2.01 - Speed of Sound - Student Section

Background

We will start our work on sound by working with speed, a concept you should be familiar with. In the history of sound study (sometimes called acoustics) the speed of sound took quite a while, historically, to determine. We are going to present you with two, or more, methods for finding the speed of sound in air. We say, "in air", because this is how we normally sense sound. We can definitely hear sound through other media like water or various metals. We'll work with these other media later on in the unit. We get bombarded by sound almost every minute of every day. To isolate one sound source from another and determine its speed, may pose a challenge. But challenges are the forces that move scientists to discover new things. If you already know the speed of sound in air, forget it for the time being and work on finding sound's speed through experiment.

The first activity will be done outside along the sidewalk along Metcalf. Each group will need a notebook to record data, something to write with, a stopwatch (hopefully, three per group with one recorder), and a thermometer. Make sure your group takes the temperature of the air at the position where the sound will come from. You will be taking time readings at 50, 100, 150, 200 m (these distances may need to be varied to match with the space we use for the experiment) away from the sound source. Your job will be to start the watch when you see the see the event and stop it when you hear the sound. It is best that you have the same people from your group do all the timings. This may take some practice and we will do a few trials at each distance. Each lab group will take its own times, average the times for each distance, and then compare with the other groups later, back in the classroom.

Measuring the Speed of Sound in Air

Aim

To calculate the speed of sound in air by collecting and graphing the average time taken for sound to travel different, measured distances.

Apparatus

Starter’s pistol, at least 12 ‘caps’ and ear muffs for starter (for this activity, the teacher will be the starter)

Trundle wheel (or measuring tape), Stopwatches (3 per lab group), Thermometer (-10 to 100 oC)

Method

  1. Use a trundle wheel to measure out and mark distances of 50 m, 100m, 150m, and 200m (and beyond if possible) in an appropriate area. This area will be selected by your teacher.
  2. The starter stands on the zero marker, gives a pre-arranged signal to ensure that the timers are ready and then strikes the object.
  3. The students use their stopwatches to measure the time between seeing the strike and hearing the sound.
  4. Repeat this procedure a few times at each distance.
  5. Repeat all of the above steps at the other distances specified in step 1 above.
  6. It would be best to do the averages with a calculator. The spaces on the data collection table are small and not too conducive for doing math by hand.

Record your individual results in the table similar to that shown below. Air Temperature should be recorded on the line below.

Air Temperature: oC

distance (m) / time (s) / Av. Time (s)
50m
Watch 1
Watch 2
Watch 3 / Trial 1 Trial 2 add (T1 &T2)/2
100m
Watch 1
Watch 2
Watch 3 / Trial 1 Trial 2 add (T1&T2)/2
150m
Watch 1
Watch 2
Watch 3 / Trial 1 Trial 2 add (T1&T2)/2
200m
Watch 1
Watch 2
Watch 3 / Trial 1 Trial 2 add (T1&T2)/2
  1. When you return to the classroom, each group will record their data on the front board, or on chart-paper, or on a whiteboard. At this point, a graph will be constructed using the times from the groups averaged. What we will then have is an average (say, 7 lab groups' times/7) of times that have already been averaged. Remember, we took the average of two trials for each of the three watches per group during the outside activity. The class must discuss outliers (those times which are clearly different, high or low, from the majority of times) and whether they should be removed. Sometimes the high and low times are not used. There are many ways to smooth numbers based on sources of error. We used one method of smoothing by having three watches averaged in each group. It is important to note that these collected times are very short and may test the reaction times of the timers!
  2. We will use the class data to produce a chart of time (s) versus distance (m). Time is the dependent variable and should be plotted on the Y-axis. Please remember we set the distances(50,100,150, and 200m) and the times varied according to the distances set.
  3. Determine the slope of this graph and use s=d/t to determine the speed of sound in air.

Summary Questions:

  1. Why did we take an average of the timing data? What uncertainty is associated with your timing measurements? Does the uncertainty vary with distance?
  2. What does the slope of the time vs. distance graph give us?
  3. Use the speed of sound from the table below for our recorded temperature and compare it to our experimental speed of sound; what is our percentage of error from the table value?
  4. Why do you think the temperature of the air was recorded when you did the activity? What effect does it have?

Your teacher will give you:

Speed of Sound in air at 0ºC =______Number of degrees above 0º(your temperature reading at the test site)=______ºC

Number of degrees above 0ºC______x 0.607= ______+ Speed of Sound in air at 0ºC=______. The classes averaged speed of sound at ______ºC.

Use the following example to calculate the % error.

Example of how to calculate percent error: You estimated your monthly car payment to be $315. The actual car payment turned out to be $300. Calculate the percent error in these payments. First, take 315 and subtract 300 = 15. Next, take 15 and divide by the correct monthly car payment. 15/300=0.05. Finally, multiply 0.05 by 100=5%. The final percent error in your car payment estimate equals 5%.

The Classes' experimental speed of sound calculation= ______m/s

Book value of speed of sound at our temperature=______m/s

Difference of the two divided by book value multiplied by 100

Percent error = ______%

Speed of Sound (This lesson is from the Vernier Lab Book, Physics with Computers(2003), Lesson#24)

Compared to most things you study in the physics lab, sound waves travel very fast. It is fast enough that measuring the speed of sound is a technical challenge. One method you could use would be to time an echo. For example, if you were in an open field with a large building a quarter of a kilometer away, you could start a stopwatch when a loud noise was made and stop it when you heard the echo. You could then calculate the speed of sound.

To use the same technique over short distances, you need a faster timing system, such as a computer. In this experiment you will use this technique with a Microphone connected to a computer to determine the speed of sound at room temperature. The Microphone will be placed next to the opening of a hollow tube. When you make a sound by snapping your fingers next to the opening, the computer will begin collecting data. After the sound reflects off the opposite end of the tube, a graph will be displayed showing the initial sound and the echo. You will then be able to determine the round trip time and calculate the speed of sound.

Figure 1

OBJECTIVES

  • Measure how long it takes sound to travel down and back in a long tube.
  • Determine the speed of sound.
  • Compare the speed of sound in air to the accepted value.

MATERIALS

computer / tube, 1-2 meters long
Vernier computer interface / book or plug to cover end of tube
Logger Pro / thermometer or temperature probe
Vernier Microphone / meter stick or tape measure

PRELIMINARY QUESTION

1.A common way to measure the distance to lightning is to start counting, one count per second, as soon as you see the flash. Stop counting when you hear the thunder and divide by five to get the distance in miles. Use this information to estimate the speed of sound in m/s.

PROCEDURE (The setup will be done for you since we will use an lcd projector to show the results of the trials and we only have one setup)

1.Connect the Vernier Microphone to Channel 1 of the interface.

2.Use a thermometer or temperature probe to measure the air temperature of the classroom and record the value in the data table.

3.Open the file “24 Speed of Sound” in the Physics with Computers folder. A graph of sound level vs. time will be displayed.

4.Close the end of the tube. This can be done by inserting a plug or standing a book against the end so it is sealed. Measure and record the length of the tube in your data table.

5.Place the Microphone as close to the end of the long tube as possible, as shown in Figure 2. Position it so that it can detect the initial sound and the echo coming back down the tube.

Figure 2

6.Click to begin data collection. Snap your fingers near the opening of the tube. You can instead clap your hands or strike two pieces of wood together. This sharp sound will trigger the interface to begin collecting data.

7.If you are successful, the graph will resemble the one below. Repeat your run, if necessary. The second set of vibrations with appreciable amplitude marks the echo. Click the Examine button, . Move the mouse and determine the time interval between the start of the first vibration and the start of the echo vibration. Record this time interval in the data table.

8.Repeat the measurement for a total of five trials and determine the average time interval.

All results will be recorded by each individual student.

DATA TABLE

Length of tube / m
Temperature of room / °C
Trial / Total travel time (s)
1
2
3
4
5
Average
Speed / m/s

ANALYSIS

1.Calculate the speed of sound. Remember that your time interval represents the time for sound to travel down the tube and back.

EXTENSIONS

1.This experiment can be performed without a tube. You need an area with a smooth surface. Multiple reflections may result (floor, ceiling, windows, etc.), adding to the complexity of the recorded data. You could clap two boards together at a measured distance from a wall and find the difference in time between hearing the clap and its echo. Remember to double the distance to the wall to allow for the echo.

2.Use this technique to measure the speed of sound in air at different temperatures. Try this by aiming a heat lamp at the tube or by blowing a hair dryer inside the tube. Record the temperature of the air inside the tube. Why don't you just measure the air temperature around the outside of the tube? What method might you use to get the speed of sound in cold air?

3.Design a method for measuring the speed of sound in a medium that is not a gas. Metal pipe railing, water or other liquid. What problems do media, other than air, have when it comes to measuring the speed of sound in them?