Film-Tech Forum ARCHIVE: Surround Sound In An Old Theater

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Posted by Edward Jurich (Member # 1787) on 07-31-2003, 07:33 AM:

Anyone installing surround sound in an old theater might want to take note of this problem.
I've been helping a small town theater put in a Dolby CP50 system. The theater was built around 1934 and the auditorium is about 3 times longer than wide. Once I got the surround sound up I discovered that with such a long auditorium, if the surround level is set good for the speakers in the back, the wall speakers near the screen can't be heard. If the surround is set so the wall speakers near the screen are good, the speakers in back are too loud. (The closer you get to the screen the louder the screen sound) We ran a wire for each wall speaker back to the booth in case they decide to install digital. With it wired this way, I was able to pad down the speakers for the back half of the auditorium so the surround sound level is even anywhere you sit.

Posted by Darryl Spicer (Member # 711) on 07-31-2003, 10:22 AM:

what is the distance between each surround speaker and how many do you have going down the side of the wall. Are you using two amps to power each side. Did you take a spl measurment in the auditorium to compare preasure levels in the front compared to the back. What is the distance from the floor to the cieling in back compaird to the front. Did the surrounds get installed using the same hieght distance from the floor or the ceiling. They should have used the distance from the floor so the surrounds maintain the same distance from the slope of the floor. All this is essential to getting even sound in the surround field.

Posted by Manny Knowles (Member # 1171) on 07-31-2003, 10:44 AM:

I suppose this is mono surround with the CP50?
Is it just one amp channel you are using?
What equipment were you using for EQ and level setting?

Posted by Ian Price (Member # 14) on 07-31-2003, 01:02 PM:

In long or overly large auditoriums I have seen installations use delays to sets of surround speakers. This addresses exactly these problems. But it takes a very well trained tech with a great set of ears to set up properly. Most large concert sound engineers are well acquainted with properly setting up delay channels. But they do it for different reasons.

Posted by Edward Jurich (Member # 1787) on 07-31-2003, 02:46 PM:

There is a big slope in the floor which puts the back of the auditorium near the ceiling. We have 8 - 12" speaker systems, 4 on each side. We actually didn't go all the way back because it's so close to the ceiling and nobody sits back there anyway. This was a budget project, they really don't have the money to get the test equipment and I don't have it. I did set the optical pickup with a scope however and it looks good. There is one amp driving all 8 surround speakers. The speakers are connected in parallel/series (all in phase) except the back 4 speakers have a pad on them. Behind the screen is left/center/right using the smaller Altec VOT's. The surround speakers are the same height down from the ceiling, it just looked better in this old theater. Although the front speakers being higher up may cause them to be a lower level, it's more the volume level being close to the screen that was a factor.
It actually sounds quite good. Could use a sub-woofer but still has good deep bass.

Posted by Steve Kraus (Member # 476) on 07-31-2003, 04:30 PM:

What kind of wire are you using that you're getting such a large level difference based on the lengths of the runs?

Posted by Manny Knowles (Member # 1171) on 07-31-2003, 08:37 PM:

My inclination is to suspect the choice to mount the surrounds near the front so high up off the floor. Not only are they further away but chances are they aren't really angled right for that height either.

AUDITORY aesthetics should be the priority when positioning speakers. VISUAL aesthetics should take a back seat. Now you have speakers that may LOOK great on the wall but they're doing very little for you in terms of why you went to the trouble and expense of putting them there in the first place.

Seeing as how this operation was on a budget...maybe they could just return those front speakers since they aren't doing much in the final analysis?

Posted by Don Sneed (Member # 955) on 07-31-2003, 09:13 PM:

When I did a large theatre with a overhang balcony, a Dolby CP50 processor (mono surrounds), QSC amps...back 20 yrs. mostly passive speakers were use..I install (1) amp for Left & center channel, (1) amp for Right & surroung channel, (1) extra ampilfier for under balcony & balcony surrounds to controlled the spl to balance the surrounds in the theatre....

I once had a theatre that would not buy a extra amp to do it correctly, I used a 100-watt L-Pad to control the under balcony surrounds, don't like doing it that way due to loss & heat but it works & still working...

Posted by Gordon McLeod (Member # 33) on 07-31-2003, 09:50 PM:

In big theatres I have had one for front one for mid and one for back surrounds and I set each to match the centre channel level in the zone in question

Posted by Edward Jurich (Member # 1787) on 08-01-2003, 10:13 AM:

Wire size, we ran 12 gauge to the front 2 (the longest runs) 14 gauge to the 3rd from the screen and 18 gauge to the 4th (shortest run to the booth). I'm going out there tonight to sit through "Pirates of the Caribbean" as part of the audience to see how it sounds.

Posted by Steve Kraus (Member # 476) on 08-01-2003, 11:17 AM:

You do have a modern Cat.150 2:4 channel decoder and Cat.280T SR NR cards in that ole box, right? [thumbsup]

Posted by Edward Jurich (Member # 1787) on 08-02-2003, 08:59 AM:

I know we have the CAT 150 cards but I'll have to check on the Cat 280. I didn't think the NR cards would matter since the surround is derived in the CAT 150.
As far as speaker placement, I went last night and sat through "Pirates of the Caribbean" and it sounded great. What's remarkable is one scene where someone walks off screen to the right and the footsteps go off screen (surround) to the right. I sat in the middle so I was getting equal (mono) surround level from both sides. I would have to assume that phasing the audio allows enough directional control to do this. I walked around at the beginning of the second show and I will have to admit that once you walk past the last surround speaker in the rear, the added dimension in the sound goes away. I guess the back is reserved for patrons who do not prefere surround sound because it makes them dizzy [Big Grin]

Posted by Darryl Spicer (Member # 711) on 08-02-2003, 10:22 AM:

quote:
I will have to admit that once you walk past the last surround speaker in the rear, the added dimension in the sound goes away.

Thats one reason why I wonder if it would be feasable in those long throw auditoriums to put a surround speaker facing out diagonaly in each back corner. To eliminat thos sound field holes in the back of the theater.

Posted by Steve Kraus (Member # 476) on 08-02-2003, 08:58 PM:

Right, the SR cards wouldn't particularly affect the surround but necessary for the overall quality of the sound since the vast majority of films are in SR. Nowadays one is more likely to encounter mono than A-type.

An SR track is recorded several dB lower in level than an A track (trading off some of the noise floor improvement for more headroom). On a system with built-in SR or the external SR adapters this level difference is handled automatically. But when running on a processor where SR is achieved simply by swapping the A cards for SR the playback level needs to go up a bit from the level set during calibration by turning up the fader.

As a practical matter, though, given how loud some movies are mixed where playing them on a regular system at calibrated level (7.0) ends up being "too damn loud" it's very likely you'll be happy just leaving the fader at the calibration point even though hypothetically this is a few dB down from the level they mixed to.

Posted by Manny Knowles (Member # 1171) on 08-03-2003, 12:06 AM:

We've always put a couple of surround speakers on the back wall. Typically, there would be three (or four) on the each wall with one (or two) on the back wall for each side. This ensures that the back row(s) aren't getting shared out.

I thought everybody did it this way until I started paying attention to the interiors of other theatres. It turns out that quite a number of theatres have their surrounds on the side walls only.

Posted by Darryl Spicer (Member # 711) on 08-03-2003, 12:12 AM:

we have ours on the back wall too. But what I am saying is in those long auditoriums put them in the corners of the walls and direct them diagonally. Basically down the sides and one in each back corner.

Posted by Edward Jurich (Member # 1787) on 08-03-2003, 09:48 AM:

The problem here is the back of the auditorium is sloped upward so high that you can reach up and touch the ceiling. We figured any speakers mounted there would be prone to tampering. We went as far back as we could without the surround speakers being close to the seats. It's just as well, even with being able to dial in delay, with an auditorium that long there would always be a noticeable echo point regardless where the delay was set, depending on where you sit. Everyone pretty much sits in the front half anyway.

Posted by Karagir Khush (Member # 1785) on 08-11-2003, 08:55 AM:

I would suggest to use large no. of small speakers, than small no. of large speakers. This will give a more evenly distributed sound for the surround channel.

Regards,

Karagir.