Green’s functions, vibrational response, structure – borne noise
Reduction of noise in a compressor is a complex criterion as many factors of machine enclosure contribute its effect on noise. When a panel of enclosure is acoustically excited, its vibrational response comprises both forced vibrational response at the excitation frequencies, and resonant response of all the relevant structural natural frequencies. These are excited due to the interactions of the forced bending waves with the panel boundaries. The non-resonant, forced modes tend to transmit most of the sound at frequencies below the critical frequency. The resonant frequencies below critical frequency are very poor sound transmitters or radiators. Thus, at frequencies below the critical frequency, mass of the panel controls the reduction in sound transmission since the low frequency resonant structural modes do not radiate or transmit sound. Above the critical frequency, it is the resonant modes that transmit most of the sound. The one qualification to the phenomenan discussed here is that the incident sound field has to be diffused – i.e. no acoustic standing waves are present in the fluid medium adjacent to the panel. Each structure exhibits certain low and high frequency response. The response of a panel depends on whether it is mechanically or acoustically excited. In this paper, acoustic enclosure for a compressor model is designed and analysis of the enclosure structure is carried out. A method is presented to reduce noise by structural modifications of enclosure. Efforts have been made to make use of experimental data as input to software and methods to do the validation of output results which matches fairly with the experimental data. In acoustic fluid-structural interactions, the structural dynamics equations were considered along with Navier-Stokes equation of fluid momentum and flow continuity equation. The radiation of sound from body can be formulated in terms of an integral equation involving Green’s functions with an imposed radiation. Green’s functions represent solutions to the wave equation – they can also be considered to be either frequency or impulse response functions between the source and receiver. Modeling of enclosure was done. The material used for panels of the enclosure was made of composite steel. The noise measurements on 10 mounting points where compressor comes in contact with the structure were carried. The results were plotted in both frequency and time domain; i.e. dynamic analysis of structure. The analysis of the structure was carried out in NASTRAN. The measured forces were applied to the enclosure structure at mounting points. Response was then measured at desired points. Authors found that, there were some variations in actual measurement data and the results acquired in software analysis. Guiding phenomenon was implemented to reduce the noise i.e. obstacles to the wave propagation were provided. The points where noise level was maximum than the required, barriers, and ribs were provided, etc. Again noise measurements were done. Numbers of iterations were performed until measured noise data tallies with analysis data in the prescribed limit. This Design approach can be used for similar problems involving structure – borne noise sources.