Sound Wave
Light, Sound and Waves

Sound shadows

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

Finding the paths of sound: predicting what you can hear.

Here you can show how a simple model of the paths that sound can follow allows us to predict where things will and will not be heard. The model is less useful than the equivalent model for light precisely because the paths that sound takes are less well defined.

What to Prepare

  • a small loudspeaker, driven by a signal generator
  • a small microphone, connected to an oscilloscope display with the time-base turned off

or

  • a computer and a data projector
  • the interactive object (see below)

Use one or more of these to explore simple examples where obvious barriers either permit, or do not permit, sound to travel from source to detector. To prepare these, assume that sound travels in straight lines and reflects as light does, with the angle of reflection equal to the angle of incidence. It is best not to have too many tricky ones, where the detector is only just in, or just out of the sound shadow.

Sound travels in much less well defined beams than light, so modelling with rays is far less useful. Sound often seems to travel around corners all by itself; light does not. However, careful thinking about the paths that sound can follow, based on appropriate simplifying assumptions, does provide insight into what you can and cannot hear.

What Happens During this Activity

Ask children to use the situations that you have prepared, which could be practical, paper based or software based, to predict where the sound can be detected. There are a few ideas for possible physical arrangements in the support file.

One place where this is very important is in seismic probing. Here a controlled explosion sends off sound into the body of the Earth and detectors are used to monitor where the sound turns up and how much of it. In this technique the bending of the paths of sound due to changes in the density of the medium (the same process of refraction as encountered with light) is also important. Much of what we know about the structure of the planet under our feet is due to seismic probing.

Resources

Download the interactive for this activity.

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