Abstract
This paper presents another application of an images group model for a special enclosure geometry and source orientation. A previous work outlined the concept via application to a special tight-fitting enclosure. Application of the concept to a fan plenum requires different mathematical descriptions for the image groups. This paper describes the sound reverberation inside a sound enclosure with mostly open sides where the primary noise sources are the air inlets and exhausts of axial type fans located at the top of the enclosure, the sound transmission through the air inlet openings, and the radiation to wayside positions. The main reverberation between the floor and ceiling is determined with an image based mathematical model. The model considers how the main reverberant part image group is amplified by its images from two parallel bulkheads and any side wall frame members. The method of images approach allows the hard surfaces of an untreated plenum to be represented by perfectly reflecting surfaces with zero sound absorption coefficients, thus not requiring any estimate or measurement for these surfaces. Numerical results show excellent comparison to experimental results for an actual plenum. The image model is also shown to be significantly more accurate than the standard large room diffuse field reverberant model.
Highlights
The propagation of fan noise in a plenum with large side openings requires an accurate method for modeling the reverberation in an acoustical enclosure where air openings are present
The reverberation estimate is performed with zero sound absorption coefficient for all hard surfaces and without having to estimate or measure the initial hard wall sound absorption
The model considers how the main reverberant part defined between the parallel surfaces of plenum floor and fans/heat exchanger ceiling is amplified by a double summation of images from two parallel bulkheads’ reflections, its images from combined bulkheads, floor/ceiling reflections, and a unique factor accounting for possible multiple reflections from the acoustically hard surfaces of the side wall frame members
Summary
The propagation of fan noise in a plenum with large side openings requires an accurate method for modeling the reverberation in an acoustical enclosure where air openings are present. The previous work by the author [10] outlined the basic group images model concept, applied it to a close-fitting enclosure, and demonstrated its differences from a large diffuse room Sabine type of enclosure Both a tightfitting enclosure with one nearby wall being an area noise source and an open-sided fan plenum enclosure with a point source have the special geometries and source orientations that do not generally fall into the class of large diffuse room Sabine types. As previously presented by Panza [10] for a tightfitting enclosure, the method of images approach for an unconventional tight-fitting enclosure is applied here to an untreated plenum containing no sound absorption with hard surfaces represented by perfectly reflecting surfaces with zero sound absorption coefficient. Radiation from the plenum side wall opening to wayside positions is computed with the Hohenwarter [11] formula which models a rectangular plane surface as Inormalcos2(θ) and represents an accurate compromise between monopole-like and dipole-like radiation
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