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MODELING

Modeling of a production hall can be quite tempting for two purposes:

  • • Assessing the efficiency of the absorptive treatment (e.g., by computing the spatial sound level decay in the empty hall)
  • • Assessing the noise levels in the hall due to the operation of various pieces of equipment (this will help point out the main noise contributors, as well as pave the way for noise reduction solutions)

While it does not seem too complicated a prospect when dealing with the empty hall (whose shape usually is rather regular), things get pretty complicated with a fully fitted hall due to the numerous fittings to be found. Computation models and strategies can be found in [14].

Sabine Modeling

Sabine modeling is a very simple way to model a hall. However, Sabine’s theory supposes a volume with a rather diffuse sound field and similar dimensions in the three directions. More to the point, it does not take into account a significant difference of treatment of a given surface (e.g., the ceiling) compared to other surfaces.

When looking at a real hall, the volume is usually considered flat, as the height is much smaller than the other dimensions. More to the point, usually there is only one surface treated to acoustic absorption, which is the ceiling.

The sound level Lp at a distance d from a sound source of sound power level Lw and directivity Q can be estimated using the formula

where R = A S/(S - A), with A equivalent to the absorption area of the hall and S the total area of the interior envelope of the hall.

Sabine modeling will overestimate the efficiency of the absorptive treatment. When performing a round-robin test on various prediction models in 1988 in an empty or moderately fitted hall, it was found that on a 30 m long, 3.8 m high volume, the predicted noise level at 20 m from the source could be underestimated by as much as 5 dB(A) [16].

Ray Tracing Modeling

It is not uncommon for production halls to feature shed-like roofs so as to benefit from natural lighting. More to the point, there often is much diffusion to be found on the walls, and even inside the hall, due to numerous fixtures (e.g., shelves, production equipment) being present.

A ray tracing model can help tackle those issues. Of course, compared to a Sabine model, things are slightly more complicated, as the dimensions and location of each surface must be entered [17, 18].

Regarding diffusion, an original solution has been proposed by INRS [17]: The sound ray is simply reflected in a random direction to take into account diffusion. Experience shows that it does work [19]. More to the point, some of those models have now started to include diffraction effects [20].

 
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