Installation Effects on the Tonal Noise Generated by Axial Flow Fans will be published in Journal of Sound and Vibration

The paper presents the results of experiments on a low-speed axial-flow fan flush mounted on flat panels of different size and shape which are typically employed in tests on automotive cooling fans. The experiments have been conducted in a hemi-anechoic chamber and were aimed at evaluating the installation effects of the whole test configuration, including mounting panel and chamber floor. The panel causes important attenuation in a narrow, low frequency range, which is related to the size of the panel and affects tonal noise more strongly than the broadband one. The propagation function presents an important dip at about 400 Hz and also a regular wavy trend associated to reflection from the floor. The analysis is performed by means of an existing spectral decomposition technique and a new one, which allows to consider different noise generating mechanisms and also to separate the emitted tonal and broadband noise from the associated propagation effects. In order to better identify the features of the noise at BPF harmonics, the phase of the acoustic pressure is also analyzed. The new technique is based on SPL spectral data measured during speed ramps, which allow obtaining both constant-Strouhal number SPL data, to which the new spectral decomposition technique may be easily applied, and constant-speed data, to which the standard spectral decomposition techniques may be applied. Based both on the similarity theory and on the analysis of Green’s function of the problem, a theoretical description of the structure of the received noise power spectrum is given. This allows to determine the conditions under which the spectral decomposition may be performed avoiding approximations. Then, the possibility of discriminating between tonal and broadband noise generating mechanisms is analyzed and a theoretical base for the new spectral decomposition technique is provided.