Perforated screen EQ curve

There are basically two types of so-called "acoustically transparent" screens: microperf (Stewart being the primary brand) and woven (championed by Screen Research). All "acoustically transparent" screens affect the sound in one way or another, so the moniker is somewhat misleading. However, manufacturers go to great lengths to minimize the effect. According to THX, woven screens generally have less effect on the sound than microperf screens, at least in the models they've certified (and they've certified both Stewart and Screen Research screens). Microperf material can be made to behave properly, but it takes more effort.

All acoustically transparent screens attenuate the overall level and exhibit a high-frequency rolloff, which can be compensated for with equalization if necessary. According to Stewart, without EQ, their microperf screens start to roll off at around 5 or 6kHz and drop by about 2dB/octave. Screen Research says their woven screen's rolloff starts at 10kHz and drops by 1dB/octave, which means they need less or no equalization.

Another critical sonic effect is controlled by the speaker's distance from the back of the screen. If it's placed too close, much of the acoustic energy is trapped between the speaker and screen, causing a comb-filtering effect that is impossible to equalize. If the speaker is placed too far away, the screen's frame becomes an issue, interfering with the sound waves. To avoid these problems, THX recommends that the speaker should be placed 6 to 12 inches behind the screen. According to them, comb filtering is much less of an issue with woven screens, which means you should be able to put the speaker closer to the screen; Screen Research claims that you can place speakers within an inch of their screen with no comb filtering.

Scott Wilkinson
Video Editor
The Perfect Vision

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Actually, the woven approach is better in general for the picture. Digital projectors can cause a potentially image-destroying problem when used with acoustically transparent screens, particularly microperf models. These projectors form their image as an array of pixels, which are separated by thin black lines. When this pattern of black lines overlays the pattern of holes in the screen, a type of visible distortion called moiré can rear its ugly head. Under the right conditions, moiré manifests itself as an interference pattern of straight or curved lines.

One factor is the display technology itself—specifically, the width of the lines separating the pixels. As this width decreases, so does the chance of moiré. In many LCD projectors, the pixels occupy about 70% of the image, with 30% taken up by the black lines; this is why LCDs suffer the most from the dreaded "screen door" effect. DLP pixels occupy 88% of the image (Texas Instruments claims 100% with their SmoothPicture technology), while LCoS pixels occupy over 90% of the image.

Also important is the precise implementation of a given technology. For example, Stewart found that they could not get the Panasonic PT-AE700U 1280x720 LCD projector to produce moiré on a microperf screen, while the Sony VPL-HS51 Cineza, another 1280x720 LCD model, exhibited lots of moiré.

Moiré can be reduced by orienting the screen's hole pattern at a diagonal angle to the projector's screen-door pattern so the two patterns do not overlap precisely. For this reason, Stewart orients their microperf holes in a diamond pattern.

Screen Research woven screens do not rely on holes per se--the sound travels around the microfibers, and the gaps between them occupy only 1-3% of the total surface area--so there is little chance of moiré in any event. The company claims that their CP1 material completely avoids moiré at any screen size with any digital-projection technology, while their CP2 material should be at least 70 inches wide to preclude moiré.

One visual advantage of all acoustically transparent screens is that they can provide the perception of better blacks because of the holes. However, there is a caveat: the area behind the screen must be dark. If this area is light in color, it can wash out the image.

To address this issue, Screen Research offers an optional black woven backing that greatly reduces light leakage from behind the screen while remaining acoustically transparent. (Like the screen material, it's THX-certified.) According to the company, this backing also improves the black-level performance of the screen without affecting whites or colors.

Scott Wilkinson
Video Editor
The Perfect Vision

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Thanks. This is very useful. I don't have a moire problem, but the reflection of the image behind the screen is noticeable. I can't paint it, so the secondary black screen approach seems ideal.

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