On the balance between the tonal and broadband noise of uninstalled propellers

This paper provides a preliminary theoretical investigation of the balance between rotor alone tonal and broadband noise. First, using well-published theory, the different source mechanisms for tonal and broadband noise are discussed and presented. Then, using these theoretical results, an extensive parametric study has been performed in which the propeller thrust, diameter and blade solidity have been maintained constant, but the blade tip speed and blade numbers have been allowed to vary. In order to account for the variations in noise against directivity and, simultaneously, across the audible frequency range, the data are presented as Overall Sound Pressure Levels (OASPLs) and as Overall Acoustic Sound Power Level (OSWLs) integrated over the frequency range. Using these collapsed data metrics, it is found that the tonal noise dominates at low frequency and/or low tip speeds but that broadband noise is the major contributor at high-frequencies and low tip speeds. The results show a clear distinction between the regions dominated by tonal contributions and those dominated by broadband contributions. These results are discussed and related to the different tone and broadband source physical mechanisms. In addition to the primary parametric study, the paper also investigates, separately, the effects of changes in blade chord and propeller thrust. The studies presented in this paper are intended to serve as noise guidelines for propeller preliminary design