Articles techniques et présentations

The ideal way to measure the acoustic performance of heavy items such as vehicles or machines is in a semi-anechoic chamber, i.e. in an environment with no reflections except from the solid floor where the noise source is standing. To accurately measure low frequencies down to 20 Hz (lower limit of the audible frequency range), the walls and ceiling of the room should be non-reflective down to 20 Hz.

This is a practical challenge as usual (passive) rooms involve absorbing material (wedges) on the anechoic walls, leading to a lower cut-off frequency related to the wedge thickness: the lower frequency limit corresponds to a thickness close to a quarter of the longest wavelength to be absorbed. For example, a typical wedge length of 0.9 m gives good measurements down to around 100 Hz. An anechoic chamber accurate to 20 Hz would therefore require more than 4 m thick wedges to provide adequate low-frequency absorption: such a test facility would be quite difficult to build and excessively expensive. An alternative has been proposed: the DADA (Dome Anti-Diffraction Acoustique). This hybrid control approach combines a thin layer of absorbent (passive) materials with an active system driving a set of loudspeakers designed to cancel out the low-frequency pressure field scattered by the walls.

In this paper, we present the scattered field control strategy for room reflections and its implementation in a COMSOL Multiphysics® model using the Pressure Acoustics, Frequency Domain interface and LiveLink™ for MATLAB® to run simulations. Results computed in a lightly damped rectangular room of dimensions 5.33 x 4.22 x 2.73 m3 over a frequency range of 20 to 200 Hz are provided to show the performance and limitations of the DATA solution in cancelling the scattered pressure field.