Domino Waves

8/2/2022

The Premise

One of the most surprising and unintuitive facts of sound is how different materials affect the speed of sound. Compared to how light is affected by the medium, it is unexpected that sound generally becomes faster with increased density. (there are certainly exceptions to this rule but this is a fine generalization for our purposes in high school)

Graph from www.explainthatstuff.com

I wanted to provide students with some sort of hands on demonstration that helps them visualize and remember this relationship rather than just memorize that sound travels faster through steel than water. In this model, dominos are used to represent the spacing between molecules in a substance propagating a sound wave.

Setting up the Dominos

To see how spacing affects the wave speed, you want to stage a "race" between closely spaced and spread out dominos. You just want to make sure that the dominos aren't too close that they don't fall at all and they aren't so far that they can't touch. It's also nice if both lines are the same length for a clear comparison.

I use a set of dominos from home (similar to this one) that allows me to give each group 18 ceramic dominos and the template shown. This way, I get to "flip the demo" and provide a nice opportunity for students investigate with a hands-on experience. The template isn't essential for this demo but it serves as a nice guide to help ensure that groups have a set up that could get them good results (see video above)

Domino Waves Template (pdf)
File Size:	27 kb
File Type:	pdf

Download File

Domino Waves Template (editable)
File Size:	45 kb
File Type:	pptx

Download File

Capturing the Result

It all happens really fast and it's really hard to tell "who won" in real time. Because of this, I have students capture this in slow-mo on their phones or tablets and watch back the footage.

Things to Keep in Mind:

It is important that both sides start at the same time with the same starting force. To do this, I usually take a pen or marker and push both sides at the same time.
I have noticed that it seems to work best with a crisp and firm starting force. If the force isn't large enough, the closely spaced dominos have too much friction to overcome.
The longer the line up, the more pronounced the difference should be. With a finite amount of dominos though, you will need to decide if it's better to have more groups with less dominos to work with or less groups with more dominos to work with.

Is this Model Perfect?

No.

As with almost every physical model representing a hard to visualize phenomenon, this model has more than it's fair share of limitations. I would argue however that at the high school level being able to relatively rank the speed of sound through a typical solid, liquid, and gas is about as much as students really need. To that end, I think this domino wave model provides a really nice experience for students to hook on to and remember.

Simulation

There was a simulation of this concept posted on reddit (thanks to @UniverseAndMore for spotting it). It does a nice job of making this difference between solid/liquid/gas even clearer.