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January 9, 2017

Projection Screen Perforation

One of the little-understood factors that affect a viewing experience is screen perforation. This refers to the small holes made in the viewing surface, which allow sound from speakers placed behind the screen to move through the material. Sound transmission loss during this movement can be tracked in decibels (dB) by sound transmission curves, which are unique for each perforation pattern. In short, different perforation patterns have different sound characteristics, allowing varying amounts of sound to reach the audience.

Here is a fairly typical example of a sound transmission curve for a type of screen perforation. The horizontal axis shows frequency measured in hertz (Hz).  Most humans can detect frequencies between 20 Hz (low) and 20,000 Hz (high). Lower Hz sounds are lower bass sounds, which have a more infrequent wavelength. Of course, the higher in the Hz range we move, the higher and clearer the sound is due to the increased frequency. On this graph, the vertical axis is expressed in the previously mentioned decibels (dB). Decibels are the unit of measurement used to describe how loud a noise is, think of it as the volume knob. The image below shows the decrease in dB relative to Hz as sound hits the screen. 

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Although sound transmission curves are a great tool to compare acoustic performance of different types of projection surfaces, they are certainly not the only element to consider when selecting the appropriate perforation pattern. Otherwise, the choice would simply be limited to the surface that is most permeable to sound. A compromise has to be made between acoustic performance and visual performance.  That is why most manufacturers offer several perforation options.

When the audience is seated closer to the screen, usually within 15 feet (4.5m), it is best to choose a micro or mini perforation pattern to make sure the audience does not notice the perforation holes. Many manufacturers also offer acoustically transparent screens in addition to perforated surfaces. Of course, no material can be totally ‘transparent’. There will always be some amount of sound lost when it hits a surface.  These acoustically transparent surfaces are made of woven materials rather than perforated opaque materials. The tiny gaps in the woven surface results in a higher percentage of void area, transmitting more sound to the audience.  Although these surfaces are more permeable to sound than perforated screens, they are not as reflective, and they cannot be coated to increase their reflective properties. Whenever a screen has more void area, it means there is less area on which the image is reflected. Projector output must be chosen accordingly. Consequently, woven acoustically transparent screens are chosen for their ability to transmit sound and when increasing screen gain is not necessary. In many instances matte surfaces are preferred for their ability to diffuse light more uniformly than gain screens, but the projector must be powerful enough to offset low reflectance typical of matte screens.

Another element one must consider is the moiré effect. Moiré is a visual effect caused by the interaction between the pixel grid and the perforation pattern. If pixels align with the perforation holes while displaced or rotated a small amount from one another, a wavelike pattern appears. You can see an example of this pattern by folding a window screen in half. The moiré effect can significantly degrade the quality of a projected image.  Perforation patterns have been established to mitigate the moiré effect for standard, 1080p resolutions as well as for 4K or higher resolution projectors to ensure that moiré does not interfere with image quality. 

Reputable screen manufacturers carefully analyze their perforation patterns and the effects that different projector resolutions have in order to offer options suitable for different projection set-ups. They also must provide sound transmission curves in order to compare the acoustic performance of each material. These characteristics of perforation patterns are of great importance to the audience’s overall audio-visual experience. 


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