The method for acoustic measurement and documentation of pipe organs d
eveloped in 1991 in VUZORT, Prague enables to describe the sound of al
l organ pipes and plena measured in situ. The real time measurement an
d digitalisation of the sampled sound, computation and processing of t
he amplitude spectrum are possible without necessity of sound recordin
g. Sampling frequencies were adjusted to the stop length and fundament
al frequency of the measured tone. The results of factor analysis of t
he octave spectra of Great organ plenum for 12 instruments are describ
ed. The Czech Baroque organs created a distinctive group in 3-dimensio
nal factor space due to their rich harmonic spectra.