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Posted by Michael McNeil on December 6, 2003 10:25:37 UTC

The original work done by Professor(?) Doppler was in Victorian times with the classic display from the observation of locomotives.

The effect is far from so pronounced (and can be said to be contradictory) when observations are made with helicopters.

There are of course fundamental differences mostly concerning traction. The helicopter's sound output varies to compenate for variations in traction as the pilot adjusts engine speed. This in turn varies the intensity of what we are observing as variations in rotor speed compress and rarify the amosphere.

Locomotives on the other hand are more like set pieces. They run on nicely calculated well maintained tracks whose sleepers are spaced with scientific precision.

Their drivers know exactly how fast they should be going on every point. Observations can be made at leisure and calculations based on data that has been controlled to allow for wind speeds etc.

To make the same contolled observations with helicopters would require wind tunnel conditions. Unfortunately the design of the wind tunnel could not mirror the shape of the laboratory in which stars shine.

What I'm trying to say with all this waffle, is that their harmonics are different.

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