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Speed of sound in air

This version was saved 13 years, 3 months ago View current version     Page history
Saved by James de Winter
on June 12, 2011 at 12:16:57 pm
 

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

(well they were, I'll try and find the link)

 

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
  • Go to FILE --> PREFERENCES  in the audacity menu

 

  • When you select preferences, you will get this box

 

 

  • You need to to two things here
    • 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 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 (plus magnifing 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.
  • 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

 

 

Developments/Thoughts

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 

 

 

 

 

 

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