The transmission of aerodynamically generated sound in ducted centrifugal rotor systems is studied in some detail, in order to explain the well-known acoustic characteristics exhibited by these machines.
In this first of two companion papers (Part I) the available data on the noise generated by ducted centrifugal rotors are reviewed, so as to provide the experimental evidence against which the results of analytical work reported in Part II may be compared. It is found that current knowledge on the subject is mainly empirical, and that explanations which depend only on the aerodynamic nature of the sound source will not satisfactorily account for all the known experimental facts. The ducted environment into which the centrifugal machine is forced to radiate is found to exert a much m0re-powerful influence on the observed acoustic characteristics exhibited by the centrifugal machine than hitherto recognized, especially at low frequencies, when the whole ducted system could act as a kind of low-pass acoustic filter. It is suggested that, at low frequencies, the classical lumpedimpedance theory could be applied to analyse the behaviour of the ducted system.