Keywords: boundary element method, closed space, acoustics, image-source.

Noise produced by equipment, such as heating, ventilating and air-conditioning (HVAC) units, is often a major concern within cities and urban areas. In many cases, those devices are installed outside the buildings, either on the roof or at specific spaces that may be acoustically treated to minimize their negative influence. Some of the noise control procedures used to attenuate the ambient airborne noise emitted by those equipments include the use of an enclosing space, built around them with the specific objective of reducing their contribution to ambient noise. However, this equipment usually requires ventilation openings, which may or may not be acoustically treated, but that constitute an acoustical weak point in the protective structure.

In the present work, the authors make use of numerical and analytical techniques to model the propagation of sound generated within such spaces by equipment, which propagates to the external space by the ventilation opening. The specific case of a parallelepipedic space with a rectangular opening is analyzed. For this purpose, a numerical model based on a three-dimensional boundary element formulation is used. For its application, the authors make use of domain decomposition together with Green's functions specifically defined to reduce the size of the involved system matrices and to allow consideration of surface absorption. Those functions are derived using the image-source technique, and automatically account for a rigid floor or absorptive floor (both inside and outside) and for other internal and external vertical surfaces. The numerical model is verified with an analytical solution known for the case of a point load acting within a parallelepipedic space, and is validated by comparing its results with those obtained experimentally.

The model is then used to simulate a number of test cases. Sound pressure levels are computed for different scenarios, in which different protective measures are used to attenuate the noise produced by a point source, simulating an equipment, near a building. Both an enclosure with a small opening and a simpler set of three barriers, defining an open protection device, are tested. Results reveal that different efficiencies can be obtained, and that the presence of absorbing surfaces within a protective enclosure can improve the performance of such devices.

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