s0610038 said:
it seems interesting experiment, can you talk a little bit more about your student experiment?
There have been seven or eight related experiments. For all, the goal is to develop an understanding of what is going to make the sound when you hit or rub a wine glass. The first experiments (from two years ago) used a Vernier microphone and LoggerPro software to record the sound made when running a finger around the rim. They used an FFT graph in LoggerPro to display the frequencies. One always produced a much higher peak, and they recorded that one. They ran a regression analysis and found a strong linear relationship between the frequency and the square of the volume of water added to the glass.
That group then repeated the experiment several times with different salt solutions and sound the same results hold for a denser liquid, but the frequencies produced are lower. They had heard that a cornstarch/water mix ("http://www.wellesley.edu/childstudy/pages/oobleck.html" ") is a "non-Newtonian fluid", so tried that and got the same results (although it was very hard to measure the volumes of the mix accurately).
They ran into a problem that you (s0610038) might see if try anything like this. The FFT analysis cannot report on every single frequency. The one they used reported frequencies in groups of about 20 Hz. That means they would sometimes make as many as four additions of water, be able to hear a change in frequency, not get get a measurable change in frequency.
Work by a student last year did not get the same results. I suspect it is because she had a different method for selecting the peak frequencies. The discrepancy is still under investigation.
Another round of experiments was just completed last week. One got the same results as the original group. Those workers also measured the depth of the water.
Two other recent experiments compared the sounds of rubbing versus tapping, focusing on the peak frequency and also on the overtones. I have attached the data from one of them. Note that this is the
preliminary phase of experimenting with tapping, and the methods are still rough. (I haven;t even carefully reviewed the report yet.) Don't read too much into the numbers. You see in the data the problem I described with getting the same frequency for different volumes. They might have done better to add 50 mLs at a time.
That group's write-up reports that a third overtone (close in frequency to the second) appeared when there were 80 mLs in the glass. It's precise frequency was not recorded. They noticed, however, that the height of the third overtone's peak gradually increased from that point on. At the same time the second overtone peak decreased. That means that the third overtone grew louder (relative to the second overtone). Another group doing similar work with Tibetan singing bowls noticed the same thing.
One interesting preliminary result seen in the data below is that the first overtone is very close to three times the the frequency of the tonic (i.e., the lowest frequency). Musically, that makes it a fifth. The relationship is not nearly so tight in the data from tapping.
I will encourage the students to try find a sensible way to average the data to deal with the repeated frequencies. I think it will turn out that the experiment needs to be run again using bigger volume increments.