Calculate speed from sound recordings using Doppler shift - RC Groups
Sep 14, 2010, 12:11 PM
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Calculate speed from sound recordings using Doppler shift

Of course there is a program that does this fairly automatically: WaveOScope.

UltimaSound II (freeware) has a sound spectrograph which looks similar to WaveOScope.

If your curious this is the math:

Speed of object = Speed of Sound * (High Frequency – Low Frequency) / (High Frequency + Low Frequency)

346 * (3768 – 3122) / (3768 + 3122) = 22 m/s = 50mph

Note that for accurate results you need to take a reading of the object moving DIRECTLY towards you, then DIRECTLY away from you. So on a high speed pass there should be a gentle turn (to avoid smashing you)

You should also be flying fairly level, because if you're in a dive (accelerating) you will be increasing the pitch of the motor with faster and faster rpms which will screw up the whole speed calculation which is Dependant on a constant pitch from the source as it moves by. In other words, you need a constant speed and rpm from the inbound and outbound sound samples.

Also, the speed of sound is dependent on the air temperature, though the accuracy doesn't affect the calculations very much.