HISTORY OF ACOUSTICALLY TRANSPARENT SCREENS

The need for A.T. screens appeared in cinema as soon as there has been a sound system reproducing speech synchronised with the image (The Jazz Singer, 1929). The most common technique consists in perforating many tiny holes in the screen material. From lengthy experiments, the size of holes has been defined around 1mm in diameter, spaced so as to cover about 3% to 6% of the overall screen surface. Even further information can be found at : www.filmsound.org/film-sound-history

Screens for the video industry, typically, were not acoustically transparent because there was no need to synchronise the sound with the image : in most professional video applications, the only sound was made by live comments upon the displayed program.

The advent of Home Cinema in the 90’s revealed the need to adapt the A.T. screens to video projection, as the projected programs were mainly movies or concert videos needing lip-sync. The first adaptation came from « micro-perforations », reducing the dimension of the holes to about 0.5 mm, and increasing their number to preserve the same surface ratio.

It was often objected that the perforation ratio was not sufficient to provide a good sound quality : The screens behaved like acoustic filters, needing HF equalisation. Further, there was a loss in light refraction (typically about 10%) which was a concern as most of the CRT projectors were operating near to their limits in light output. However, the benefit of placing the centre speaker in the proper place, behind the centre of the screens, allowing sound & image coherence and correct lip-sync was so obvious and dramatic that the micro-perforated screens became the standard for high-end installations.

In the late 90’s, however, due to their tremendous improvements, the fixed-pixel projectors (especially the DLP) became the new standard of the industry. This aroused a major problem, as the projected grilles of pixels came to interfere with the rows of micro-perforations, creating unacceptable moiré effects. The micro-perforated screens were simply not compatible with most video projectors.

An unexpected solution had been found in Japan, as far back as 1994, when a screen fabric made of woven cloth was developed (JAES, Vol 42 N°12), with the main scope of improving the audio performance. More recently, Screen Excellence developed its Enlightor woven fabric. This new design has combined uncompromised video performance, including with fixed-pixel projectors, with a level of acoustic transparency unmatched by any micro-perforated screen.