Lab Instructions

Objective: To determine the speed of sound in air.

Method: The speed of sound is a very important number. It determines the pitch of wind instruments, the time lag for echoes, and is important in the acoustics of concert halls.

We will send sound waves down a pipe by clapping near the pipe's end. The sound travels down the pipe and reflects from the other end of the pipe. The WavePad app on a cell phone will be used to detect this returning sound wave and show a graph of it on the screen. The up and down line on the graph screen shows variation in air pressure in the sound wave. (Note that the sound wave will reflect from the far end even though this is an open end.)

The graph on the phone screen will show a large response when the sound wave is first created and passes by the phone's microphone as it enters the pipe and later another large response when it arrives back after reflecting. The sound wave then reflects at the end with the phone and goes back down the pipe again and reflects again. This way we see multiple peaks each time the sound wave returns to the phone as the wave "bounces" back and forth. The time scale in the app, given in milliseconds (ms), will allow us to measure the time that it takes the sound wave to move down the pipe and return. Knowing the distance traveled and the time of travel, we can calculate speed.

To see the very quick rise and decline of sound waves, we will need to expand the time scale, spreading out the wave so we can see the details of detection and reflection. To allow enough time separation between the waves, we need a pipe that is at least 10 feet long.

Questions: 1. Did it matter how far from the pipe the clap was done? Why or why not? 2. Did it matter how far from the pipe the phone was located? Explain. 3. Suppose the pipe was stored outdoors during a cold winter's day, and then brought in, and the experiment was then done immediately. Would anything change? Explain. Procedure: 1. Our pipe is 10 ft = 120 inches long. the standard for PVC pipe. and this is L for this experiment. Convert this into meters. Enter only SI metric values into your data table. 2. Complication: The reection at an open end actually happens a small distance beyond the physical end of the pipe. This extra distance is 0.3 times the inside diameter of the pipe and the actual length of the pipe must be corrected by adding this amount. This standard PVC pipe has an inside diameter of 1.5 inches. Calculate this correction (uLl and convert it into meters. 3. Add the correction to the L and then calculate twice the pipe's corrected length in meters. This is the distance the sound wave travels. 4. Examine the image (below) of the phone screen with the graph of the sound wave on it. This has been expanded to show the time scale in as much detail as possible. The rst set of long vertical lines on the left represents the detection of the rst burst of sound waves as it passes the microphone on the way into the pipe. Find the largest vertical line (representing the maximum wave amplitude) and determine when (according to the time scale in the app) it moved past the microphone in the phone. To do this. follow that largest line down to the time scale and record the time in your data table as t1. This may not line up exactly with one of the time markers on the screen. You will probably need to estimate the fraction of the distance between two given time markers. For example. if the line in question is half way between the 120 and 125 markers. then the time is 1225. Another example: if the line is 2Z10 of the way between 160 and 170. then the time is 162. The units given are ms (milliseconds). (Reading "between the lines" is known in science as interpolation.) Sometimes the time is given in seconds. Note that 0.300 5 equals 300 ms. 5. Note the next group of vertical lines representing when the sound returned after being reected at the far end. You can see that the lines are a little shorter than the rst group because not all of the sound reected back; some escaped out into the space beyond the pipe. 6. Remember that we gain in precision when we measure larger quantities. In this case. instead of looking at the time over one interval (from the rst wave to the second wave]. let's look at the time interval for multiple waves. Remember. we are counting the number of intervals between the waves spikes. not the spikes themselves. (From the rst spike to the second spike is counted as "1."l Each interval represents one round trip for the sound wave. to the other end and back. Record the number of waves (intervals. not spikes). 7. Determine the time aftera number of wave spikes. which we'll call t2. 8. The amount of time t for all the reections and movements is the difference t = t2 - t1. Calculate this difference. Convert this time into seconds is): it will be a small decimal. 9. Calculate the total distance the waves traveled for the entire motion. including reections and back and forth travel. 10. Calculate the speed of sound: 2.! = 11. The next step is to determine the standard (accepted) speed of sound. The speed of sound in air depends on the temperature of the air. The temperature in the room where I did the experiment was 25.0C. measured using a small thermometer. The speed of sound is found using the formula. 1; = 331 + 0.6 - Tc where Tc is the temperature in degrees Celsius: the result is in ms. Calculate the speed of sound under the conditions of the experiment. 12. Calculate the percent error of your results. using the calculated speed of sound from step 10 as the standard value. Note the abbreviations in the data table. "stan" means standard (the standard or accepted value for this temperature): "exp" means experimental (your result found from distance and time measurements). The formula for percent error can be found in the "Lab Information" module. 13. Completely optional: If you happen to have a 10 ft piece of pipe and want to try this by yourself. it might be fun and shouldn't be too hard. You'll have to download the free WavePad app and experiment a little with it to see how it works. Here is a screen shot of my phone with graph from the experiment: WavePad \"Untitled 17.an Upgrade Now Home Edit Levels Effects Tools Vi. New File Open Save Export ' 0:00:00.26 ' "I\"'f :' ':\" $GNUP Advertise to Local Customers -