Acoustically Transparent Screens

Why an Acoustically Transparent Screen?

Image by courtesy of Votre Cinema

In every movie theater (or Cinema) we see and hear an AT (Acoustically Transparent) screen at work. The seamless synchronising of image and sound is fundamental to the film art of suspension of disbelief.

All three front soundstage loudspeakers are installed directly behind the screen with the centre channel being positioned where is belongs, in the centre of the screen.

So the right question is not so much “why an A.T. Screen  ?”, but “how much do we lose in not using an A.T. screen  ?”. To this question the answer would be far more than you might imagine.

When  a non-A.T.is being used the centre channel loudspeaker must be located either below or above the actual image. As we view the film we become aware that the origin of the sound is clearly different to that suggested by the image. As the screen size increases this error becomes even more obvious.

This mismatch between sound and image location is all we need to realise it is not an actor speaking, but a sound system located somewhere else.

There are a few other problems induced by placing the speakers below or above the screen : When a sound is following a moving action, you may see the visual supposed source of sound moving horizontally (say, from left to right via the centre) when the sound source is obvioulsy going down first and then up again. This is very funny, but not quite realistic…

Another issue is the immediate floor or ceiling sound reflection of a speaker placed too close from a hard surface, acoustically deteriorating the sound quality.

But none of these two shortcomings are as severe as the total discrepancy between the sound and the image when the centre speaker is not placed where it should be : behind the screen, at its centre.

The lip-sync problem

Whether or not you are aware of it, we can all read lips and do notice when sound and lip movement are not synchronised. This synchronisation is essential to overall intelligibility. In its absence a common practice to obtain a good level of intelligibility is typically to raise the level of the centre channel to try to restore the balance.

This level increase, often as much as 6dB (that is, multiplying power by 4) has significant implications in speaker distortion and the 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 to have a Home Theatre or a true theatre in your Home ? Only an Acoustically Transparent screen can realise your best wish.