Abstract
Low-frequency electroacoustic absorbers have recently been developed as a solution for the modal equalisation. Firstly investigated in waveguides, the technique consists in matching the acoustic impedance at a closed-box loudspeaker diaphragm to the characteristic acoustic impedance of air. Extending the results in a duct to rooms brings up several challenges. Some parameters, such as the position and orientation of absorbers, the total area, as well as the acoustic impedance achieved at the diaphragms may influence the performance, especially in terms of modal decay time reduction. In this paper, the optimal values of a purely resistive acoustic impedance at an absorber diaphragm, whose area varies, are first investigated under normal incidence and grazing incidence in a finite-length waveguide. The optimal acoustic resistance values are then investigated for a given position, orientation, and total area of absorbers in rooms of different size. From these results, the target acoustic impedances with multiple degrees of freedom are defined with a view to assign to the absorber diaphragms. These impedances are then optimised from a global criterion, so that these impedances approach at best the different optimal resistance values found to minimise the modal decay times. Finally, an experimental evaluation of the performance of the electroacoustic absorber in a waveguide is provided.
Highlights
Room modes cause uneven distributions in space and frequency of the sound field and alter the temporal acoustic response, resulting in long decay times [1]
The absorbers were modelled by flat disks, where a purely resistive acoustic impedance was assigned to each absorber
Two numerical studies in a duct showed that the absorber causes a significant decrease of the modal decay times, whether its diaphragm is oriented normal to the propagation dimension
Summary
Room modes cause uneven distributions in space and frequency of the sound field and alter the temporal acoustic response, resulting in long decay times [1]. The target acoustic impedance should be determined from a representative criterion that maximises the performance of the electroacoustic absorber for the modal equalisation at any location Such a quantity could be derived so as to minimise the dynamics of the sound pressure level at different locations as in [11]. The minimisation of the modal decay times is quite representative of the performance for the modal equalisation in rooms This global criterion will be used in the following, in order to find the target acoustic impedances that will be assigned to the electroacoustic absorber diaphragms.
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