Articles and whitepapers

Technology: Acoustically-Transparent Screens (4/7/2012)

By Patrice Congard, Screen Excellence

The film experience is about suspending disbelief, and this is determined not only by the film itself, but by the way it is reproduced in the cinema or home. The fewer the distractions, the better.

The primary reason why the acoustically-transparent (AT) screen was developed was to allow the sound, particularly dialogue, to originate from where it appears, i.e. from the screen. After all, when you see an actor speaking, you expect the sound to come from their mouth, and from nowhere else. Hence all commercial film theatres and professional film sound mixing studios use AT technology.

With an acoustically-transparent screen, the primary focus is the screen. There are no other distractions.

AT screens are also available to bring this immersive experience to the home theatre, but in the past, they have stirred up some controversy. Traditionally, the main arguments against them focus on image and/or sound quality.

In this article I aim to address these reservations and explain why acoustically-transparent screens are the best choice for the home cinema specifier.

Sound Cohesion

When you see an actor speaking, you expect the sound to come from the location of his or her mouth and from nowhere else. The issue of such cohesion is also crucial when a sound-producing visual travels across the screen.

With an acoustically-transparent screen, the image and the sound remain in perfect coherence, resulting in an immersive cinematic experience.

In commercial movie theatres and professional sound studio facilities, the dialogue channel loudspeaker is always located behind the centre of an AT screen. This results in the sound corresponding to the action being located on the screen where the action takes place, and dialogue intelligibility being enhanced thanks to lip-sync being correct in space as well as time.

When a solid screen is used, the speakers must be placed outside the viewing area, but the acoustic result of a loudspeaker located very near to the floor or to the ceiling can be disastrous.

With a solid screen, the image travels across the screen following a straight path, whereas the sound in this example, travels down and then up.

The permanent lack of cohesion between the sound and the image destroys the illusion, and results in what is commonly called the 'EXIT sign' effect, whereby you get distracted from the film and see the familiar green EXIT signs.

Sound Degradation

One of the reservations that specifiers have about AT screens is that they may not be truly acoustically transparent. This, it is argued, can result in a serious loss in sound quality, with the AT screen also acting as an acoustic reflector, causing the sound to bounce back and forth between the screen and the rear wall.

While these initial objections may have been valid a few years ago, current state-of-the-art AT screen materials mean that AT screens no longer need to suffer from such problems.

There are two types of material that are used for AT screens, namely perforated and woven. With perforated screens the sound can only go through the holes punched in the screen material, and these represent around 6% of its surface. This means 94% of the sound is reflected backwards.

With a perforated screen, most of the sound is reflected backwards.

Perforated AT screens act like reflectors, giving a 'phasey' sound and an uneven response that cannot be corrected with simple equalisation.

The response of a perforated screen showing a marked drop off at the higher end of the frequency range. The red trace is the response of the speaker alone, while the black trace is what comes through the screen.

With woven fabric screens on the other hand, the sound diffracts around the woven threads to make its way through the screen surface. This generates a bit of loss by absorbing some energy, but no sound is reflected backwards.

Sound diffraction through a woven screen.

Woven fabric screens only suffer minimal high frequency loss due to absorption, which is possible to correct with basic equalisation, or even tone controls.

The response of a woven fabric screen showing a consistent but minimal loss. The red trace is the response of the speaker alone, while the blue trace is what comes through the screen.

Moiré Effects

Early AT screens of the perforated type were prone to moiré effects due to interference of projector pixels and screen holes. Micro-perforated screens were then introduced which, along with fabric screens, offered sufficient small steps to avoid interference.

Moiré artefacts are the result of interference between two sets of fine pattern grids.

With the arrival of HD, the distance between pixels became smaller too, and so moiré problems returned, however current fabric screens, such as the Screen Excellence Enlightor 4K, have been engineered to completely solve this problem.

Examples of different screen materials.

Other Reservations

There are some more refined arguments which are still pertinent within the present state of the art. One is that that AT screens have a surface granularity that blurs the image and limits resolution. This is true to some extent, but newer fine-weave fabrics have been engineered so that viewing from normal distances is not limited.

The meaningful criterion here is the resolution limit: if a pixel is not distinctly seen from a very close distance to the screen, then the screen is the limiting factor. If a pixel can distinctly be seen, then the projector is the limiting factor, which is most often the case with very fine fabric screens.

Another argument is that AT screens do not provide gain or, like the latest solid screen technologies, contrast enhancement. This is true, however, such screens are mostly used to solve severe problems in bright environments. Top-grade home theatres are always installed in controlled lighting environments in which screens that provide unity gain (or very near to 1) are the best choice.

The final argument is that AT screens are more expensive than solid screens. This is true, but is has to be put in perspective in terms of the cost of a screen compared with the total cost of the installation. Typically this will be just a few percent, or at most 10%, but given how important the screen is, and the difference that an AT screen can make to the whole experience, is it not worth spending a few percent more?

Screen Size

In addition to its primary advantage, another benefit of the AT screen is that it can be significantly larger. The solid screen, by its very nature, cannot obscure the speakers, and so its height and width are restricted by the speaker positions. The AT screen on the other hand, is designed to have the speakers behind it, and so can be much larger. Indeed the effect of covering up all of the distractions behind can be very impressive.

The acoustically-transparent screen can be much larger than its solid counterpart.

Conclusion

No screen is perfect, and AT screens have some minor limitations, but ultimately, whatever the picture quality is, if the sound isn't right, the whole experience is ruined.

In my experience, installers who have adopted AT technology have done so because the benefits so clearly outweigh any drawbacks, and in practice their customers are delighted with the cinematic experience achieved.

Patrice Congard is the Founder and CEO of Screen Excellence, a British company that manufactures quality acoustically transparent projection screens.

www.screenexcellence.com