Gas station without pumps

2015 April 21

Loudspeaker lab went well

Filed under: Circuits course — gasstationwithoutpumps @ 22:34
Tags: , , , , , , ,

I’m falling behind in my blogging about the applied electronics class, and so the notes on the lectures may be a bit skimpy.

Last Friday, I had several topics:

  • Some metacognition talk about answer-getting vs problem solving, with an emphasis on the descaffolding (removing the detailed what-to-do instructions and worksheets) that needed to happen as undergrads, and how too many of their classes had not been doing it, so I had to descaffold them faster than was really optimal.  I warned them that this would be uncomfortable.
  • Test equipment as part of the circuit. One of the main points of the lab they had just completed the day before was to make them aware that test equipment (like oscilloscopes) become part of the circuit that they are testing, and can affect that circuit. I’d not made that clear in the book (there is a chapter that needs a massive rewrite this summer), so I spent some time in class on the idea.
  • Inductors.  I didn’t do much with magnetic fields, but I did give them V= L \frac{dI}{dt} and derived the impedance j \omega L.  There was an unplanned digression about transformers, in response to a question. I did have students give me the impedance of a resistor in series with an inductor, which they had to plot vs. frequency as one of their pre-lab assignments over the weekend.
  • Loudspeakers. I showed them a labeled diagram of a loudspeaker, and the magnetic field near the voice coil. I reminded them of the EMF right-hand rule, and showed them how current through the voice coil would push the voice coil up or down (in the orientation of the pictures).
  • I then quickly went over measuring the magnitude of impedance with a frequency generator and a pair of  AC voltmeters. This is almost identical to the lab they just did for DC characterization of a microphone, but I would have liked to spend more time on it.

The students did a prelab exercise over the weekend, but I forgot to tell them to do the prelab exercise for Thursday’s lab as well as Tuesday’s, so I had to tell them that in class on Monday (assigning it for Wednesday).

Monday’s lecture was supposed to cover several topics, but I didn’t get to them all:

  • feedback on the previous week’s design report.  This took far too long, in part because I went over some fundamentals—I’m not sure, based on their writing, that all the class understands the difference between voltage and current. I introduced the hydraulic analogy (voltage=pressure difference and current=flow), but I’m sure that there are still several who think of both voltage and current as some incomprehensible measure of “electricity”.
  • Another fundamental that came up as a result of the report is what a “model” is.  Too many people had been trying to stick voltmeters into the middle of the model for the loudspeaker.  I think that the physics classes are doing a poor job of getting students to think in terms of models, but I’ll do what I can to remedy the deficit.
  • I also spent some time on writing fundamentals, like who the audience is for a design report, and that the document must be written for someone who does not have the book or the assignments.  Students are so used to fill-in-the-blank answers that they often don’t get the idea of a self-contained report—but that is what they will have to create as engineers, so they need to start learning how to do it. I had a full page of notes on feedback for the students, much of it specific to the writing assignment I was returning.
  • I went over how to determine the shunt resistor for the current measurements in the loudspeaker lab. I was surprised to find out that most had never heard of a geometric mean and had no idea that it was the appropriate way to average numbers when errors were measured by ratios not by differences. I’m not sure that the idea came across well, because I had not anticipated having to teach them that part of high school algebra (silly me—they’ve needed to be retaught everything else they should have learned in high-school algebra, so I should have known that they would never have heard of geometric means).
  • I never did get to the main topic for the day: hysteresis.  I’ll have to cover both model fitting and hysteresis on Wednesday, though I’d only originally planned to do model fitting.  Fitting the models for the loudspeaker is a bit tricky, since the main R and L values have to be fitted away from the resonance peak, then the resonance peak needs to be fitted on just that region. I hope that there is enough time on Wednesday to cover hysteresis and relaxation oscillators as well as the model fitting.

Lab today went fairly smoothly.  Everyone managed to get good data from 3Hz to 1MHz, with lots of data around the resonance peak, so they have good data for fitting models to.  There were three problem loudspeakers: Two had bad insulation-displacement connectors on the ends of cables to the loudspeakers.  They didn’t fail as open circuits, but as about 70Ω resistances that remained fairly stable until the wire was jostled.  This meant that the students had inconsistent results that were hard to debug. I think that the cheap connectors were slightly oversized for the wire, so that there was not a gas-tight connection and some corrosion had formed between the wire and the connector.  I finally managed to figure out what was going on for one of the groups, and a little while later applied the same thinking to another group.  Unfortunately, it was getting rather late by then, so that group only got one of their two loudspeakers characterized—they’ll probably try to finish up on Thursday, after doing the next lab.

The third problem loudspeaker behaved normally as far as resistance was concerned, but its resonant peak was nowhere near where it was supposed to be.  Instead of a resonance at 137Hz (as reported on the data sheet), it was out around 700Hz—way too high for a mid-range loudspeaker.

Incidentally, I was wrong about which loudspeakers the staff had bought this year—they got the 15W midrange speaker rather than the slightly cheaper 5W one, so we don’t need to worry about burning out loudspeakers on the power-amp lab.

1 Comment »

  1. You mentioned before that students can take this class before taking 2nd semester physics? You might look for any correlation between students who slip through that crack and issues you raise in this post.

    Friday bullet #2 on test equipment:

    I was glad to see that something I have added to our labs has potential long-term value. One of the first lab activities I have my second semester students do is to see how an ammeter changes a circuit and that you get “errors” in quite a few simple experiments if you assume the voltage from the power supply is the voltage across the single resistor connected to it. They get used to largish errors in the first semester labs, so they don’t realize that they are seeing physics rather than crummy data in the electrical labs. You might talk to your physics department about that.

    Monday bullet # 2 on model:

    Are you referring to a “box” containing multiple elements that cannot be separated from one another? Yes, I agree that they get limited exposure to that in the labs we do, but we do one where they have to separate things like L and R in a coil or, for another example, V and R in a battery. Haven’t tried getting them to look at the AC power supply.

    They also have essentially no exposure to model building.

    Monday bullet #3 on writing:

    Huge problem. Part of it might be, IMO, mental compartmentalization mixed in with a “burn your notes” ceremonies at the end of the school year (apparently common in some of our high schools). They don’t carry over what they learn from one class to another. And when it comes to lab reports, there is a huge inertia associated with who is doing the grading in intro classes. I know someone working on this, and it is a challenge.

    Comment by CCPhysicist — 2015 April 23 @ 06:23 | Reply


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