|MadSci Network: Physics|
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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