Scanning with sound

What the Activity is for

Creating a map with sound.

Sounds travel at a known speed, so you can find distances from the trip times. One common use of this is to fire a sound out and listen out for its echo. Bats use this in echo-location; we use it for ultrasound scans. Similar methods are used in sonar scanning for locating the bottom of the ocean. In this activity we show how a regularly spaced series of echo locations can map a terrain.

What to Prepare

• a sonic tape measure (These are available from many hardware stores. This sends out an audible pulse, then reports the distance to the object you are facing.)

Per group:

• plasticine, about 150 g
• a straw
• a ruler
• graph paper
• a prepared piece of electrical ducting

What Happens During this Activity

Introduce ranging by sound using the sonic ruler. It will work well over the ranges commonly available in the classroom. With better classes you might like to try softer targets, like a staggered row of pupils, to find the limits of the device. Which reflection does it latch onto?

Then to some practical activity.

Explain that the class are about to model the plotting of a sea floor by sonar. The top of the ducting represents the sea surface, along which their survey ship will travel. The holes show where they will take samples. The straws will be used to make the depth readings, simulating the sound reflecting off the sea floor. Their job is to produce a map of the sea floor.

We suggest that one pair use the plasticine to make a landscape (seascape?) inside the ducting, then clip the drilled lid on to hide the landscape. Then they should swap their creation with another pair to see how close each pair can get to a good map by making measurements.

There are subtleties, which you may leave them to figure out or choose to rehearse beforehand. They will measure depths, but their map probably wants to show heights. Some simple calculations can fix this.

More fundamentally, there is a distinct limit to the sensitivity of their measuring devices, as they cannot measure how far the straws go under the sea all that easily. Plus there is the issue of a limited number of samples, so that the resolution of their map is distinctly limited in the horizontal plane as well.

Both of these two are well worth discussion as they can help with an appreciation of the limits of any measurement process.