The French "Basson" and the German "Fagott" are two descendants of the baroque bassoon. They evolved differently by the addition of tone-holes, the elongation of the main bore, and the modification of the reed shape. These two cousins are played with different fingerings and have a slightly different sound color: the French Basson having the reputation to have a more nasal and less homogeneous timbre along the pitch range. The aim of this presentation is to identify the sound differences between one Fagott and one Basson and to relate them to their geometry through acoustic considerations. The two studied instruments have been played by a professional Fagott player, familiar with the French Basson. Each bassoon is played with two reeds: its own cane reed and a unique plastic reed, all equipped with a pressure sensor. The external sound and the reed pressure are recorded for musical excerpts and chromatic scale allowing, for each signal, the computation of the mean spectrum over specific note ranges (e.g. first register). These recordings are completed by a set of geometric measurements (main bore and holes) and impedance measurements for each fingering of both instruments. Some notes are also played by an artificial mouth on both instruments with similar control parameters, avoiding the adaptation of these parameters to the played instrument by the musician. This set of measurements allows the quantification of the difference between the two instruments in terms of acoustic properties and radiated spectrum. In addition, it gives the possibility to compute the transfer function between the reed spectrum and the external sound spectrum. The evolution of this quantity along the frequency axis can be related to manufacture elements such as the length of the holes' chimney, the dimensions of the radiating openings and the associated radiation impedance, the tone-holes lattice, etc. This transfer function is also computed from the geometry of the instrument by using simple wave propagation model.