Acoustic enclosures for machines

Enclosures are undoubtedly the most common means of reducing noise pollution in industry. Even if noise reduction at source remains the priority, they can be found around machines, turbo generator sets, encapsulating motors or forming an integral part of equipment by constituting its casing. The reason for this is that noise levels are still considered too high. For example, a vacuum pump without a cover used in a hospital will produce excessive noise levels for its surroundings, despite the care taken in its design.

Paradoxically, designers who have to design formwork have few tools at their disposal to guide them in their work. Fahy in 1985 explained perfectly in a few sentences the difficulty of finding simple models to perform these calculations: "Theoretical prediction of the performance of enclosures has not been successful to date, and designers still rely heavily on empirical data. There are three reasons for this:

1) the walls of the enclosure and the source are strongly coupled by the fluid, so that the radiation impedance of the source is affected by the dynamic behavior of the cavity;

2) source geometries lead to very complex acoustic fields in the cavity that are difficult to model deterministically;

3) the dimensions of the cavities are not large enough for statistical models to be applied with confidence".

Although computing resources and methods have continued to evolve over the last twenty-five years, the tools available to manufacturers have not. Indeed, the difficulties of producing reliable models remain the same. Design software has become more sophisticated, but requires a great deal of precise data, which engineers generally don't have at their disposal. For these reasons, and even if the conditions are not met (large cavity dimensions), models based on diffuse reverberant acoustic field theory are often employed. There are, however, fairly simple models that can be used in conjunction with each other, depending on the configuration, to give results that are not too far removed from reality. They should be seen as an aid to hood sizing by the engineer, who can thus easily grasp the influence of the various parameters.