This paper addresses the characterisation of cello sounds. In order to study to which extent a jury perceives differences between two cellos in playing conditions, we carried out two blindfolded hearing tests, involving two instruments and two professional musicians: (1) An evaluation test of the couple musician-instrument on a structured scale, for 5 different attributes defined by adjective-pairs. The assessment of the jury was based on the same musical sequences, played by the musicians; (2) A comparative test, based on the ranking of the couple musician-instrument on 4 different attributes. The same short musical fragment was played successively by the musicians. All the played musical sequences have been recorded, and metrics based on the acoustic signal (playing frequency, spectral centroid, signal/noise ratio) were calculated in order to interpret the perceived differences. The results show that for the evaluation test, the inter-subject differences of the jury are too large and do not allow the definition of a significant instrument effect. For the comparative test, the agreement between the subjects is better and significant differences between the instruments and the musicians can be observed, explained by the signal processing of the played sounds.