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Speed of sound in air

Page history last edited by James de Winter 5 years, 8 months ago

Other versions are available, eg  here :http://www.cs4fn.org/physics/speedsound.php

This link comes from Ian Galloway who I seem to recall showed me this method a few years ago.


NOTE: newer versions of audacity have a sample count at the bottom rather than a timer. Annoying. Just set the sample rate to a multiple of 1000 and then convert from samples to seconds. Not ideal but works.


Measuring the Speed of Sound in Air Accurately for 99p


You need

1) A cheap pair of stereo headphones (spend more if you want but I don't care)

2) A computer with Audacity installed on it, it's free and GREAT

3) Something that can make a clear sharp noise. [I have found a metre ruler slapping on a desk is best]


What to do to get ready

  • Install audacity
  • Plug your cheap headphones into the MICROPHONE socket of the computer (usually pink)
  • Go to FILE --> PREFERENCES  in the audacity menu


  • When you select preferences, you will get this box



  • You need to to two things here on the AUDIO I/O tab
    • a) Make sure that the RECORDING DEVICE is set to the headphone socket - I cannot tell you what this one is as all kinds of computers are different. Just set it to one of the options and then click record, shout into the headphones and see if you get a trace. You need to shout in both headphones to see that you have a stereo signal.
    • b) set the input to 2 (STEREO)


Doing the Experiment

  • Pull the stereo headphones as far apart as you can (15cm apart can work but the further apart the better)
  • Stick them to a metre ruler with the headphones facing the same way
  • Start recording
  • Make a sharp loud bang to one side of both headphones
  • Stop recording
  • You should get something like this, that has two spikes, one bigger than the other (can you guess why?)




  • Now, they look the same but when you zoom in (+ magnifying glass at the top), you see that one spike happens earlier than the other




  • ahh, you can see where this is going...
  • It's a bit tricky to be clear about the spikes but what you want to do is highlight the section between the first spike of the top graph and then the same one on the second one. This is why there needs to be a clear, sharp noise. The image above is a bit rubbish, it can be much clearer.
  • Now, if you look at the little time box at the bottom of the screen you will see that Audacity tells you the time between the two points



  • so, as you know the distance between the headphones and the time it took the sound to get from one to another, you can do some sums and get a speed of sound in air that is pretty close and reliable.




  • Ideally you should hang the headphones in air so you don't get a speed of sound through the table value.
  • I never bother and always seem to get 330m/s ish so haven't changed the way it do it.
  • I really want to set a pair of headphones in jelly and then find a way of measuring the speed of sound in jelly, maybe one day!



Photo of setup

SASP Physics group 2009/2010 

The chap on the right is James F who came up with the groovy wind turbine thing. 




Troubleshooting/some common problems

  • Some laptops (i've got one) and possibly desktops (haven't had this problem but might be one)  do not have a stereo input on the sound card. As such whatever you do it will NEVER work, sorry.
  • Some laptops have stereo build in microphones and so you get results (i.e. different traces)  but a bit rubbish - fiddle with the audio preferences I/O stuff mentioned above and it may work
  • Some windows computers have the microphone input set to mute within the windows settuing. Click (probably right click) on the little speaker icon in the bottom left hand corner and have a rummage in the settings. You are looking for a MUTE tickbox ticked - but I guess you worked that out.
  • I know I put it in capitals, but did you plug the headphones into the headphone socket by mistake - it doesn't feel right to put them into the microphone one.




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