This paper presents another application of an images group model for a special enclosure geometry and source orientation. A\nprevious work outlined the concept via application to a special tight-fitting enclosure. Application of the concept to a fan plenum\nrequires different mathematical descriptions for the image groups. This paper describes the sound reverberation inside a sound\nenclosure with mostly open sides where the primary noise sources are the air inlets and exhausts of axial type fans located at the\ntop of the enclosure, the sound transmission through the air inlet openings, and the radiation to wayside positions. The main\nreverberation between the floor and ceiling is determined with an image based mathematical model. The model considers how\nthe main reverberant part image group is amplified by its images from two parallel bulkheads and any side wall frame members.\nThe method of images approach allows the hard surfaces of an untreated plenum to be represented by perfectly reflecting surfaces\nwith zero sound absorption coefficients, thus not requiring any estimate or measurement for these surfaces.Numerical results show\nexcellent comparison to experimental results for an actual plenum.The image model is also shown to be significantly more accurate\nthan the standard large room diffuse field reverberant model.
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