The Whisper instrument provides two functions : (i) the measurement of the total density of the plasma, via an active radio frequency technique, and (ii) the continuous survey of the natural plasma emissions in the 2 kHz to 80 kHz frequency band. Each of those will serve the three dimensional exploration of the magnetosphere carried on by the Cluster mission. The absolute density values recorded from the four spacecraft will form a reference for other techniques. They will be converted into gradient vector quantities to be used for further studies. The frequency/ time spectrograms of the natural emissions can be sorted as electromagnetic emissions when present on all four spacecraft, or electrostatic ones, when seen only from a given satellite. Compared direction findings from each spacecraft can be used for remote studies about the sources of electromagnetic emissions.