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Low-Cost Wave Generator Apparatus

posted Jan 20, 2017, 11:16 AM by Mark Schober   [ updated Jan 20, 2017, 12:49 PM ]
There's something magical about standing waves, resonance, and the tangibility of the nodes and antinodes. Students love working with the waveforms, and they love the direct connection to the musical instruments they play, how harmonics work, and the physical principles behind them. Unfortunately, most science suppliers sell a wave driver apparatus in three parts: 1) the thing that does the shaking, 2) an amplifier, and 3) a function generator. Depending on the supplier, one full setup costs between $500 and $1000. That's a bit steep for me, so here's a homemade version that costs about $30 in parts and takes advantage of the capabilities of your smartphone.

Mechanical Waves Make-n-take Workshop Description

This workshop is based around a low-cost wave-driver and amplifier driven by a free smartphone app for investigating wave propagation in strings. Similarly capable equipment from science suppliers ranges from $500-$1000 per lab setup, while ours costs just $30. We will begin by using the wave driver to examine properties of standing waves on strings, followed by a lab to quantitatively investigate how string tension and string mass per unit length affect the wave speed. You will then build your own wave drivers to use with your students. Not only does this set of labs establish the fundamental properties of mechanical waves, it provides an excellent experimental environment for students to collect and analyze data for a phenomenon that depends upon multiple variables. 
I'm leading a make-and-take workshop to build these for your classroom on March 5th, 2017, from 10 am - 1pm. If you are available to join me at Teacher's College, Columbia University, I'd love to see you there. Registration for the workshop is $20, and wave drivers are $30 each. You can sign up for the workshop here:

In the workshop we will run the labs you can do with this equipment as well as build the apparatus.

Design Features:

The apparatus consists of a 4-inch speaker, a 50 Watt mono amplifier circuit, a 12-volt power supply, and some laser cut acrylic pieces that tie things together. All parts are mounted to a central acrylic plate that includes notches for cable management when not in use.

You need to supply a ring stand with a flat metal base. The metal rod acts as one anchor for the string and the acrylic pyramid mounted to the speaker cone shakes the string. The magnet in the base of the speaker sticks nicely to the metal base, and a hole in the acrylic plate locks it onto the ring stand rod. This makes the overall apparatus much heavier so that it doesn't go wandering all over the lab table.

There are a variety of smartphone apps that feature tone generators or frequency generators for free. I've been using one called "Function Generator" on an iPhone which works fine, though it's filled with pop-up ads. You want a program that allows you to increase or decrease the frequency by tapping add 1 Hz or add 10 Hz. Interfaces dependent on a slider or having to type in the frequency are difficult to use for this application. I want to give a shout out to the Physics Toolbox app that gives you access to all of the sensors in your phone. (Check it out!) The Physics Toolbox has a tone generator for Android that allows you to change frequency by tapping, but it does not work that way on iOS.

There isn't anything that complicated here that you couldn't do with a saw and a drill, but once I refine my prototype for the workshop, I'll post the cutting templates and a detailed parts list here to save you some time. I'll also share the curriculum materials that go with the lab and the followup analysis.