Prev: A constant speed of light in all reference frames? Surely youcan't be serious.
Next: Solar wind and planetary drift
From: Tom Roberts on 24 Feb 2010 09:30
Marshall wrote:
> [... measuring the speed of sound from a moving source]

Get a pair of microphones, some long cords for them, and a laptop with stereo
microphone in. Go set up near a train track, with the microphones separated as
far as possible alongside the track. Wait for a passing train. Use either its
basic noise, or its horn.


Tom Roberts
From: Tom Roberts on 24 Feb 2010 23:50
Zinnic wrote:
> On Feb 24, 8:30 am, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote:
>> Marshall wrote:
>>> [... measuring the speed of sound from a moving source]
>> Get a pair of microphones, some long cords for them, and a laptop with stereo
>> microphone in. Go set up near a train track, with the microphones separated as
>> far as possible alongside the track. Wait for a passing train. Use either its
>> basic noise, or its horn.
> I fail to see how this set up would allow one to assess more than the
> speed of the train. Explain how your separated microphones can be
> used to assess the speed (in air) of the sound wavefront emitted as
> the train approaches and recedes.

It should be obvious: Set up in a place such that the train will blow its horn
while approaching both your microphones, as you are recording them. From the
recording you can easily determine the time the horn is first heard by each
microphone. From that time difference and the distance between the microphones
you can compute the speed of the horn's sound in the air.

You could also do this just from the running noise of the train,
but that would probably require a correlation analysis to
determine the time delay. If you don't know what this means,
ignore it and be sure to use the horn.

For a 20-foot separation (easily achieved), the time difference will be roughly
20 milliseconds, which for recording at 44.1 kHz is more than 800 samples. That
implies an accuracy of perhaps 0.2% or about +-2 ft/sec -- trains move much
faster. You must measure the separation to better than 1/4 inch to achieve that
accuracy. Larger separation would permit better resolution. Try to find fast
trains. You could also do this beside an empty country road with a friend
driving a car and blowing its horn at the right place. Be sure to measure the
wind, temperature, humidity, and barometric pressure.


Tom Roberts
 | 
Pages: 1
Prev: A constant speed of light in all reference frames? Surely youcan't be serious.
Next: Solar wind and planetary drift