were to change my question to the measurement of surface waves rather than sound waves, could I use my idea of the dowell and the grape juice, or would I have to go about measuring it in another way. Just kidding, I have another question: what is the purpose of the phonograph cartridge that you think I need? Is that for use as a speaker, or is that to use as a way to measure the amplitude of the waves?

Thanks Again,
hurleyan

hurleyan,

A phonograph cartridge is designed and built so that the needle follows the wiggles in the groove of the record. In the same way if you place the needle in contact with the side of a wine glass and the glass vibrates, the needle will move too. Inside the cartridge the wiggles are turned into electrical signals. It's like a microphone except the microphone has a part that follows movements of the air around it while the phonograph needle follows the groove in the record. I've been assuming that you are familiar with old-style phonograph discs with the grooves in them. Your current question reminded me that it's been so long since they were widely used that maybe you've never seen one. You can still find descriptions on the web of how the old-style records worked.

In any case, the vibrations of the wineglass or the waves on the liquid surface are likely to be very small and I think you may have a really hard time measuring them with a ruler or other visual method.

Have you looked at the project titled, Singing Wine Glasses?

I hope I haven't led you away too far from your original idea, but I emphasize that your method for measuring the surface wave amplitude may not be sensitive enough. Please give it a try soon, and find out if it works. Then you can either get right into your experiment or you'll know to try something else.