Spatial Equalization of Sound Systems in Cars
This paper discusses the equalization of sound reproduction systems in cars. The whole equalization process can be seen as a cascade of multiple filters, each with different purposes: frequency response regularization, time-domain realignment, spatial displacement of the sound sources, linearization of the loudspeaker's distortion, and so forth. A modular mathematical formulation is derived, making it possible to approach separately each stage of the equalization process. The greater effort is devoted here to spatial equalization, which makes it possible to move the apparent position of the loudspeakers, and to modify the subjective spatial impression due to the volume in which the reproduction takes place. However, the other equalization stages are also discussed. From the proper implementation of the above theory in a set of suitable software modules, it was possible to transport a measured sound field in a very different reproduction space. This was useful in performing two opposite tasks: reproducing the sound field of good listening rooms or theaters inside the compartment of a car, or reproducing the sound field of different cars in a listening room. The second task was useful for making blind subjective comparisons of the sound system of different cars, while the first task made it possible to recreate the preferred sound field inside another car. In both cases it was first necessary to measure binaural impulse responses in the origiual space and in the reproduction space (making use of the same dummy head), after which a proper set of digital inverse filters was created, capable of transforming the second ones in the first ones. These filters are applied to the signal to be reproduced (music from CDs) by convolution using to real-time convolution software. The results were judged subjectively by an automated interactive questionnaire implemented as another software tool. This paper describes in detail the theory developed for building the numerical filters for spatial equalization, the software developed for measuring the binaural impulse responses and for processing them, and the results of the subjective listening tests.
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