MadSci Network: Physics

Re: what is a good way to test the speed of sound in air?

Date: Mon May 21 17:49:55 2001
Posted By: Denni Windrim, Director of Education, Sylvan Learning Centre
Area of science: Physics
ID: 987709950.Ph

Well, it depends on how fancy you want to get. You are helped by two things: 
both light and electricity travel at speeds orders of magnitude above sound. 
The simplest thing I can think of is to have you and a partner work 
together. After measuring off a distance along a clear line of sight, say 
two or three kilometres, your partner stands at one end and you atand at the 
other. Your partner simultaneously flashes a bright light in your direction 
and creates a loud sound with something like a starter's gun. When you see 
the light, start the stopwatch. When you hear the sound, stop it. Since 
sound travels roughly 343 m/s, a two-kilometre gap will give you enough 
distance that, if you repeat this experiment several times, you should get a 
rough estimate of the speed of sound. Also, since you are correct about air 
pressure having an effect on the speed of sound, you might want to do this 
experiment over several days.

If you have the money, the wire, and the expertise, you could get a more 
accurate measurement by connecting the light, the sound source, and your 
timer electrically. When your partner pushes a button, the light and the 
starter's gun go off together and also starts the timer at your end. When 
you hear the sound, turn the timer off. But that's a lot of wire. 

In both cases, taking human reaction time into account is important. From 
the time you hear the sound until the time you press the stopwatch button 
may be between an eigth to a quarter of a second, so make sure to take that 
into consideration. With that, you should be able to pin down the speed of 
sound at your location to a decent 10 percent, plus or minus. 

I think this could be lots of fun, and a great reason to get out into the 
fresh air!

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