Sound Principles

Today I gave my first large presentation to my two classes. On an earlier occasion I did a short talk on the LCROSS mission and had the class do basic calculations. That didn’t go over so well because I think many of them don’t share quite the same enthusiasm for space as I do. A few weeks ago I handed out a small piece of paper asking the students to write down a topic that they’d like me to talk about. I left the field open to any subject as I didn’t anticipate having any trouble finding math in even the most obscure suggestions. One of the most frequent suggestions was music; though many simply wrote ‘rap’, there were others that mentioned music, and how to build instruments. I decided that a presentation that covered the basic principles of sound and how it can be described mathematically would be the best place to start.

I used a few principles to guide how I put together the material. I decided on two “acts” in the talk. The first was rather basic and tried to imbue a qualitative understanding of the necessary principles. The second act used those principles as a foundation for real world topics that are difficult to analyze without an advanced degree, but would hopefully be understood intuitively. I also tried a form of multimedia blitzkrieg; almost every slide had a visual and audio component with several short videos near the end. It was my hope that this would keep the students’ attention much better than a straightforward, dry lecture.

My talk began with a discussion of what exactly sound waves are and some of their properties. I made a conscious effort to avoid words like transverse and longitudinal, but instead demonstrated the concept using a Slinky. By hooking my laptop up to the overhead speakers, I then played back various sine waves and combinations thereof. I did my best to show them how sines could be manipulated in much the same way as regular numbers.  I mentioned how the simple ideas behind superposition could be used for tuning instruments or for noise-cancelling headphones. These examples helped a fair amount in grabbing the attention of the students. I ended this first act with mentioning how combining a sufficient number of sine waves could added in such a way as to make any sound imaginable. This was a natural precursor to drum machines and electronic music which in turn led to music in general.

In the application portion of my talk, I went over many subjects. The first one was on turntablism. Here I used a microscopic view of record grooves to explain why scratching sounds like it does. I also used a video of an oscilloscope output to show how music is really just a collection of sine waves. My next target was how to use speaker response curves to select a good speaker. This slide did not go as smoothly as the turntablism one, probably because I did a poor job of discussing why a flat response curve is a good one and many found the topic as an excuse to discuss what speakers systems they wanted for cars. I also used a slow motion video of guitar strings vibrating to discuss how stringed instruments make noise and why many of them have large resonant bodies. I wrapped up the discussion with a slightly tangential video, but one that the students really enjoyed. I chose the collapse of the Tacoma Narrows bridge as an example of waves gone wrong. Like the resonation in a guitar body, I showed them how the bridge resonantly reacted to the winds.

Overall, the talk went very well. While there were a number of kids that got distracted, talked out of turn, or passed notes, many seemed genuinely interested. The best part of my day was when several told me how interesting the presentation was at the end of class. One student who is usually hostile and taciturn was very engaged by the talk, listened to me and volunteered answers. I was nonplussed. My only regret is not having a more interactive presentation; I’ve been meaning to make an ultra toriton for a long time and it would’ve been a perfect accompaniment to the talk. I’m planning to do my next talk on football which will likely be a bit more challenging to put together, but I’m hopeful. In the mean time, anyone who would like to look at my slides, can be download them  here (careful it’s nearly 20 MB).