10.14.07

In Your Classroom: Sounds of Silence

"Sounds of Silence" is a perfect introduction to a unit on sound in the physics classroom.  Kids always want to know about human extremes and this is a way to tease them into learning about concepts of frequency, period, and velocity.  I like to begin my sound unit by hooking up a function generator to a large speaker and amplifier.  If you do not have this you can easily download free software to generate sounds through your computer speakers.  A simpler java applet that plays music and a smaller range of frequencies is available here.

I ask the kids to raise their hands and keep holding them high until they can no longer hear the sound I generate.  I will sweep the signal up to about 20,000 Hertz, the high limit for most people and the hands will suddenly go down.  Usually one or two kids keep their hands up as I sweep into the 22,000 Hz range, which then elicits a number of jokes about how they are similar to dogs and bats who can hear in this ultrasonic range.  Most people are comfortable with this because we learn it in grade school.  However, sweep the frequency down and when you reach around 20 Hertz, the hands will quickly go down.  We cannot hear beneath this frequency. This technically is the beginning of the infrasound region, although scientists are usually listening to frequencies even lower than one hertz.   As an interesting activity I will do the same thing with a blinky light, and increase the frequency until students can no longer see it blink.  This is a great extension into the difference between the speeds of chemical receptors in our eyes compared to the mechanical receptors of our ears.  The ears are much more sensitive.  You can extend that lesson even further with the classic demo of placing a meter stick between a person's fingers and dropping it.  You can measure their reaction time by the distance the stick falls.  Do this with their eyes open first.  Then ask them to close their eyes and repeat the experiment by giving them a verbal clue of when you drop it.  There is a large difference and it is fun to graph this as a class exercise.

Imagine you could really hear infrasound.  What would it sound like?  If you go to the University of Hawaii Infrasound Laboratory you can actually listen to it.  There are several sound bites in their "Infrasound Zoo" that have been recorded and sped up so that they are in the audible region.  This is identical to spinning an old fashioned turn table at a faster rate to change the pitch.  The sounds of magma flowing or meteors slamming into the atmosphere is incredibly uncanny and dissonant.  I highly recommend doing this for your students.  An interesting classroom extension into infrasound might be to look at how animals sense the audible spectrum.  While some animals hear in the ultrasound region it is known that elephants can communicate in this region.  A great site for this is The Elephant Listening Project.

I recently traveled to the island of Maio where I helped in the installation of a detector there as part of the ARMADA project.  You can see some pictures by reading my online journal.  It is essentially a large, 2km grid of steel pipes that channel air into a sensitive micro-barometer.  This information then is radioed back to a central recording facility for analysis.   A comprehensive resource that joins researchers around the world is the Inframatics website where you can find out the latest news in infrasound.

For the more ambitious teacher you might want to build your own infrasound detector and recording circuit.  When I returned from my trip I spent time with my students making our own infrasound detector.  My students used a circuit that was available online by a site called Infiltec.  I was very pleased with the sensitivity of the Honeywell microchip we used that detected small changes in atmospheric pressure.  Students built an array out of flexible sprinkler irrigation pipe that spanned an area of about one hundred square meters.  The arm of the array funneled into a central chamber where the pressure sensor was located.  They amplified the signal and recorded their results using a digital oscilloscope.   Their science fair project involved comparing the infrasound signals in the valley to that of the Pacific coastline about one hundred miles away.  They found a clear difference due to the pounding of the ocean surf.  I've attached a few pictures of their project and I welcome your comments.   Additional "Wired Science" Video Segments
Don't forget to check out our Video Section for other segments from "Wired Science" that you can use in your classroom.

 

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