Room response equalization has been applied for improving the objective and subjective quality of sound reproduction systems in cinema theaters, home theaters, and car HiFi systems. Room response equalization systems act by shaping the room transfer function from the sound reproduction system to the listener with a suitably designed equalizer.
Both minimum-phase and mixed-phase room equalizers have been proposed in the literature. Minimum-phase room equalizers acting on the minimum-phase part of the RTF phase response can be used in order to shape the RTF magnitude response. In contrast, in mixed-phase room equalizers the non-minimum-phase part of the RTF phase response can be corrected too. In principle, in mixed-phase room equalizers, some of the room reverberation can be removed, but particular care must be taken to avoid "pre-echoes" caused by the errors in the non-causal part of the equalizer.
Room equalizers can also be divided in single position and multiple position equalizers. In the first case, the equalization filter is designed on the basis of a measurement of the room impulse response in a single location. These equalizers can achieve the room equalization only in a reduced zone around the measurement point (of the size of a fraction of the acoustic wavelength). Indeed, the room impulse response varies significantly with the position in the room. Moreover, the room impulse response varies also with time and thus the room should be considered a weakly nonstationary system. Multiple position room equalizers are capable to enlarge the equalized zone by measuring the room impulse response in multiple locations.
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Our studies in this area are devoted to the derivation of innovative and computationally efficient techniques for multiple position room equalization. Both minimum phase room and mixed phase room equalizers are researched. Objective and subjective evaluations are used to assess the quality of the proposed room response equalization techniques.
The research is done in cooperation with the Department of Biomedical Engineering, Electronics and Telecommunications (DIBET), Università Politecnica delle Marche (UNIVPM), Ancona, Italy.