Student misconception: Sound travels infinitely fast.

Okay, most will deny that if you ask if it’s true, but it’s still an interesting or odd experience for many students to actually witness and notice the delay due to sound’s finite speed. Not only is the speed of sound not infinite, it’s slow enough that we can notice it in not-too-uncommon situations.

Some situations that are evidence of this delay: Seeing the smoke from a starter pistol before hearing the sound across the field at a track meet, seeing the batter hit a ball then hearing the crack from the stands at a baseball game, hearing thunder after lightning. Although, that last one requires the person understand the relationship between thunder and lightning.

Even with these examples, it’s interesting to actually see the spread of the sound wave. The video in the following tweet provides a good visual.

Original Tweet (should open in a new window):
(Also includes interesting conversation of the questions you might ask with it)

Video for download if you can’t access Twitter from school (please don’t just stream this whenever you need it – save it to your computer and play it from there)

Context (since you never see from that video what’s going on on stage):

Discussion Questions

  • What do you notice about the crowd’s timing? (Of their responses, I would then invite students to focus on the “spreading” or delaying aspect)
  • Why do you think the back of the crowd claps later than the front of the crowd? (Also can be many reasons)
  • Is it possible to calculate the speed of sound using this video?

I tried to answer that last question myself. This led to more questions: What information might we need/use, and how do we get it?

Yep – distance and time.

For the distance…

Location on Google Maps
(I found this um… manually. Thankfully there weren’t that many places in Reykjavik with big boats in a harbor.)

The stage is on the west (left) end of the park and the back of the crowd is on the east end. In the wide shot video, we can see as far back as the statue.

To measure distance, right-click on the map and select “Measure distance”. I measured roughly 250-300ft.

For the time…

I tried to do it visually by identifying how many frames between someone clapping in the front and someone clapping in the back (using VLC video player), then calculating based on standard video being 30 frames per second. However, it’s really hard to tell when the clap gets to the back (no, not that clap). Just watching the video and using a stopwatch might be easier haha.

I would use this one qualitatively instead of quantitatively. It may be a good way to lead into a more quantitative lab, such as doing a more controlled experiment timing echoes (I might post that one day).