Loudspeaker lab

The loudspeaker lab was a new one this year, but I think it went fairly well.  Students set their measurements up quickly, because the ideas were essentially the same as for the electrode lab.  Because there are fewer measurements to make with the loudspeaker (only one loudspeaker to measure, not several different concentrations), this lab is actually quicker than the electrode lab.  I think that next year I should move the loudspeaker lab before the electrode lab.

I did make one mistake—I had included a bonus section to characterize some ferrite-core inductors, but I forgot that the students did not have the inductors in their parts kit—I had bought them myself last year, and just brought them in for the class-D power amp lab.  I left them at home today, so students could not do that bonus lab. I did remember to bring in my speaker, which was a good thing, because the student working without a partner this week also forgot to bring the parts kit.  I was able to lend out my speaker and another student loaned a breadboard, so the lab still got done.

All the students managed to complete their data measurement (most of them finishing early), and I managed to get all the students to plot their data as they collected it (or, at least, to alternate collecting a bunch of points then looking at them).  Looking at the data allowed them to get several data points right around the resonance peak, and to spot anomalies in the data right away, and go back and remeasure data points near the anomalies (many of which were transcription errors in recording the data by hand).  I’m hoping that students get into the habit of checking their data as they collect it, but that is probably too much to expect from students who were not doing even the most trivial of sanity checks in previous labs.

One pair of students got very weird data for their loudspeaker, but I could not find anything wrong with their test setup, and I confirmed the measurements using a different instrument ( a digital scope rather than the digital multimeters), so I suggested that they try again with a different loudspeaker.  They did that, and got the expected results with the other speaker.   The bad speaker had a normal DC resistance (around 7Ω) but had an impedance around 20 Ω at frequencies below the resonance. Above the resonance, instead of quickly leveling off at 8Ω, the impedance dropped slowly to 8Ω and immediately went back up, without the usual wide flat region. I have no idea what might cause this unusual behavior.

Tomorrow in class, I have two topics to cover:

• fitting the model L+R+(C2||L2||R2) to the data (and why we can’t really get it right—though I don’t have a good physics explanation for why the impedance goes up more slowly than the first power of frequency).  I asked the students to try to do the fitting before class, so that we can concentrate on the tricks for fitting the two regions separately before combining them.
• sampling and aliasing, which is Thursday’s lab.  There is not much to write up in Thursday’s lab, which should help compensate for the time they’ll be spending on redoing the quiz as homework.