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What's so special about this new radio experiment?

To try to illustrate the difference between the two main methods for meteor radio detection - between a simple echo monitoring experiment and one which performs a frequency analysis of the echo - Jean-Pierre Lebreton performed the following simple experiment:

I recorded on a simple tape recorder the sound of a scooter (motor + horn) approaching and passing by me. I made two similar recordings. The two consecutive recordings have been transformed in a single audio *.wav file.

Listen to the WAV. file

I then analysed the wav file with the audio analysis programme made available at http://www.monumental.com/rshorne/ and the result of the analysis is shown in the image below.

Click for a larger imageClick for a larger image
The top plot shows the simple time sequence of the recorded sound. The bottom plot shows the spectral analysis of the signal. The motor sounds for the first 4 sec; then the horn sounds between 4.5 and 8.5 sec. The scooter passed in front of me at 7 sec.

The horn spectrum is rich in harmonics. Note the change in tone (frequency) at about 7 sec. Before then the scooter was approaching me; after that the scooter was going away from me.

The experiment is repeated in the second part of the Figure.

You can clearly note that the frequency increases when the source is approaching the listener (me) and the frequency decreases when the source moves away from the listener. Note the big frequency change around the time of the velocity direction change with respect to the listener.

Measuring the relative change in frequency before and after passing by me, you can deduce the speed of the scooter ( try it and you should get about 9 m/s, assuming a sound speed of 330 m/s).