Focus

ACTIVE NOISE CONTROL APPLIED TO BUILDING ELEMENTS

The need to improve the acoustic performance of the indoor premises and the will to overcome the limitations imposed by traditional techniques forced ITC-CNR to apply active noise control to building elements such as windows and vertical partitions. Active noise control (ANC), in which additional secondary sources are used to cancel noise from the original primary source, is based on the principle of destructive interference.
ITC-CNR designed two prototypes of an active window and of an active opaque partition respectively. These prototypes were tested both in laboratory and in an experimental building.
The components conceived do not have any limitation for what concerns their integration in different building technologies with respect to the traditional systems.
The active window is a standard-size window modified in order to house microphones and loudspeakers on the perimeter of the intrados. This configuration controls the disturbing noise passing through the open window.
The active opaque partition consists in an external panel acting as a support for the loudspeakers and has been conceived to be inserted in double partitions used to separate different offices.
The heart of these configurations consists in a commercially available digital board, which allows to set up an active noise control system, based on a FXLMS algorithm. Several control parameters such as number of inputs and outputs, filter taps, factors of convergence and leakage, can be varied.
In 1992 Murst (the Italian Ministry of Scientific Research and Technology) accepted the project and inserted it into a wider plan. Due to problems in other part of the plan, the project started only in 1999.

Conclusions and possible applications
The good performance of the open window ANC presents this configuration as the only possible solution to a common need: insulation without having to live in a closed box. In the Mediterranean region most people would enjoy keeping the windows open in spring and summer without being annoyed by incoming noise. In fact, in areas near airports, railways, highways and industries a lot of people complain about having to keep all the openings closed in order to prevent noise from entering: they feel it as a discomfort.
The active opaque partition is mainly addressed to offices located near disturbing sources when light mobile partitions have to be used.
Although the best results were achieved in presence of narrow band noise sources, this doesn't limit excessively the field of application. Actually these devices could be applied when noise emitted by transformer, conditioning systems, blades, rotors and engines in continuous running must be controlled.
Although the Murst plan ended, the research is still going on under different projects.

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