Determine the Speed of Sound

Introduction: Determine the Speed of Sound

I will go through a process that will allow you to determine the speed sound travels. This is a classic physics lab that in the past would have required lots of expensive equipment. The only expensive equipment involved here is a computer with a microphone input. Which by itself can be expensive, but most classrooms these days have at least one in the room.

Step 1: Stuff You Need

1. Computer with microphone input
2. Microphone you can plug into the computer
3. Long tube. I typically use discarded carpet tube (which is free), but you can use PVC pipe. 1.5 m to 2 m in length is best.
4. An audio recording program. I recommend Audacity. []

Step 2: Record Your Sound

Stand the tube up on a hard surface or place against a wall. Hold the microphone right at the opening of the tube. Start recording in Audacity. Snap your fingers near the microphone. A weak, pathetic snap tends to work better than a strong one. Snap a few more times so that your odds of having a good one will increase.

Note, you want your hand above the microphone to get the most precise answer.

Step 3: Zoom In

Click and drag over one of the peaks and zoom in. The easiest way is to click the "Zoom to Fit" button.

Step 4: Zoom in Again

Now you should see two sets of peaks. If you still only see one, highlight a smaller area and zoom in again.

The first set of peaks is your snap. The second is the echo of your snap.

Highlight just the two sets of peaks and zoom again.

Step 5: Really Precise Stopwatch

Now we can easily see the recording of our snap and the echo of our snap. All we need to do now is determine how long it took the sound to travel from the mouth of the tube to the floor and back again.

The easiest way (as pointed out by one of my students) is to click and drag from the beginning of the sound to the beginning of the echo. If you look near the bottom of the window you'll be able to see how much time you've highlighted.

For mine I get a time of 0.009057 seconds. I figure four significant figures is pretty good. Now I simply measure the length of my tube, which in this case was 1.548 m. Since the sound has to go there and back again I double this distance (almost everyone forgets to do this the first time).

Speed can be calculated by dividing distance by time.

So v = d/t; v = 3.096/0.009057

v = 341.8 m/s or 764.6 mph

The accepted value at sea level is 340.29 m/s (the value in my classroom can be anywhere between 340 m/s and about 348 m/s depending on the weather).



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    14 Discussions

    question(s) - 1)what if you wanted to measure the distance from the object(that created the sound) to the microphone that captured the sound ? 2)what if you placed two or three other microphones in the room in each corner, could you use them as well ? or would they cause any foreseeable problems in this project ? Im trying to pinpoint the distance and vector of an orignating sound. Is this possible ?

    2 replies

    I've not tried using multiple microphones with Audacity before. I'm not sure you could do that. What you could do would be to wire two mono microphones to the stereo input and record as a stereo track though.

    i am still a student taking up bs physics!! just want to ask wer can f find audacity program in our computer system! pls help me tanx

    1 reply

    well i dont know what the hell you just asked but if i get what your saying, no audacity does not come standard on your computer. google "audacity download" and it should take you to the audacity homepage. just download the version that your system requirements meet.

    Why do people assume you need high-tech equipment to do decent science? You can calculate the speed of sound with two lumps of wood, a stopwatch and a convenient wall.

    3 replies

    How long do you think you can get away with having a comment like this and no Instructable showing us how?

    Doh! OK, I know what I'm doing with my Science Clubbers tomorrow night...

    You're right, you don't need high-tech equipment to do decent science. In many cases I do very low tech labs. It all depends on what I'm trying to teach. The tech tools can often be more engaging for students, which is one of the reasons I use them. Additionally we use Audacity for some later labs and this provides a great introduction to the program.

    Makes me wonder two things. 1) Is there anything open source can't do? :) 2) How'd they do it before the computer made it this darn easy?

    2 replies

    It is also worth pointing out that the pulse comes back inverted - that first "down" peak comes back as an "up" peak - since the end of the air tube can be considered as fixed (cement has larger impedance than the air). If you can detect a reflection from the end just held up in the air it may be erect (assuming air not surrounded by the tube has a lower impedance that air in the tube). Neat!

    1 reply