Perfect Projection Surface...

Perfect Projection Surface...

This article describes a perfect surface for a home theatre screen.

As I walked my dog down the lovely smooth flat sands of East Anglia, I thought it was a perfect surface for playing ball; it bounces perfectly!  

East Anglia Beach, UK

So what is a perfect surface? In Tennis, is it Grass, Clay or Hard court?  My preference is grass as it adds unpredictability to the game, however, unpredictable results in a home theatre would not be welcome.

For a home theatre, that produces a true cinematic experience, the surface should be a white fabric that is acoustically transparent with little or no gain.

The perfect surface is a surface that diffracts light evenly, without artifacts, hotspotting or the like. It should not have any visible structure, and in no case larger than the smallest pixel of the projected image This last criterion is particularly difficult to combine with a proper acoustic transparency, especially if considering Ultra HD –that is 4K – projection.

It should not incur any moiré artefact by interfering with the pixels grille as projected.
It should not distort the colours.

Screen composition...

Most acoustically transparent screens are made woven of fibre glass strands coated with PVC. Although such designs bring a considerable improvement in terms of acoustic transparency over the traditional microperforated vinyl surfaces, they are limited by the mere diameter of the thread with respect to resolution.

In short, they are fine when the screens are very large (> 6m), but as the smaller the screen, the smaller the pixels are, and the image resolution is no longer limited by the projector but by the screen itself.

Further, when the step of the fabric structure is of the same order as the step of the pixels grille, moiré is very likely to occur.

We have addressed all these issues as far back as in 2009 when we launched the ultimate projection surface, the Enlightor 4K

Made of a very fine yarn (about 0.08 mm) which is twisted to avoid any visible structure, it has no visible difference with a solid projection surface.

Still its acoustic transparency is unique, as its frequency response is intrinsically flat, providing a uniform 2.5 dB attenuation throughout the 200Hz to 20 kHz frequency range.
This attenuation is only due to absorption, not incurring any backwards reflection, no time domain alteration, no directivity modification.

Enlightor 4K Fabric

This uniquely woven fabric is available exclusively from Screen Excellence.  Its look height allows us to build screens up to 290 cm high, reaching 260” base width in 2.40 aspect ratio.

Why Acoustically Transparent?..

In every movie theatre (or Cinema) you can experience an AT (Acoustically Transparent) screen at work. The seamless synchronisation of image and sound in the same location is fundamental in the art of suspension of disbelief. It’s the ability to absorb the audience in the ’reality’ of large-scale pictures that gives the cinema its enduring appeal.

In a professional cinema, all three (L/C/R) front soundstage loudspeakers are installed directly behind the screen with the centre channel being positioned where is belongs, behind the centre of the screen.

So the right question is not so much “why choose an A.T. Screen  ?”, but “how much is lost by not using an A.T. screen  ?” There’s far more to the answer than you might imagine.

If   a non-A.T. screen is used, the centre channel loudspeaker is located below or above the image. When viewing the film, we become aware that the origin of the sound is clearly different to that suggested by the image. The larger the screen, the more obvious it is. The mismatch between sound and image location is all we need to realise we’re listening to a sound system rather than a live scene. The suspension of disbelief is gone.

Another issue is the poor sound quality due to immediate floor or ceiling sound reflection of a speaker placed too close from a hard surface.

But neither of these two shortcomings are as severe as the total dislocation of sound and image when the centre speaker is not placed where it should be: behind the screen, at its centre.

The Lip-sync

Whether or not we are aware of it, we can all read lips and we notice when sound and lip movement are not synchronised. Lip-synchronisation is essential to overall intelligibility. In its absence a common solution for improving intelligibility is typically to raise the sound level of the centre channel speaker.

Such a level increase, often as much as 6dB (that is, multiplying power by 4) has significant consequences in both speaker distortion and perceived balance of the front soundstage.

Next time you go to Cinema notice how easy it is to accept sound and image as one. Now, ask yourself, do you wish for a Home Theatre or a true theatre in your Home? 

Only an Acoustically Transparent screen can help to realise your dreams.

Freqency Response

For a true cinematic experience you want the sound to pass through from behind the screen. The viewer will see and hear the difference.

Some manufacturers do this with micro-perfing (small holes in the screen) which is not noticable in large screens like an IMAX theatre screen. However, as you bring the screen size down the hole patterns can create a moire' effect which distorts and blurs the image. Additionally, a micro-perf will require EQ to compensate for the loss in db and specific frequencies. This cannot reduce the time-related distortion due to the sound bouncing back off the rear surface of the screen, then on the front panel of the speakers or the rear wall, and then on the screen again…

Using our Enlightor 4K projection surface, we are allowing the sound to flow from behind through the screen with minimum absorption and no distortion or frequency response degradation.

Gain...

"Overall, the benefits of placing the loudspeakers behind the screen in order to have spatial coincidence between picture and sound by far outweigh the picture and sound degradation issues." - Widescreen Review

With a low to medium gain in AT screens you will want to use a bright projector in a darkened room. Ideally, you’d select an Ultra HD,4K projector to enjoy the maximum available resolution by today’s standard.

In conclusion, for a true cinematic experience you will require an acoustically transparent screen capable of not limiting the resolution of the projector by any standard, whatever the size of screen you choose.

Screen Excellence provides AT screens in a variety of standard configurations as well as custom screen sizes to meet your exacting needs.

www.screenexcellence.com