In most professional, commercial and domestic cinemas the loudspeakers are mounted behind the screen so that the visual images on screen coincide with the position of the associated sound images. However, this requires the screen material to be both light reflective and acoustically transparent, which is difficult to achieve. The resultant, imperfect sound transmission gives rise to the sound from the loudspeakers being reflected back from the screen and then forwards again as it reflects from the loudspeaker. The interference between the initial sound and the subsequent reflexions gives rise to comb-filtering which depends upon the reflection coefficients of the screen and loudspeaker and the spacing between the screen and the loudspeaker (see Figures 1 & 2). A change in separation will cause the respective phase difference between the direct and reflected waves to vary, thus resulting in a different resultant wave at the point of the receiver (see Figures 2 & 3). Little research has been carried out to date on the audibility of this comb filtering and in particular, given a certain screen / loudspeaker setup, the effect of varying the spacing has not been investigated thoroughly. By modeling the system in Matlab, and using existing experimental data as a comparative reference, the behavior of the perforated screen when subjected to an incident sound wave can be predicted. Once this is achieved, the effective filter for different separations between screen and loudspeaker can be used to create a database of simulated output signals. These signals represent a portion of cinema audio being played through the screen / loudspeaker system in different geometric setups. These signals can then be used to determine the audibility of the comb filtering effect at different separations via a comparative subjective testing technique.
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