Outdoor Surround Sound System

[bgavin]

What the tops require is voltage. Power is really secondary, and indicative of the limitation imposed by your amplifier.

Determine your voltage requirements for each top
A conservative value is 50% of the rated input power.
For a Deltalite-II 2512, this is 250/2 = 125w.

Volts = Sqrt (RatedPower * RatedOhms)
Using Excel: =sum((125 * 8 ) ^0.5) is 31.62v
You need 31.62v applied to each 2512 in your system.

Watts = (Volts * Volts) / Impedance
250w = two 2512 parallel at 4 ohms

An amp as small as the RMX-850 will drive up to four 2512 in parallel (2 per channel) to half power.
It is pointless to run much higher than 50% rated power, due to power compression and reduced headroom.

[maxmercy]

with 40 Watts into each, you would get around 118dB at 1 meter with an Otop12, if I am reading the charts correctly. 105dB is the Dolby Reference Level for peaks for the L/C/R and surround channels (calibrated at the venue by playing pink noise recorded at -20dBFS and adjusting speaker level until an SPL meter reads 85dB at the center of the listening area). That means you would be at 106dB for an audience 4 meters (just over 13 feet) away, and 100dB at 26 feet away.

That would leave you with 3dB of headroom at around 15 feet...I know, not much, but have you ever heard a movie at reference level? It is pretty damned loud (esp. summer action movies), and I have read that you should get 3 or 4 'way too loud' complaints for every 100 people that watch a movie at reference. That's folks that will take time-out to see the theater manager, not folks who just thought "I'm never coming to this theater again", and decided to say nothing. I do have to say, most movie houses I have experienced are simply not equipped to handle reference levels, especially at the lower octaves. I have been to more than one cinema where loud, but highly distorted sound was all we got out of the theater loudspeakers (maybe young projectionists careless with the controls?).

Also, surround volume is where lots of people will differ. There are many that turn down their surrounds so that surround content will not be as 'in your ear' and 'dominant' for people sitting closer to the surrounds, and there are others who really like hearing what the surrounds have to say, even to the point of watching guests turn their heads during certain scenes...away from the action on the screen.

I'd say try it, and if it is a no-go, getting a receiver with pre-outs should not be a huge problem. After all, you are planning on building a bunch of Otop12s and 4 THTs 36" wide, if I remember correctly...

If you will get a new receiver, get one with the Audyssey MultEQXT software. It will do an RTA (and more) and equalize (and I believe, time-align) every speaker for you...no need to buy separate EQs...and some MultEQ equipped receivers do all the Audyssey processing BEFORE the pre-outs, so your power amps will be part of the chain that MultEQ measures and adjusts for...

JSS

[ben.in.salzburg]

You could use a DI-box to bring down the voltage to PA line level... might sound interesting though.

[RBIngraham]

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.

If you are still looking for screen material, I have seen a few set designers use that white plastic boat cover material they put on boats to cover them up in the off season as a projection screen. They use it of course because it's cheap and easy to get around here and when you are done with they just toss it. But maybe it might be worth while to look into that. It would at least be inexpensive, and if it got ruined you wouldn't want to cry. Just break out the roll and put some more over the frame. Wouldn't help with any wind issues however.

Just a thought.

[RBIngraham]

Pardon my ignorance, but how do movie theaters get away with surrounds (other than a high mounting position)? There are theaters in the 100 foot range dimension-wise...with seats wall-to-wall...although I will have to say, I am no fan of sitting near the side-walls....the surround speakers do intrude and are too noticeable. But yet there are people seated next to walls every time the theater is crowded...

JSS

Movie theatres do it by having numerous surround sound speakers all along the outside and back walls and each one of them has some time allingment delay added to them, so they don't produce an echo from any audio signals that are also sent from your main L-C-R speakers. Everything is time alligned to the screen. Doing this outdoors to the same extent would likely be very expensive and lot of work to set up properly. But I think you'll still get a nice effect. The main thing is time allignment delay should be used for the surrounds and you just don't go crazy with the volume levels of the surrounds. Make them subtle so that if someone is sitting close to one they don't get blown away by the surround tracks and then miss all the dialog which typically comes out of the center channel only, unles it's 7.1 rather than 5.1.