Skip to content


Standards Based Grading, Week 3

February 2, 2013

Day 7 (Monday):

We closed out Chapter 15, activating the last remaining standard.  In hindsight, there were some “neat” things we didn’t do in this chapter (derivations and a few extra demos).  But in the interest of time and really trying to focus on the standards, they simply didn’t make the cut.

I also haven’t given a pop quiz yet.  That will happen next class, maybe.  As the time I thought was right for a pop quiz today, we ended up using it as a mock assessment.  I drew two wave plots on the SmartBoard and asked them to write the corresponding wave function.  They were also supposed to include values for amplitude, period, wavelength, wave number and periodic wave.  After going over the answers (which were on small whiteboards), it was clear no one would have gotten a “3” or a “4.”  Students agreed that the scores earned would have been a “2” or “2-ish.”

Day 8 (Tuesday):

Today (in lab) we pulled from Chapter 15 a bit to serve as an introduction to sound traveling in an air column. While a string under tension is different from sound, the expressions for the normal modes are the same for strings under tension and sound if both ends of the sound column are open or closed.  So I started by drawing diagrams of the standing waves for both ends closed (string and sound); noting nodes at closed ends, antinodes at open ends.  We derived the expressions for the normal modes and showed they were equivalent.  Then, a series of diagrams of sound tubes with one end open, one end closed revealed we’d need a different expression where n = 1, 3, 5, . . . applies, rather than n = 1, 2, 3, . . .

And <pouf!> Standard 16.4 (a lab standard) became active.

To demonstrate how sound consists of a source undergoing vibrations, I showed the second half of this YouTube video:  It’s worth watching, and it has a little bonus at the end about filming rate.  I wish the narrator made reference to beat frequencies as an analogy to what is being observed (f_ beat = f_tuning fork – f_filming rate|).

Anyway, the task for students in this lab was to determine the speed of sound empirically given two tuning forks of known frequencies and the apparatus below:

Jokes and confusion ensued as everyone fumbled around with how the 2nd normal mode is the 3rd harmonic and the 5th harmonic is the 3rd normal mode, etc.  Kind of a “Who’s on 1st” banter.

A student mentioned having seen a video of a series of pendula showing neat normal modes.  I knew just the video he was talking about:  I made the comment that constructing a demonstration like this (having determined string lengths and normal modes ahead of time) would get a 4/4 on one or more standards 🙂

Day 9 (Wednesday):

Well, I forgot the pop quiz.  Again.  Good thing these posts aren’t published until the week is over!  Pop quiz to be given on Friday, maybe 🙂

Spent time today on wave interference.  Having two large tuning forks attached to wooden rectangular boxes (with one end open) really helps demonstrate how one wave can drive another.  Adding a movable mass to one of the tuning forks illustrates beat frequencies well, too.  One student brought up tuning instruments in band, trying to hear the beats everyone was talking about.  A perfect example from previous experience to tie in to class!

Discussion of Bose sound systems, noise cancellation features of automobiles and noise cancellation headphones led us to working a problem involving two speakers creating interference for an observer some off-center distance away.  Having reasoned out the relationship ahead of time, we found a typo in the textbook’s example.  (f = nv/d where n = 1, 2, 3, . . . for constructive interference; f = nv/2d where n = 1, 3, 5, . . . for destructive interference).

While it wasn’t announced today in class, this essentially activates Standard 16.2.  I’ll have to announce it officially in Friday’s class.

So is it ok to activate standards out of sequence?  I think the answer is yes–at least that’s what’s happening this semester and the flow of the course doesn’t feel weird, anyway.

It just seems natural to talk about beat frequencies immediately after addressing and working with normal modes of waves.  We’ll get to intensity and intensity levels soon enough, just not in the order Young & Freedman have it printed in the text.

Question: I know that you can sustain hearing damage if subjected to intense sound, even if out of the audible range.  However, if you are wearing noise cancellation headphones and there is a very loud sound they are cancelling out by producing waves that interfere destructively, are your ears more susceptible to damage by receiving a pair of out of phase sound waves?  Or, does the cancellation really mean a decrease in vibrations received by the ear?

Day 10 (Friday):

Well finally, Friday arrived and I remembered to give the pop quiz!  The problem was given at the board.  No numerical values:

You walk in front of a concert stage with two speakers at either end (separated by a distance L).  As you walk from in front of one speaker to in front of the other speaker (parallel to the stage), plot the volume of sound you would hear as a function of L.  Assume the speakers are emitting a constant tone of the same frequency.

I’d secretly planned to ask a student to come to the board — just hoping for a 3 or a 4 performance for our first standard.  And we got it!  By the end, we were all chiming in.  And the diagrams drawn by the student were pretty thorough, addressing:

  • Interference patterns if the tone emitted were perfect and no other interference/reflections were occurring
  • Interference patterns if the speakers were playing music with multiple tones
  • Correct scale in terms of L as to where minima and maxima would occur
  • What the idealized interference patterns would look like if the frequency of the tone were increased

There are a couple of points I’d like to make as to why today’s class made an impression on me and my students:

  1. At the beginning of class, someone said “No one likes pop quizzes” <shaking head with a quiet groan>.  However, it soon became clear that quizzes are (in this class) an opportunity to submit a standard.  And hey, if you don’t do so great, then you can learn from it and resubmit.  The pressure to perform was alleviated.
  2. If someone is absent from class and misses the quiz, that just means they’ll just need to submit that standard on their own.  Students will likely share what they did in class during the absence

Anyway, I had to leave campus early in the afternoon, so I wasn’t able to get a screen capture of what was done in class to post and share.  But that’s what I wanted to do, to brag about how the discussion went and how well questions were fielded; all of which is the subjective litmus test for assigning it a 4/4 (per Andy Rundquist!).

  1. Andy "SuperFly" Rundquist permalink

    My semester just started so I got to tell them about my bragging litmus test. They’re usually a little shell shocked because it comes right after saying that just getting the problem right is a 3, which they realize is a C. How are you eventually converting your numbers into a grade?

    • I believe my students are more pleased with the chances for reassessment than concerned about the scale. As the syllabus is written now, points earned on standards will count 40% of their final grades; I’m just going to sum them up at the end and use a weighted average. I told them at the beginning this was new to me, so we may revisit this policy from time to time to keep things in check.

      It’s a very small class though, and I know them really well. I have concerns about implementing this in a larger class for the “but a 3 is only a ‘C’ ?” issue.

  2. Glad they were ready by Friday! My last friday sounded like your Monday… we’ve been working on exponential equations and I thought most of them would be ready for a quiz, but walking around as they were taking it I could see they all still struggled with making a context when given an equation. No one had a context that actually fit. So we did that part of the quiz as a class, I told them it was just practice this time, and we’ll try again next week. They were close though. And once we started talking about which parts of their context didn’t quite fix they were able to give me a correct example quickly.

    • I’m really hoping this opens the flood gates for submitting standards. We are approaching the 2-week deadline on our first set of standards this coming week. I need to emphasize that they should submit something even it not entirely proficient in the material. If they want to improve they can always resubmit (provided they submitted the assignment).

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: