The goal of this lab activity is to determine the speed of sound by using the relationship between the distance traveled by sound versus the time taken to travel that distance.

In order to be ready to take data, you need to have done several things first.  These are described below.  You don't necessarily need to do them in order.

A. Gather your materials

You'll need the following.

1. Calculator
2. 30-cm ruler:  The smallest division must be millimeters.  Inches and fractions thereof aren't used for measurement in science classes.
3. Protractor:  This needs to measure in degrees.
4. Graph paper:  For this lab, you must draw the graph by hand (no software allowed!). Right click on the following link and select Save Target As.  Then save the file on your computer. Once saved, you can open the file in Microsoft Word or Open Office.  Print the graph paper.

Download Graph paper

5. Set of 8 photographs for analysis:  These photographs are contained in a PDF file that will open in a browser window with Adobe Reader.  Print the photos for analysis.  One set of 8 is sufficient for 2 students.

Download Photos

B. Watch the video

You'll need to watch two video demonstrations so that you’ll see how the data was obtained. You'll not only see the method used, but you can also get an understanding of some of the errors inherent in obtaining data.  These clips last a total of about 15 minutes. In order to view the demonstrations, RealPlayer must be installed and working on your computer. If you need the RealPlayer install file, that's available in the software folder.

Don’t try to continue with the lab without watching the videos first. Since the videos also have sound, you’ll need speakers or earphones. Watch the videos in the order below.  It would be a good idea to take notes.  You'll be asked questions about the videos to answer in your lab report.  Each link below will download and then open in RealPlayer. By the way, for your reference, the video clips for this course are all listed here. There is also a link to the list on the Weekly Schedule page. The clips are several megabytes in size, so you'll need a high-speed connection to download them in a reasonable amount of time. If you're using a phone modem at home, we recommend doing your downloading at school first.

Measuring Frequency with a Stroboscope

Measuring the Speed of Sound Part 1

C. Study Chapter 1 of your text and do the Review and Study Assignment (E.1.1 all parts).  (If you don't have a textbook yet, you can still do the Review and Study assignment, because the online review summarizes the important points of the chapter and gives example problems.)

D. Read the introduction below.

E. Do the prelab problem that follows the introduction.  As part of the problem, you'll submit your answers to the instructor on the form at the bottom of the page.

Average speed is defined by the following relationship:

Average speed = (Distance traveled) ÷ (Time taken to travel that distance)

In shorthand notation, v_av = Dd/Dt, where v_av represents the average speed, Dd is the distance traveled, and Dt is the time to travel that distance.  The Greek letter, D, is commonly used to represent the difference of initial and final values of a quantity.  Therefore, Dd means d_f - d_i, where d_i is the initial distance (frequently 0) and d_f is the final distance.  Note that one always subtracts the initial value from the final value.  Likewise, Dt = t_f - t_i.  It's important to remember that average speed is the ratio of two differences.

A common way to determine the average speed of an object is simply to measure how long it takes the object to travel a known distance. This is called a time-of-flight technique. Sound waves are known to travel at a speed of about 340 m/s in dry air at room temperature (20 °C). This speed is generally too fast to measure accurately by time-of-flight techniques. In one method, students stand on the goal lines at opposite ends of a football field. At one end, a student smashes two metal trash can lids together. At the other end, another student starts a stop watch when he/she sees the lids come together. (See below.) He/she then stops the watch upon hearing the sound from the lids. The time interval between sight and sound is only about a third of a second. This isn’t much longer than a typical human’s reaction time in starting and stopping the watch. Therefore, accurate results can’t be expected with this method.

The method used to collect data for the present experiment is described in the video clip Measuring the Speed of Sound Part I. Two wood blocks are struck together to produce a loud sound. The sound is detected by two microphones a distance d apart. Each microphone is connected to an amplifier which automatically discharges a flash unit. The flashes of light illuminate a disc rotating at a constant frequency. The disk, which serves as a clock to measure the time interval between the flashes, is black except for a white radial which serves as the hand of the clock. The two flashes of light produce two images of the hand separated by an angle.  A typical photograph of the clock face is shown to the right.