Measuring the Speed of Sound

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The following physics lab activities are new versions of old experiments for measuring the speed of sound--what makes them new is their use of electronic interfacing, which can be exciting for students.

Experiment 1: The "echo tube"

When I was setting up my new physics lab a few years ago, I purchased a set of 1.2 m long aluminum tracks for dynamics experiments. These tracks came packaged in long cardboard tubes, and it occurred to me that they would make neat "echo tubes." An echo tube can be used for echo detection and is comprised of a long cardboard tube, clothespin, sound sensor, and the associated computer-interfacing equipment.

I designed a simple lab for measuring the speed of sound in the classroom using these echo tubes. Students measure the time delay between a snap of the fingers or "clack" of a clothespin and the arrival of its echo after reflecting off the floor. To measure this delay, a computer is programmed to begin the timing sequence at the sound of the snap and to end after one second. Knowing the tube's length (total distance) and finding the time delay (total time) allows for a calculation of the speed of sound in air using the formula below:

Speed of sound = Total distance (down and back) Total time (down and back)

With the help of a temperature sensor, it is possible to check the experimental results adjusted for temperature.

The trickiest part of this activity for students is getting the computer to start at the "clack" sound of their clothespin. Classroom noises, as well as competing lab stations, can cause the computer to trigger prematurely. As students experiment in closely spaced teams of two, they must work cooperatively and take turns "clacking" so as to trigger only their own computer. Students need practice to identify the echo return point--which is used to calculate total time--on the computer display, but after a few trial runs, they quickly learn how to find it on the screen. If teachers want to get really fancy, they can also use a motion sensor to measure the distance from the sound sensor to the floor (the length of …