Tracking Huygens Signal
The sound is a tone whose frequency depends on the strength of Huygens' signal as received by the receiver on board Cassini. This in turn depends on the distance between Cassini and Titan, and on the angle Cassini is seen from the probe.
Radio energy emerges from Huygens' antenna in a slightly nonuniform pattern - like the petals of a flower rather than as a smooth shape such as a circle, or ellipse. Thus the rapid variations in the tone indicate changes in Huygens' orientation due to both its steady spin (the rate of which changes during descent) and swinging motions underneath the parachute.
The motion becomes slower and steadier later in the descent. After landing, the tone has much less richness, since the probe is no longer moving, but there is a slow change due to Cassini flying through the lobes or 'petals' of the antenna pattern. Just before the end, the weakening signal even drops out for a moment and returns. Overall, the signal was very robust, with lock not being lost during descent.
Huygens investigators will use this data, together with tilt sensors and accelerometers on the probe, and its camera DISR, to reconstruct the probe's attitude motion during descent.
The sound file was made by Dr Ralph Lorenz, an Assistant Research Scientist at the University of Arizona and a co-Investigator on Huygens' Surface Science Package, from data formatted by Miguel Perez of ESTEC.