Ambient-air Hushbox for D-ILA projector

By Tom Morrow, tmorrow_us at (@) yahoo dot (.) com,  10/24/02

D-ILA projectors throw great images but they are among the noisiest projectors around.  There is a commercially available Hushbox for the D-ILA, the Whisperflow, and I was on the verge of buying one.  The designer Dave Beatty seems like a great guy and he produces a very attractive product.  I went and saw a local home theater that had a D-ILA in a whisperflow and I was impressed at the quality, but it just didn't get it quiet enough for my high standards.  I fancy myself a bit of an audiophile, and I'm used to just not hearing the projector.  So I wanted something that was really going to be as quiet as possible, pulling out all the stops.  The other concern I had with the Whisperflow was thermal performance; the Whisperflow doesn't physically separate the projector exhaust from the intake, which means that over time the whole chamber can heat up.  That can be solved by adding fans to increase airflow, but then it's noisy so what's the point.

The normal way to go about this would be to duct the projector to another room through the ceiling.  But I live in a condo and the only walls I could duct to would require cutting through a firewall or an outside wall, which is against my HOA rules and therefore against the law!  The other consideration is that my ceiling is the popcorn acoustic type which is basically impossible to restore once you have damaged it significantly.  As much as I love this projector, I don't want to pay several thousand dollars to redo the whole ceiling when it becomes obsolete and the next one rolls along.  

So I had some different restrictions than many others who have build D-ILA hushboxes and I came up with a different solution.  My hushbox does not require any external ducts or air supplies.  It takes air in one end, and expels hot air out the other end.  Because the inlet and outlet are open to the room rather than buried in a duct somewhere, I had to get the sound as low as possible before it gets to that inlet or outlet.  This means putting the fan and projector in the middle of the box, and having "folded baffled" ducts on either end covered in sound absorbing material to absorb the sound.

Like all the other good hushbox designs I've seen, it is designed to work with the projector exhaust system, so that air is forced in the projector intake, and the projector exhaust is isolated from the projector intake so that the hot air goes outside rather than recirculating into the projector.

The cool things about this hushbox are:
Check out this diagram that shows the top view.  The dimensions finished were about 48"x21"x13".  Sorry for the Soy sauce stain on the diagram!  The box is completely closed up except for an inlet port on the back, an outlet port on the back, and a window for the light to shine out of, covered with anti reflective glass.


For the fan, I went with the Panasonic Whisperwall fan, model FV-08WQ1 for $180.  I bought from EFI, which is a great organization whose environmental consciousness is worth supporting. At the time I bought it was rated at .50 sones, which is one of the lowest sound ratings you'll find on a fan anywhere.  Now I see their web page is saying 1.1 sones, which may be more accurate.  At any rate, it is very quiet; when you turn it on you only hear white noise from the air going through, you don't hear any motor noise.  In general the wider the fan diameter the quieter a fan will be because it can move more air per rotation, meaning it can rotate slower.  This is one of the widest exhaust fans around. I figured it's much better to spend a little more for a quiet fan than to spend a lot of time and money overbuilding the hushbox to use with a noisy fan.  After having used it, the only concern I have is that it may not move quite enough air... since it is designed for through the wall use it probably isn't designed for high duct loads.  It might slow down when faced with the load of moving air through all the channels of the hushbox.  It's hard to say how another fan might perform though.

In order to keep dust out of the projector chamber, I duct taped a 3m filtrete furnace air filter over the fan grille inside the box.  (It was fun to finally use duct tape for its intended purpose!)  Cut a 10x20 filter in half, and the 10x10 piece fits just right over the grille.  

Fan Switch:
There is a $20 Craftsman device available from Sears that everyone uses to switch their fan on when their D-ILA powers up.  It's available online, but I don't have the info here.  Try searching avsforum for it.  Basically you plug the projector in one outlet and whenever it senses power going through that outlet it powers up the other outlets (which you plug the fan into).  There is a switch on it, which I was a little scared of.  I opened up and fired up the soldering iron to jumper a wire around the switch so that I won't have to worry about bumping the switch off while messing with cables.

I went with 3/8" plywood for most of the box, except for the bottom which I used 1/2" plywood for.  It was all cheap non finished plywood since I covered the whole thing later on.  I used 1x2 pine wood for most of the corners and guides.  I really wanted to keep this thing light, because I wanted to be able to put it up and take it down myself if necessary.  In the end I happened to have 3 folks around to help get it up, but I think I could have put it up with just myself and some chains to temporarily hold it in place.

Acoustic Foam:
The foam I used is Hush cloth from 800-no-noise .  I ordered a whole roll of the 1" Embossed Foam with PSA (Pressure Sensitive Adhesive).  It was 45 square feet, and cost $100 plus $28 for shipping.  It seems to have good specs, and was fairly easy to work with, requiring just a knife to cut it.  Most importantly it lays flat, so it doesn't impede the air flow.  I did find that it was easiest to make clean cuts by dragging a sharp knife with light pressure next to a ruler a few times, rather than using heavy pressure.  I covered just about every surface inside and outside with the foam, and ended up not having anything left besides a few scraps.

Window Glass:
Like just about everyone, I got a 4x5 piece of anti reflective glass from Edmund's Optics for about $25 plus shipping.  It really is good stuff... Unless there is dust on it it doesn't even look like it's there.  It covers the hole that the light shines out through, so that sound doesn't go out and air doesn't leak in.

The picture below shows the box when it was ready to be put together.  In the picture, the top of the box is laying behind the box so you can see the grooves where it all fits together.  While building the box it's helpful to not glue or fasten anything until you have fit everything together in grooves and verified that it all fits together right.  I ended having to move a few guides around a bit to get it to fit together just right.

You can see the fan mounted on the right.


Here is a view above the fan, where you see the dust filter duct taped on.

 The whole grille can be removed from the fan when you need to replace the filter, but honestly there's not much space to work your hand in and get it off.  It might be a good idea to leave more room than I did.  This is the view of the fan grille and filter, from the back of the box.  The air gets sucked in the back from the left, and then goes around a 180 degree bend and then into the fan.

After the inside was covered in foam, the back is ready to go on.  This is what it looks like.  You can see there is an electrical box with the handy Craftsman power switch plugged in:

Here's the view from the back just before the back gets screwed on.  You can see that the fan blade is rather exposed.  After this picture was taken I added more foam around the fan to "sink" it.  I was concerned about wires getting caught in the blades and stopping them from moving.  The electrical box is a part that came with the fan and I wired the cord into it.  In this picture you can also see how the inside chamber is completely sealed with silicone caulk to keep air leaks from happening.  That is important because any leaks will mean air is getting wasted rather than going through the projector.  You can also see the weather-stripping that I used between the back and the rest of the box, again to prevent leaks.  The baffle parts don't have to be completely airtight, but the main chamber does.

This view, from the back of the projector, shows the holes that will mate with the projector exhaust.  The seals are closed cell foam designed for sealing leaks around automobile doors.  There is one hold for each of the two exhaust ports on the D-ILA.  Later on I added thermometers in the middle of each of them.  The frames are made with cardboard and packing tape.  It's kind of an art getting these things to fit just right, involving a few iterations of trying different sizes and materials.  I ended up reinforcing the smaller one with Plexiglas since it was getting destroyed as the projector slid out past it.  The whole idea is to seal the projector so the exhaust goes out rather than into the main chamber, and yet to keep the pressure on the projector light and even enough that the projector stays pointed straight over time.

This shows the box assembled except for the front, and shows how the projector can slide out.  The sliders are heavy duty drawer sliders from Home Depot.  You can see that the dust filter is accessible without removing the projector.  Also, the projector can actually slide completely out by flicking levers on the slides; the slides separate into two pieces.  That is the way to install the projector:  you screw it into the mounting plate outside the projector, and then pick the plate and projector up and slide it in.  You can see that there are metal angle irons in order to reinforce the mounting plate; without them it wouldn't be strong enough to hold the projector without flexing a lot.   You'll notice that one of the feet is missing; my used projector didn't have that foot so I used an allen bolt and some washers.  The mounting plate is just a piece of plywood with holes drilled through it for the projector mounting feet to be screwed into.

Here is the box with the back on.  Notice the oval holes in the top.  Those are where the ceiling hooks go in.  The positions of everything are all calculated in advance so they go into ceiling studs that run across my room, spaced 16" apart.  4 hooks in the ceiling connect to 4 turnbuckles, which connect to 4 hooks on the bottom of the box.  By tightening the turnbuckle, you raise the box up into place.

Here is the back of the box when it is all up on the ceiling.  Notice the yellow gummy stuff around the wires.  That is the stuff used for hanging posters without damaging walls.  It is sometimes blue and called "Blue Tack".  It seals the space around the wires and yet doesn't make a mess.  In the picture the inlet is on the left, and the outlet is the white metal exhaust redirector vent that aims up toward the ceiling.  The white vent thingie came with the Whisperwall fan.  It had an internal swinging cover that I removed.  The insides are covered with acoustic foam, and I figure it serves as one more corner for the sound to go around before it exits.  Also it directs the hot air high, which helps keep it away from the slightly lower intake on the other end.  Thermometers show temperatures at inlet and exhaust.

Here is the projector in place and ready to project.  Notice that the IR transmitter for my Xantech system hangs down in front of the projector.  That way it can swing out of the way when swinging the projector out for service.

This shot is the projector extended for service.  You can also see the turnbuckles that were used to pull the projector up to the ceiling.  They permit very fine adjustment of the angles and you can get it right next to the ceiling without actually touching the ceiling.

Projector inside and ready for action... but someone take the lens cap off!

Here is with the front cover on.  You can see that the front cover just screws in with a bunch of drywall screws.  It's not the most elegant attachment method, but it provides a very tight seal.  And it only take 4 minutes with a screw drill to open or close.  You can see the window with the anti reflective glass covering the lens.  It's not necessary to expose the whole lens, just the part that projects images.

Later on I bought a bunch of Polder brand thermometers.  I have one on each of the two projector exhaust ports, and one near the projector inlet.  They have max temp alarms so that if the fan stops for some reason I'll know to shut down the system.  They have magnets on the back so they stick to the metal vent.  Unfortunately, they are almost as noisy as the projector in the enclosure, making a clicking noise that is kind of objectionable.   So I wouldn't necessarily recommend them.  The Wal-mart thermometers I had didn't read high enough for the exhaust fans, and the Polder ones go up to 300 something degrees.  I keep the old thermometers around mostly just because it'd be a pain to unthread the probes.

Here's the back with a scrap of acoustic foam left over.  It's kinda ugly but nobody looks there except me.

Here's the finished product viewed from the front.  You can see I covered the front and bottom with acoustic foam.  I was very careful to cut it neatly and exactly on those outside pieces.  The foam on the front is attached to thin hardboard.  The ends are actually thin hardboard covered in felt.  The front and ends are attached with hardware designed for hanging pictures on walls.  I didn't want to use acoustic foam on the ends because I want the surround sound speakers near there to reflect around the room rather than be absorbed.

The dark color of the acoustic foam complements my dark couches nicely. This is after all my living room, and the box also serves to hide the projector. 


Subjectively, it really is quiet.  I just took my radio shack db meter out to measure on C weighting.  It's lowest level it can measure is 50 db, and it could not measure anything right next to the bottom, front, or sides.  On the back, it could measure about 54db within a foot or so of the inlet/outlet, and the highest measurement was 72db right at the edge of the inlet.   Keep in mind that there are walls and shelves 1-2 feet away from the back which reflect the sound around a bit back there.   At seating level below the projector it is not loud enough to measure.  Those numbers make it sound noisier than it actually is.  The sound is very low pitched, kind of a low rumble, which makes it subjectively very easy to ignore.  Much better than the high pitched whine of the normal fans.  And much better than I subjectively remember the D-ILA in the Whisperflow box.

I've used it for about 1500 projector hours so far with no reliability problems.

The temperature at the inlet generally starts about 10 degrees over room temperature when the projector is run for 10 minutes.  After a few hours it goes up to 20 degrees above room temperature.  Typically the inlet temperature is at 95 degrees after a few hours of use, before I shut the projector down.  After the projector shuts off, it actually gets even hotter, but that is okay since the projector internals are getting cooler.  It can go as high as 30 degrees over room temperature during the cool down.  I have an alarm at 135 degrees on the inlet which hasn't gone off yet.

The small exhaust port typically gets up to about 130-135 degrees after a few hours.  I have an alarm at 154 degrees.
The large exhaust port typically gets up to about 180-190 degrees after a few hours.  I have an alarm at 204 degrees.
When the projector shuts down the large and small exhaust ports immediately begin dropping.

I did a test of running the projector in the box but without the front cover, which simulates running in free air because the box is not constricting airflow much at all. I found that the temperatures at the exhaust ports were about 15-25 degrees lower than with the box closed up. This seems reasonable... as long as I keep the temperature in the room around 70 degrees, a 15-25 degree rise is like operating the projector in 85-95 degree free air, which surely should be within the safe operating limits for the projector. It would be nice to have a fan that forced a little more air through the system.  Subjectively, the amount of airflow with the whisperwall and the projector fan seems about the same as what the projector fan would move in free air.

I think I spent about as much on materials for this as I would have spend on a brand new Whisperflow with all the options, about $650.  And I spent basically all my free time for about 2 weeks.