The audibility of comb-filtering due to perforated cinema screens

Abstract

In cinema audio, the sound needs to be combined to try and create an atmosphere in which the visual and audio images work in conjunction with each other to help engage the viewers. The loudspeakers positioned behind the cinema screen are integral in creating a localized image that matches the positions of the video sources. This means that current cinema screens need to be optimized for both image reflection and sound transmission. Placing the loudspeakers above or below the screen results in a less satisfactory experience with other attendant problems [1]. Because placing the loudspeakers behind a normal, non-perforated, screen would give rise to large amounts of attenuation at high frequencies, perforated screens have been developed to “let the sound through”. Currently no material exists that has the qualities required for both perfect reflection of light and perfect transmission of sound. This means that trade-offs need to be made to enable a screen to meet the requirements of both the reflection of light and transmission sound at acceptable level. For years, the quality of the image has dominated cinema specifications, partly due to the fact that early films did not include particularly high quality sound. However, the arrival of digital soundtracks, and as the science of psycho-acoustics has become better understood, the realization that audio quality is greatly important for the overall realism of the cinema experience has led to more research into improving the sound whilst trying to maintain the high definition required for the projection of a high-quality image on to a large screen. The presence of the screen gives rise to a phenomenon known as ‘comb filtering’,arising from the addition of delayed versions of an original signal to itself. The severity of the comb-filtering will vary depending on the geometric setup of the system. In particular, varying the distance between the loudspeaker and the cinema screen can change the nature of the comb filtering that the system exhibits. This, in turn, will alter the frequency content of the final signal reaching the listener, and it is this final signal that is essential to the cinema experience. This paper focuses on characterizing the actual audibility of a comb-filtered signal by means of signal processing. The cinema screen’s acoustic properties are used to create a model that simulates the effect of playing a sound through the loudspeaker/screen system. The audibility or otherwise of the comb-filtering is then assessed via a subjective listening test.

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ORCID for Keith Holland: orcid.org/0000-0001-7439-2375

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