Outdoor Surround Sound System

[TomG]

This is my first post on the forum as I am seeking advice for a crazy idea that we are contemplating. I currently host a free “Music in the Park” series for my city. We project the video image on the side of a building and use a budget PA system for the sound. We usually attract a crowd of 200 to 300 people in an area about the size of half a football field.

Our next grand idea is to build a screen and surround sound system onto our twenty-foot trailer so that we can show movies at different locations. The screen, subwoofers, and front channel speakers will all be permanently mounted to the trailer, so size is not really an issue. Our thought was to build 4 tuba 36’s for the subs and 5 Omni 12 Fullrange speakers for the surrounds.

As in most cases, we are on a limited budget and we deal with a young audience that wants to feel the ground shake as T-rex enters the scene. I have done a lot of searching on line but, other that a few backyard systems, I cannot find much information on this kind of a system. It could be a fun project. Does anyone have any ideas?

[gdougherty]

O12 full-rangers would be completely unnecessary is my thought. Something like DR200/250's whould be perfectly fine for all your tops. If you wanted something easy and closer to full-range, OT15's with good compression drivers instead of piezos would get you there. DR's would do better and DR280/290's would probably be the best option with full-fat response down to a lower crossover frequency.

[Bill Fitzmaurice]

I don't see why OTop12s wouldn't be fine for the mains, centers and surrounds, since they go just as low as TLAH and SLA do. I'd lean towards Tuba HTs for the subs. Assuming size is not an issue they could be 30 to 36 inches wide, outdoors four of those would convince an entire neighborhood that real T Rex's were wandering about.

[AntonZ]

How high is the trailer? Won't the subs be just a tad too high from the ground on a trailer for the lowest freqs (cancellations at 3 or 4 feet from the ground)? If so, it makes sense to high pass at the limit for these cancellations and use T48's instead?

[TomG]

Thanks for the responses! The trailer is an old boat trailer that sits very low to the ground. The completed subwoofers will only be about 2 feet off of the ground. If necessary, we could even add a removable piece of plywood that would couple the subs to the ground. Once again, the subs will be permanently mounted to the trailers so physical size would not be a issue. All this work because of a big old T-rex!

[maxmercy]

How big is this screen going to be? If I read right above, all three (Left Center Right) channels and subs and screen are going to be permanently attached to the trailer?

BTW - I love the idea of a 'movie in the park', especially since you are taking the sound part of the presentation seriously...

JSS

[TomG]

“Movie in the Park” is a great community event. The movies are free and everyone is welcome to bring a blanket or lawn chair to enjoy the movie. Currently we project the image on the side of a two-story building. This makes for a large picture size. Our trailer is twenty feet long. We are planning to make a frame out of PVC or metal conduit and stretch a screen across it. If we mount the right and left channel speakers on each side of the trailer, it would allow us a screen size of about 16’ by 9’. If we mount the speakers below the screen, we could even go bigger. The one main problem we face with screen size is that the screen acts like a sail. The larger the screen, the greater the wind load. We may have to attach guide wires that stake into the ground.

Everyone seems to love the large picture and the “drive-in movie” atmosphere. Right now, our budget P.A. system just does not do it justice. We have volume, but not much else. A quality surround sound system would really make it a great experience. When I searched on line, I was surprised that I could not find much information about people doing this kind of a thing.

[maxmercy]

Please keep us posted on your progress, I am keen to see the finished product, especially how you will deal with the 'screen as a sail' issue...

With a 16x9 screen, most of your audience will be around 22 feet or more away from the screen for a 40 degree viewing angle (the closest THX recommendeds for 1080p HD resolution screen; audience can be closer for a film presentation). so I'm guessing the sweet spot for audio will be around 20-50 feet from the screen (roughly 6-16 meters). With 4 36" wide THTs, I think you can get around 112dB from 30Hz on up at 8 meters out, and 106dB at 16 meters out, if setting up in the middle of a field, with 100 watts into each THT (if I am calculating correctly). Dolby reference level is 115dB for the LFE channel. You could have a sound system that most theaters would like to have for your 'movie in the park' shows! I am usually disappointed in most theater sound systems, especially out here in West Texas...THX certification (and possibly even digital sound) has yet to step foot out here...

For main and surround channels, Dolby reference level is 105dB, which you could easily hit with OTop12s. As usual, Bill's recommendations are spot on...

Are you using blu-rays and a 1080p projector, or a 35mm film projector? Just curious...I have had an idea like this in my head for a while as well...also, do you get municipal funding for this?

JSS

[David Robinson]

BTW - I love the idea of a 'movie in the park', especially since you are taking the sound part of the presentation seriously...JSS

They do something similar at the Beach here during summer months. There's a little amphitheater-ish stage where they place the screen. Families bring blankets and lawn chairs. It's really a nice way to spend an evening. I like your idea of a mobile thing, though. We've got several "pocket parks" throughout Jacksonville that go largely unused after dark...I think it's time for me to call the City Council!

[davygrvy]

Height off the ground for the subs will diminish the loading. Consider placing the subs on the ground, it will make a large difference.. try it once you've made your subs! 2 feet is actually a lot and enough to drop 1/2pi loading to probably 1/3pi. You'll lose your last precious octave. Best bass would be against a wall such as the bottom of the two-story building you mentioned. 1/2 space is more difficult and will require twice as many subs to equal a wall loaded setup.

[Bill Fitzmaurice]

Height off the ground for the subs will diminish the loading. .

With a 100 Hz crossover the subs need to be within 2.8 feet of the ground for full loading. So if the trailer isn't too high there's no problem. But four feet up would certainly be detrimental.

[maxmercy]

I imagine the THTs should be v-coupled for best output, correct? Here's a place where I found a screen that seems made for this type of thing:

www.airscreen.com

I'm sure DIY with a frame and screen stretched on it you could save some money, though.

JSS

[davygrvy]

Height off the ground for the subs will diminish the loading. .

With a 100 Hz crossover the subs need to be within 2.8 feet of the ground for full loading. So if the trailer isn't too high there's no problem. But four feet up would certainly be detrimental.

Are you saying there wouldn't be any measurable difference if I took my stack of subs off the ground and put them up on chairs?

[Bill Fitzmaurice]

Height off the ground for the subs will diminish the loading. .

With a 100 Hz crossover the subs need to be within 2.8 feet of the ground for full loading. So if the trailer isn't too high there's no problem. But four feet up would certainly be detrimental.

Are you saying there wouldn't be any measurable difference if I took my stack of subs off the ground and put them up on chairs?

Not down low. Boundary reinforcement takes place within 1/4 wavelength. But there would be no advantage to lifting them either.

[TomG]

Thank you to everyone who is willing to share suggestions and ideas. It has really helped me in the initial planning of this project. I will be sure to keep you all posted as we move along with the construction. Also, thank you Mr. Fitzmaurice for creating this forum. I am relieved to know that I am not the only crazy person out there who gets so much enjoyment out of these kinds of projects.

“Movies in the Park” is a wonderful event to do in any community. The only expense is the cost of the equipment. Beyond that, it is a very inexpensive way to bring a community closer together. Due to licensing agreements, we are not permitted to charge an admission fee, however we do generate some revenue from doing a concession. We are located in The City of Flagler Beach, on the east coast of Florida. Movie nights are the last Saturday of the month during the spring and the fall. We do not show movies during the winter because the weather is too unpredictable. The temperature may be in the 70’s (today is 80 degrees) or in the 30’s. We also do not show movies during the summer time because we have mosquitoes and gnats which could probably devour a small child in the time it takes to view the coming attractions. That’s the price we pay for living in paradise.

At this time, we are using a standard Infocus high definition projector. The movies are projected on the side of a building in one of our parks. It is a stucco wall which is painted a light peach color. The picture quality is not too bad, but obviously we can make some improvements. This is one of the reasons for building a “real” movie screen.

As far as my screen dilemma, I first looked into the inflatable screens. They seem to have the advantages of being light and portable, but they became very expensive as they become larger. I also came across another kind of mesh screen material on line. The claim was that the mesh holes were large enough allow wind to pass through yet small enough that it would appear solid so that it would not degrade the picture quality. It seemed to have gotten mixed reviews from other people who had posted on-line comments.

I have also contacted some people who have had success with “blackout cloth”. It is the material used to line the back of curtains to make them opaque. Blackout cloth is inexpensive and comes in a 12 foot width. I am still not convinced that it would be the best material for outdoor use. Our best alternative might be canvas. If we can locate a sail maker, we could probably get a screen made to a custom size.

Of course, the bigger the screen, the more wind will become an issue. Out maintenance supervisor suggested that a galvanized frame secured by guide wires should keep the screen secure. Another suggestion was to position the screen up close to the side of a building. Not only would the building act as a wind break, but it will also help to load the subwoofers. Finally, we just will not set up the screen if the weather forecast calls for strong winds.

As for the subs, I measured the trailer and it is 17 inches from the ground. The width of the trailer will enable us to V-load 4 subs in a row or stack them in two columns. I am not sure which way would provide a better bass response, however, keeping the subs in a row will result in a lower center of gravity for the trailer, which will provide better support for the screen.