Measuring the Speed of Sound

By Brown, Jeremy | The Science Teacher, Summer 2008 | Go to article overview
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Measuring the Speed of Sound

Brown, Jeremy, The Science Teacher

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 the tube); however, a meterstick works just fine. Figure 1 shows actual student data gathered for this activity, in which each lab group performed four trials.

An interesting extension of this lab would be to heat the air inside the echo tube with a hair dryer and then have students repeat the experiment for a new temperature. By plotting class data for measured speeds of sound versus the various temperatures, a linear plot should indicate that the speed of sound increases with temperature. The slope of the line should give the correction factor, 0.6 m/s/[degrees]C. Figure 2 shows the plotted student data graph.


Experiment 2: Closed pipe resonance

A common high school experiment for measuring the speed of sound involves closed pipe resonance, which occurs when a pressure (sound) wave travels down the inside of the pipe, reflects off the closed end, travels back up the pipe, and constructively interferes with another incoming wave. When this happens, an increase in energy or loudness occurs. This increased loudness is quite noticeable, even to the unaided ear, and occurs in multiples of one-quarter wavelengths.

Typically, a closed pipe resonance experiment involves a large cylinder of water and a piece of plastic pipe. A tuning fork is sounded over one end of the plastic pipe, and by raising or lowering the pipe and the fork while the pipe is immersed in the cylinder of water, a resonance can be heard at multiples of one-quarter wavelengths. However, this version of the classic experiment does not require water and the associated mess. Instead of reflecting sound off the surface of water, this new method uses an empty sewing thread spool mounted on a stick, which can be slid inside the tube and acts as the reflecting surface. Several science equipment supply companies sell nonwater versions, but I decided to make my own.

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