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Through a Quiet Window

I have a tradition every year at CEDIA EXPO. At some point during the show, I find a quiet corner (an increasingly difficult task) and pull out one of my seemingly endless copies of the CEDIA Designers Choice Awards booklet. You know the one Im talking about; it contains beautiful glossy pictures and glowing descriptions of all of the projects that various firms have submitted for awards. Once settled, I search through it from cover to cover looking for front-projection video systems where the noise produced by the projector has been properly controlled. Guess what? I usually dont find one, and thus my tradition continues.

Every year, I hope to discover that the message we teach in the acoustics classes has gotten across: No projectors without noise control in home theaters. Unfortunately, Im still waiting.

Why is projector noise control so important that I would give it this much attention during the hectic CEDIA week? Two words: dynamic range. By definition, a front projector must be in front of the screen, which usually puts it out in the room with the viewers. Because projectors generate heat, they have noisy cooling fans, and those fans out in the room cause an instant dynamic range problem. Consider for a moment that the potential dynamic range of a 20-bit movie soundtrack (the difference between the loudest and softest sounds on the soundtrack) is potentially 120dB. A home theater will be stretched to reproduce the entire range of a soundtrack under the best of conditions. A good home theater sound system should produce 105dB peaks, and the noise floor should be somewhere around 15-20dB weighted on the NC scale. Even the quietest projectors produce around 30dB of noise, which is more than enough to raise the noise floor to an unacceptable level.

In the golden era of CRT front projection, hush boxes were used to control projector noise and restore dynamic range. The projector had to be in the roomusually right at the prime viewing locationso a soundproof box was built around it, with port holes cut in it for the light to pass through. Glass was put in the portholes to contain the fan noise. The same thing can be done with modern lamp-based projectors. In fact, it is often easier because they are smaller and can go on the ceiling. Unfortunately, hush boxes have a big ventilation problem, especially with lamp projectors that produce more heat than CRTs. Many times the hush box itself must have cooling fans, and then youre right back where you started. The most common remedy is to vent the hush box through the ceiling or the floor directly into the HVAC system. If you do this, be sure to clear it with the HVAC engineer. You dont want your tap into his/her system to create a pressure problem that will cause the whole system to crash.

The ultimate solution for front projector is a projection room, just like the cinema. The projection room houses all of the sound and video equipment in a soundproof environment with its own climate control. The equipment has no aesthetic impact on the room whatsoever, so even the most dcor-conscious clients will have nothing to complain about. A projection room doesnt have to be big; something as small as a dormer window will usually suffice.

Projection windows (called ports) are the key to making both hush boxes and projection rooms work effectively. Ordinary window glass is inadequate for projection ports because it lacks color purity and has very high refractance. If you shine a projector through it, the color balance of the image will be tinted green because of iron in the glass, and a good portion of the light will reflect off the glass and land somewhere other than the screen. The light loss is about 10 percent, which is completely unacceptable. You would never seriously consider subjecting a projector that has been endowed by the manufacturer with a camera-grade lens to such treatment. For a projection port, you need low-iron, high-transmission float glass, such as Optiwhite, Eurowhite, or Starphire, that has been coated with a high-efficiency, anti-reflection coating, and only shaves off two to three percent of the light. Home Depot is not going to have it, so start looking for a specialty glass supplier. They will know what youre talking about.

Once youve got the glass, you have to mount it in such a way that it blocks the most amount of sound and the least amount of light. Typically, you want to use two pieces of glass with different thicknesses. They should be a few inches apart and slightly angled so that they are not parallel to each other or perpendicular to the projection beam. Ideally, the space between them should be a vacuum, and the framing should be sealed with acoustic caulk or expanding foam. If that kind of construction project sounds a little ambitious, you can always opt for a ready-made projection port from the same manufacturers that make them for the cinema. In most cases, youll get the added advantage of being able to slide the glass out of the way if you ever want an unobstructed path to the screen.

Given how important dynamic range is to home theaters, its amazing how many installers put in front projectors with no thought to noise control. If you think Im fibbing, just grab the CEDIA Designers Choice Awards booklet like I do and look at the projectors in the home theater entries. If you find one that is properly sound isolated, I promise to vote for your home theater entry next year…but only if youve controlled the noise from your projector.